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Overview

Comprehensive Description

Turtle Grass (Thalassia testudinum) is an important seagrass found from Bermuda and southern Florida south to the Gulf of Mexico, the West Indies, Central America, and Venezuela. It can form very extensive beds in protected shallow waters that serve as both habitat and a food source for a tremendous diversity of organisms, among them sea turtles, which graze on T. testudinum and are the source of its common name. (Dineen 2001 and references therein)

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Seven species of seagrasses (Thalassia testudinum, Halodule beaudettei (formerly H. wrightii), Syringodium filiforme, Ruppia maritima, Halophila engelmannii, Halophila decipiens and Halophila johnsonii) occur in the Indian River Lagoon, Florida. An illustrated key and guide to their morphology and distribution is presented by Eiseman (1980).Orpurt and Boral (1964) redescribed the flowers, fruits and seeds of Thalassia testudinum and detailed fruit development and seed germination. It was estimated that it takes about 8 weeks for fruit to mature after pollination. Tomlinson and Vargo described the vegetative morphology of Thalassia testudinum (1966) and further described root functional morphology (1969 a), floral morphology and anatomy (1969 b), and leaf anatomy and development (1972).
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Distribution

Thalassia testudinum is restricted to the Gulf of Mexico and the Caribbean and has been recorded from Bermuda (the other species in the genus, T. hemprichii, is widely distributed in the coastal waters of the Indian Ocean and the western Pacific) (Larkum et al. 2006).

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National Distribution

United States

Origin: Unknown/Undetermined

Regularity: Regularly occurring

Currently: Unknown/Undetermined

Confidence: Confident

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Global Range: Bahamas, Bermuda, West Indies, Cayman Islands, FL to TX, to S. America.

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Range Description

Thalassia testudinum occurs in the western central Atlantic from Florida, USA to Venezuela, throughout the Gulf of Mexico and the Caribbean Sea. It is also found in Bermuda.
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Fla., La., Tex.; Mexico; West Indies; Central America; South America (Colombia).
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Thalassia testudinum is distributed from just north of Sebastian Inlet, Florida south to the Gulf of Mexico, Bermuda, the West Indies, Central America and Venezuela (Eiseman 1980).Several factors, such as temperature, salinity, water depth, turbidity and wave action can potentially limit the distribution of Thalassia testudinum. The absence of T. testudinum beds along the Louisiana Coast is thought due to increased turbidity and low salinity.Along the northwestern Cuban shelf, Thalassia testudinum was by far the most abundant seagrass accounting for 97.5% of seagrasses present, and was found at depths to 14 meters but occurred more abundantly in the first 5 meters of depth. When occurring alone, Thalassia was more abundant in substrata composed of mud and sand, colonizing better on coarser bottoms (Buesa 1975). This study also reported that red light (620 nm) promoted optimum growth of Thalassia. Thalassia testudinum is the dominant seagrass in southeast Florida as well as the Florida gulf coast. Seven species of seagrass occur in the India River Lagoon. Of these, 6 are known throughout the tropical western hemisphere, while Halophila johnsonii is known only from coastal lagoons of eastern Florida, . Halodule beaudettei is the most common. Ruppia maritima is the least common and is found in the most shallow areas of the lagoon. Syringodium filiforme can be locally more abundant than H. wrightii. Thalassia testudinum occurs in the southern portion of the India River Lagoon (Sebastian Inlet and south). Halophila decipiens, Halophila engelmannii and Halophila johnsonii can form mixed or monotypic beds with other species. Because of their abundance in deeper water and high productivity, the distribution and ecological significance of the 3 Halophila species may have previously been underestimated (Dawes et al 1995).The northern area of the Indian River Lagoon supports the most developed seagrass beds, presumably because of relatively low levels of urbanization and fresh water inputs. Four species of seagrass - Halodule beaudettei, Syringodium filiforme, Halophila engelmannii and Ruppia maritima - can be found north of Sebastian Inlet, while all 7 species occur to the south (Dawes et al 1995). Seagrasses were ranked in order of decreasing percent cover by Virnstein and Cairns (1986) as follows: Syringodium filiforme, Halodule beaudettei, Halophila johnsonii, Thalassia testudinum, Halophila decipiens, Halophila engelmannii and Ruppia maritima.Thalassia testudinum occurs in the southern half of the Indian River Lagoon at mid-depths. T. testudinum can be locally abundant, often occurring in monotypic stands and appears to be increasing in abundance in the Indian River Lagoon (Virnstein 1995). In 1980, Eiseman reported that Thalassia testudinum was distributed sparsely in the Indian River Lagoon: small patches were found near St. Lucie inlet and from Fort Pierce Inlet to Vero Beach, Thalassia testudinum occurred relatively abundantly, but only in scattered patches from Vero Beach north to Sebastian inlet.Philips (1960) reported on Thalassia testudinum in the Indian River Lagoon occurring near St. Lucie, Fort Pierce and Sebastian Inlets and speculated that Sebastian Inlet was probably the northern most limit of Thalassia on the east coast of Florida.The distribution of 3 species of seagrass was mapped in a 15 ha area in mid-Indian River Lagoon. Halodule beaudettei and Syringodium filiforme were more abundant in shallow and deeper water respectively. Thalassia testudinum occurred in patches. Areal coverage (%) of monospecific stands of these three species was 35% for Syringodium, 14% for Halodule and 6% for Thalassia. Mixed beds, mostly Syringodium and Halodule accounted for 25% coverage. Biomass (above-ground) was greatest during the summer and minimum in late-winter. In this same study area, drift algae, primarily Gracilaria spp. was initially mapped and then sampled in order to estimate its abundance. It was concluded that, at times, drift algae can be quantitatively more important than seagrass in terms of habitat, nutrient dynamics and primary production (Virnstein & Carbonara 1985).Depth: Phillips (1960) reported depth distributions of Thalassia testudinum in Florida by various investigators. Depths ranged from the intertidal zone to 100 feet on Molasses Reef off Key Largo. He concluded that assuming favorable temperatures, water clarity is the major factor in determining depth distribution of Thalassia.When occurring in a mixed seagrass flat, Halodule beaudettei occurred closest to shore. Ruppia occurred in slightly deeper water. Thalassia testudinum, although probably preferring continuous submersion, was limited by the neap tide low water mark, whereas Syringodium was limited by the spring tide low water mark and will be found in the deepest parts of the mixed flat (Phillips 1960).Turtle grass was reported at depths deeper than 30 feet in clear waters of the Bahamas and only to 6 feet in murky conditions (Tampa Bay) (Stephens 1966). Thalassia is not tolerant of strong wave surge, growing only in protected areas (Moore 1963).Distributional Changes: Changes in seagrass distribution and diversity pattern in the Indian River Lagoon (1940 - 1992) are discussed by Fletcher and Fletcher (1995). It was estimated that seagrass abundance was 11 % less in 1992 than in the 1970's and 16 % less than in 1986 for the entire Indian River Lagoon complex (Ponce to Jupiter Inlet). Decreases in abundance occurred particularly north of Vero Beach. In this area of the lagoon, it was also estimated that maximum depth of seagrass distribution has decreased by as much as 50 % from 1943 to 1992. Alteration of such factors as water clarity, salinity and temperature could affect the diversity and balance of seagrasses in the Indian River Lagoon system and should be considered when developing management strategies for this resource (Fletcher & Fletcher 1995).Mapping: Sources of mapped distributions of Indian River Lagoon seagrasses include the following: 1) Seagrass maps of the Indian & Banana Rivers (White 1986); 2) Seagrass maps of the Indian River Lagoon (Virnstein and Cairns 1986); 3) Use of aerial imagery in determining submerged features in three east-coast Florida lagoons (Down 1983); and 4) Photomapping and species composition of the seagrass beds in Florida's Indian River estuary (Thompson 1976). Data from the first two sources (White 1986; Virnstein & Cairns 1986) is now available in GIS format (ARCINFO) (see Fletcher & Fletcher 1995).
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Physical Description

Morphology

Description

Rhizomes elongate, 3--6 mm thick. Leaves 10--60 cm ´ 0.4--1.2 cmm, margins entire proximally, minutely serrulate near apex; veins 9--15. Inflorescences: staminate inflorescences 1--3-flowered; peduncles 3--7 cm; margins of spathes connate on 1 side; pistillate inflorescences 1-flowered; peduncles 3--4 cm; spathes connate on both sides. Flowers: staminate flowers: pedicels 1.2--2.5 cm; stamens 9; pistillate flowers nearly sessile, styles 7--8. Fruits bright green to yellow-green or red, 1.5--2.5 cm diam., dehiscing in 5--8 valves; beak 4--7 mm.
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Size

Beds of Thalassia testudinum, destroyed from thermal effluent in Biscayne Bay, FL, were restored by planting "thousands" of seeds in late summer. Approximately 3 & 1/2 years later, blade density in restored areas averaged 2030 blades per square meter, almost equivalent to control areas. Also, after this time interval, flowering occurred in the restored bed in the spring with subsequent fruiting in late summer. This temporally defined sexual maturity in T. testudinum: 3.5 years from seed to flower and 4 years from seed to seed (Thorhaug 1979).T. testudinum undergoes seasonal fluctuations in productivity. Productivity, standing crop, blade length and density reach a maximum during warm summer months. Blades of Thalassia testudinum can grow rapidly, up to 1 inch per week under ideal conditions (Stephens 1966). Average growth rates for Thalassia were also estimated at 2 - 4 mm/ leaf per day, with maximum growth at 12.5 mm/leaf per day (Zieman 1975).Shoot longevity and rhizome turnover, rather than capacity to support dense meadows, are key elements in determining either pioneer species (Halodule beaudettei and Syringodium filiforme) vs. climax species (Thalassia testudinum) of seagrass (Gallegos et al 1994). Because of stored starch in the rhizomes, Thalassia can withstand environmental stress for some time (Zieman 1975). However, it was estimated that it takes approximately 2 - 5 years for a Thalassia testudinum bed to recover from physical disturbance of the rhizome system, most often caused by motor boat propellers. Disturbance of this nature results in a loss of the fine sediment component and a lowering of pH and EH (Zieman 1976).Growth and Light: Growth of Thalassia testudinum, Halophila engelmannii, Ruppia maritima, Halodule beaudettei and Syringodium filiforme was investigated in the laboratory, at various light intensities. Optimum growth for all five species was obtained at light intensities of 200 - 450 foot-candles. At light intensities above or below this range, growth was much slower for all species (Koch et al 1974).Because of the seasonal and spatial (flowering plants more abundant in the Miami area than in Tampa Bay) nature of flowering, often occurring when summer solstice occurred, the relationship of temperature and photoperiod relative to reproduction had been suggested (Phillips 1960). However, water temperature, as opposed to photoperiod, appears to be more influential in controlling floral development as well as subsequent flower density and seed production in seagrasses. Laboratory experiments showing flowering induction under continuous light suggests that photoperiod probably plays a limited role in sexual reproduction (Moffler & Durako 1982).Restoration: Beds of Thalassia testudinum, destroyed from thermal effluent in Biscayne Bay, FL, were restored by planting "thousands" of seeds in late summer. Approximately 3 .5 years later, blade density in restored areas averaged 2030 blades per square meter (m2), almost equivalent to control areas. Also, after this time interval, flowering occurred in the restored bed in the spring with subsequent fruiting in late summer. This temporally defined sexual maturity in T. testudinum: 3.5 years from seed to flower and 4 years from seed to seed.In a transplant feasibility study, fragments of Thalassia testudinum and Halodule (Diplanthera) wrightii were transplanted to both aquaria and flow-through seawater systems. In the aquaria, Thalassia survived for 7 months, whereas Halodule survived for only 3&1/2 months. In the flow-through seawater tanks, Thalassia survived 12 months and produced new leaves, roots and rhizomes. Only a few Halodule plants survived in the flow-through system. These results suggested that transplantation of Thalassia fragments could provide a means of restoring seagrass beds impacted adversely by coastal development (Fuss & Kelly 1969).Thorhaug (1979) discussed restoration and mitigation efforts of seagrasses in the Gulf of Mexico, Florida and the Caribbean. Thalassia testudinum was the dominant species throughout much of the Caribbean and Gulf of Mexico. It was concluded that: restoration efforts including seeding, plugging and turion planting of various seagrasses can be successful in one area, but not in another; both Halodule and Syringodium can be successional stages to a Thalassia community; food webs can differ between Thalassia and Halophila; and faunal diversity and abundance as well as epibionts and associated macroalgae can also differ between Thalassia and Halodule in many locations (Thorhaug 1979).
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Type Information

Isotype for Thalassia testudinum Banks & Sol. ex K.D. Koenig
Catalog Number: US 934188
Collection: Smithsonian Institution, National Museum of Natural History, Department of Botany
Preparation: Pressed specimen
Collector(s): M. Bang
Year Collected: 1890
Locality: Vicinity of La Paz, Lake Titicaca., La Paz, Bolivia, South America
Elevation (m): 3962 to 3962
  • Isotype: St. John, H. 1964. Caldasia. 9: 111.
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Isotype for Thalassia testudinum Banks & Sol. ex K.D. Koenig
Catalog Number: US 32340
Collection: Smithsonian Institution, National Museum of Natural History, Department of Botany
Preparation: Pressed specimen
Collector(s): M. Bang
Year Collected: 1890
Locality: Lake Titicaca, La Paz, Bolivia, South America
Elevation (m): 3962 to 3962
  • Isotype: St. John, H. 1964. Caldasia. 9: 111.
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Ecology

Habitat

Comments: Submerged in coastal waters in large colonies protected by coral reefs.

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Habitat and Ecology

Habitat and Ecology
Thalassia testudinum is the most abundant seagrass in the Caribbean and forms dense rhizome mats below the sediment, creating extensive meadows on shallow sand or mud substrates from the lower intertidal to a maximum 10-12 m depth. It has also been reported below 20 m. In Cuba, this species was found to a depth of 14 m and accounted for 97.5% of total angiosperm biomass (190 g/m²). Optimum temperatures for this species range between 20-30°C (Phillips 1960).

Thalassia testudinum typically dominates seagrass vegetation in reef lagoons, where it often coexists with Syringodium filiforme, Halodule wrightii and calcareous rhizophytic green algae belonging to the order Caulerpales, amongst which Halimeda spp. are the most conspicuous members. This species plays an important role in production of sediments (Zieman 1982, UNESCO 1998, Hemminga and Duarte 2000, Green and Short 2003, Larkum et al. 2006). It is typically found in low density in oligotrophic areas and replaced by other species when there are continuous high nutrient inputs (Fourqurean and Rutten 2004).

Thalassia testudium is an important food for green sea turtles and manatees, as well as for a number of other fish and invertebrate species.

The dense beds in the Florida Bay are susceptible to primary die-off, which usually occurs during late summer to early winter. Primary die-off is usually followed by reduced water clarity and increase epiphytic growth, which can lead to secondary die-off among neighbouring local seagrass (Nuttle et al. 2003).

Seeds are viviparous and do not form seed banks. After major eradication, recolonization is dependent on import from seeds from other areas or from vegetative fragments (van Tuessenbroek et al. 2006). Seedling success can be variable, and the generation length of this species has been estimated as eight years (for recruitment).

Systems
  • Marine
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Ocean floor consisting of organic matter, rock matter, coral sand, or dead reefs; -10--0m.
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Depth range based on 672 specimens in 1 taxon.
Water temperature and chemistry ranges based on 656 samples.

Environmental ranges
  Depth range (m): 0 - 54
  Temperature range (°C): 26.864 - 27.678
  Nitrate (umol/L): 0.161 - 0.781
  Salinity (PPS): 35.179 - 36.127
  Oxygen (ml/l): 4.630 - 4.746
  Phosphate (umol/l): 0.034 - 0.125
  Silicate (umol/l): 1.406 - 2.300

Graphical representation

Depth range (m): 0 - 54

Temperature range (°C): 26.864 - 27.678

Nitrate (umol/L): 0.161 - 0.781

Salinity (PPS): 35.179 - 36.127

Oxygen (ml/l): 4.630 - 4.746

Phosphate (umol/l): 0.034 - 0.125

Silicate (umol/l): 1.406 - 2.300
 
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Thalassia testudinum grows in shallow coastal waters that are protected from strong wave surge. In clearer water it can be found at greater depths than in murky water. (Dineen 2001 and references therein)

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Trophic Strategy

Photosynthetic rates were determined for three species of seagrass in the Indian River Lagoon. Photosynthetic rates (mg C/g dry wt-h) ranged between 0.009 - 0.395 for Halodule beaudettei, 0.005 - 0.79 for Thalassia testudinum, and 0.009 - 1.72 for Syringodium filiforme (Heffernan & Gibson 1983).The protein, carbohydrate and trace element composition, energy content and nutritive value of Thalassia testudinum and Ruppia maritima were investigated. It was found that relative to other aquatic plants, Thalassia and Ruppia contain substantial amounts of protein, carbohydrate, energy and minerals, but that nutritional value of these plants can vary seasonally (Walsh & Grow 1973).Habitat: Various substrata have been reported to support stands of T. testudinum: e.g., hard packed to course, muddy sand; soft marl or mud; silt and clay-sized sediment; very fine, loose grayish calcium carbonate. Common to all these substrata was the presence of calcium carbonate with the substrata itself presenting anaerobic conditions (Phillips 1960).The rhizome of Thalassia testudinum is usually buried from 2 to 4 inches in the substratum (Phillips 1960) but was also observed at 25 cm and more in Florida Bay (Ginsburg & Lowenstam 1958).
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Associations

Other Seagrasses: Although Thalassia testudinum can be locally dominant, it is often associated with other species of seagrass. For example, although preferring slightly shallower water, Thalassia is often associated with Syringodium below the low tide line. Halophila engelmannii (Moore 1963) can co-occur inconspicuously with both Thalassia and Syringodium, because of its small leaf size. Halophila is apparently tolerant of shade conditions and can occur at depths of 73.2 - 91.0 meters (Moore 1963). Grazers and Epiphytes: Turtle grass beds serve as both habitat and food source for marine animals. Direct grazing on Florida seagrasses is limited to a number of species, e.g., sea turtles, parrotfish, surgeonfish, sea urchins and perhaps pinfish. Other grazers, e.g., the queen conch, scrape the algae present on seagrass leaves (Zieman 1982). At least 113 epiphytes and up to 120 macroalgal species have been identified from Florida's seagrass blades and communities respectively (Dawes1987). Although few animals graze directly on seagrass, its epiphytic community (bacterial films, diatoms and algae) provide food for small animals at the base of the food chain to be consumed by young fish and caridean shrimp (Moore 1963).A species list of seagrass epiphytes of the Indian River Lagoon, FL, was provided by Hall and Eiseman (1981). Forty one species of algae occurred on the seagrasses Syringodium filiforme, Halodule beaudettei and Thalassia testudinum. Epiphytic algal diversity and abundance was generally higher in winter and spring and lowest during late summer and early fall. Macrobenthos: In Card Sound, FL, although molluscan biomass (2.31 g dry/m2) associated with turtle grass beds exceeded the polychaete and pericaridean crustacea biomass (1.74 g dry/m2), it was thought that the former taxa accounted for the main interaction between primary consumers and higher-level predators. The main fish predators in this system were the syngnathids and the gold-spotted killifish (Brook 1977).An Indian River Lagoon, Fl, study compared the abundance of macrobenthic invertebrates and epifauna in seagrass (Thalassia testudinum, Halodule beaudettei and to a lesser extent, Syringodium filiforme) vs. adjacent sandy bottom habitats (Virnstein et al 1983). Both groups, especially the epifauna, were found to be both more abundant in seagrass habitats and also more heavily preyed upon and hence more trophically important than seagrass infauna. The primary transfer path to higher trophic levels occurs through the epifaunal macrobenthos in seagrass habitats and through the infauna of sandy habitats (Virnstein et al 1983).A comparison of faunal communities between thermally impacted and stable Thalassia testudinum beds was undertaken in Biscayne Bay, FL. Species abundance and diversity between restored areas and those that had not recovered from thermal impact were statistically significant. No differences were seen between restored areas and those that were not impacted. Certain groups of animals, e.g., pink and caridean shrimp as well as juvenile fish were numerically higher in restored areas than at control sites, and a magnitude higher than at non-recovered areas (McLaughlin et al 1983).A high standing crop of Thalassia testudinum does not necessarily indicate macrofaunal abundance. For example, when five turtle grass communities were sampled (4 in Biscayne Bay and 1 in the Everglades), abundance of macrofauna ranged from 292 to 10,728 individuals per m2. Other factors such as sediment type and total organic carbon (TOC) could affect organisms living in the sediment water interface as well deposit feeders (Brook 1978).Amphipods: Amphipods are capable of detecting differences in density of seagrasses and will choose areas of high blade density, presumably as a prey refuge. When 3 different species of seagrass, Thalassia testudinum, Syringodium filiforme and Halodule beaudettei were offered to amphipods at equal blade density, amphipods chose H. wrightii because of its higher surface to biomass ratio (Stoner 1980). Decapods: A study of decapod crustacea associated with a seagrass/drift algae community in the Indian River Lagoon, FL showed remarkable diversity. The seagrass community sampled was composed of 4 species, 3 of which were abundant: Syringodium filiforme; Halodule beaudettei; and Thalassia testudinum. Brachyuran crabs and caridean shrimp comprised the majority of decapods. In all, 38 species in 28 genera and 17 families were sampled. The crustacean community was regulated by above ground plant abundance i.e., a function of habitat complexity. It was concluded that competitive exclusion rather than predation was more important in regulating habitat diversity of the macrocrustacean community in these seagrasses (Gore et al 1981). Habitat Diversity: Virnstein (1995) suggested the "overlap vs. gap hypothesis" to explain the unexpectedly high (e.g., fish) or low (e.g., amphipods) diversity of certain taxa associated with seagrass beds. In a highly variable environment such as the Indian River Lagoon, diversity of a particular taxa is related to its dispersal capabilities. For example, amphipods, lacking a planktonic phase, have limited recruitment and dispersal capabilities, whereas highly mobile taxa such as fish (which also have a planktonic phase) would tend to have overlapping species ranges and hence higher diversity (Virnstein 1995). For an extensive treatment of seagrass community components and structure including associated flora and fauna, see Zieman (1982).
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Seagrass beds provide food and shelter, both directly and indirectly, to many ecologically and economically important fish and shellfish species. Over 100 species of fishes and over 30 crustacean species are found in Florida Bay, including both permanent residents and temporary residents using seagrass habitat as a nursery ground, such as spotted seatrout, redfish, snook, tarpon, snappers, and grunts. Important shellfish species include pink shrimp from the Tortugas bank, blue crabs, and spiny lobsters. (Robblee 1991)

Kirsch et al. (2002) studied grazing by smaller herbivores (e.g. the bucktooth parrotfish Sparisoma radians) on Thalassia tedudinum in Hawk Channel, in the northern Florida Keys (USA). They found that that seagrass grazing varied greatly both spatially and seasonally but, on average, grazers consumed virtually all of the aboveground production at 2 of the 3 sites studied. When experiments were repeated in the summer of a second year at 6 sites, seagrass grazing again varied greatly among sites, but at 3 of the sites most of the daily production of seagrass shoots was consumed by small herbivorous fishes. These results suggest that while it is undoubtedly true that modern day grazing by manatees, turtles, and waterfowl on seagrass is reduced relative to historical levels due to declines in populations of these large grazers, small vertebrate grazers nevertheless consume a substantial fraction of seagrass production in the northern Florida Keys.

A variety of sea urchins may graze heavily on Thalassia testudinum, sometimes even overgrazing (i.e., grazing at a rate that exceeds the seagrass growth rate), which may dramatically reduce seagrass biomass, leading to a restructuring of the local ecosystem (Eklof et al. 2008 and references therein).

Tussenbroek and Brearley (1998) found a burrowing isopod, Limnoria simulata, in sheaths of Thalassia testudinum in the Puerto Morelos reef lagoon, Mexican Caribbean, and this isopod likely has this habit across the Caribbean (Tussenbroek and Brearley 1998).

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Known predators

Thalassia testudinum (turtle grass) is prey of:
Cheloniidae

Based on studies in:
Polynesia (Reef)

This list may not be complete but is based on published studies.
  • W. A. Niering, Terrestrial ecology of Kapingamarangi Atoll, Caroline Islands, Ecol. Monogr. 33(2):131-160, from p. 157 (1963).
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General Ecology

Ecology

The widespread decline of seagrass, in particular Thalassia testudinum, in Florida Bay (Florida, U.S.A.) in 1987 was followed by a cascade of ecological effects. By 1992, frequent phytoplankton blooms began to appear in the central and western bay where none had been recorded previously. Negative impacts extended to higher trophic levels as well, including 100% mortality of some sponge species. Spiny lobster and pink shrimp catches at Tortugas Banks plunged in 1988 to their lowest levels in decades and game fish catch also declined. Algae blooms persist and the bloom ‘‘footprint’’ has expanded to include the eastern bay. (Madden et al. 2009 and references therein)

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Life History and Behavior

Cyclicity

Flowering/Fruiting

Flowering spring--summer.
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Reproduction

Plant increase and growth of Thalassia testudinum can occur either by sexual or vegetative reproduction. Seasonality of both growth and biomass is exhibited by all species of seagrass in the IRL, including Thalassia testudinum, being maximum during April - May and June - July respectively. However, since vegetative reproduction occurs at least to some extent during 9 months of the year, it was felt that this type of reproduction probably accounts for the maintenance and spread of (Thalassia) seagrass beds (Phillips 1960). Zieman (1975) also concluded that sexual reproduction in T. testudinum is not that extensive and that vegetative reproduction probably accounts for significant spreading of turtle grass beds.Flowering: T. testudinum has both staminate and pistillate flowers. Reports of flowering in Thalassia testudinum indicate reproductive seasonality. In Biscayne Bay, FL, flowers were seen only during the third week in May, with fruits appearing 2 - 4 weeks later. Fruits remained attached to the parent plant until the third week in July at which time they detached and floated off. In Tampa Bay, FL, although evidence of bud development in Thalassia testudinum is apparent in May - June, when water temperatures increase, early bud development was observed in January (Moffler 1981).T. testudinum was seen flowering in the Dry Tortugas in July (1916) and both male and female flowers were seen in early June (1926) (as cited in Phillips 1960). Among several sites investigated by Phillips (1960), 10% of plants collected in the Florida Keys in late May (1958) were flowering and temperature ranged from 25.5 to 33.5 °C. In Tarpon Springs in July (1958), 5 - 15 % of Thalassia plants collected had female flowers, temperature range was 27.2 - 31.6 °C. Flowering plants (female inflorescence) were found in Tampa Bay in June (1959). It was noted that when Thalassia flowers were found, only one sex was observed (Phillips 1960).Reproduction and flowering of Thalassia testudinum was compared between clones placed in laboratory culture under controlled conditions of light, salinity and temperature, and those in Redfish Bay, Texas. Thalassia could not be induced to produce flowers in the laboratory, nor was Thalassia observed flowering in Redfish Bay. In contrast, Halophila engelmannii produced flowers continuously in the laboratory (January - September), as well as in the field (April - mid-June) implying that conditions inducing flowering in Halophila do not affect Syringodium similarly (McMillan 1976).
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Growth

Development

Under favorable conditions, Thalassia testudinum can grow several centimeters per day (Dineen 2001 and references therein).

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Molecular Biology and Genetics

Molecular Biology

Barcode data: Thalassia testudinum

The following is a representative barcode sequence, the centroid of all available sequences for this species.


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Statistics of barcoding coverage: Thalassia testudinum

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 1
Specimens with Barcodes: 1
Species With Barcodes: 1
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Genomic DNA is available from 1 specimen with morphological vouchers housed at Australia Museum
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Conservation

Conservation Status

Although the IUCN lists the conservation status of Turtle Grass as "Not Evaluated", there are significant conservation issues known to be associated with this marine plant. Notably the Manatee and other marine fauna are dependent on relationships with Turtle Grass, and correspondingly a number of protected areas have been established within the range of the Turtle Grass species.

As an example, The Chetumal Bay Manatee Sanctuary is home to one of the largest remaining populations of Mexican Caribbean manatees, approximately 150 to 200 individuals, an animal that early Spanish explorers thought were mermaids. The sanctuary is situated in Chetumal Bay a national protected area of the Quintana Roo province of Mexico. The bay is a mixture of terrestrial and aquatic environments creating a landscape of exceptional beauty. The protected area also harbours other threatened and endangered species such as crocodiles (Crocodylus moreleti), the river white turtle (Dermatemys mawii) and jaguar (Pantera onca).

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National NatureServe Conservation Status

United States

Rounded National Status Rank: NNR - Unranked

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NatureServe Conservation Status

Rounded Global Status Rank: G4 - Apparently Secure

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IUCN Red List Assessment


Red List Category
LC
Least Concern

Red List Criteria

Version
3.1

Year Assessed
2010

Assessor/s
Short, F.T., Carruthers, T.J.R., van Tussenbroek, B. & Zieman, J.

Reviewer/s
Livingstone, S., Harwell, H. & Carpenter, K.E.

Contributor/s

Justification
Thalassia testudinum forms extensive dense seagrass beds and is thought to be the most important habitat-forming seagrass species in the Caribbean. This is an abundant species that is relatively robust to disturbance, and the overall population trend is stable. Localized threats to T. testudinum include coastal development, eutrophication and sedimentation, which have contributed to some local declines. This species is listed as Least Concern.

Increased coastal development has the potential to cause more widespread declines. Since this species is a major habitat-forming species that cannot be replaced functionally by another species, its available habitat should be closely monitored.
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in 1987, a mass die-off of Thalassia testudinum began in Florida Bay (Robblee et al. 1991). Robblee et al. estimated that over 4,000 hectares of seagrass beds had been denuded and an additional 23,000 hectares were affected to a lesser degree. About a third of the dense seagrass beds of western Florida were impacted over a period of just a year. Until the 1980s, Florida Bay was widely viewed as a healthy and stable ecosystem, with clear water, lush seagrass beds, and highly productive fish and shrimp populations. By 1992, the ecosystem appeared to have changed from a clear water system, dominated by benthic primary production, to a turbid water system, with algae blooms and resuspended sediments in the water column. (Rudnick et al. 2005 and references therein)

A defining feature of Florida Bay is its shallow depth, which averages just one to two meters. Light sufficient to support photosynthesis can reach the sediment surface in almost all areas of the bay, resulting in dominance of seagrass beds as both a habitat and a source of primary production (i.e., capturing energy from the sun through photosynthesis). In some portions of the bay, salinity can rise rapidly during drought periods due to water loss from evaporation exceeding input from precipitation and freshwater inflow. Following observations of Florida Bay’s dramatic ecological changes in the 1980s, it was commonly assumed that a direct cause of these changes was a longterm increase in salinity, which in turn was caused by the diversion of freshwater away from Florida Bay via South Florida Water Management District canals. However, subsequent research has indicated that these ecological changes may not be attributable to a single cause. While decreased freshwater inflow and resultant increased salinity have been part of the problem, it appears that other human activities, as well as natural forces, may have also played a role. (Rudnick et al. 2005 and references therein) Duarte (2002) notes that the causes of this die-off continue to be debated, and may include, among others, increased anthropogenic (i.e., human-caused) nutrient loading, the effects of climatic changes involving a long time interval without hurricanes affecting the area also causing unusually low freshwater discharge, and the effects of the increased accumulation of detritus derived from loss of large grazers. Difficulties in experimenting at the appropriate scale of entire seagrass meadows to test these hypotheses have made it difficult to assess to what degree the decline was due to human or natural causes, or a combination of both. (Rudnick et al. 2005 and references therein)

Regardless of the cause of the mass-mortality event, once it was initiated, the ecology of Florida Bay changed. Continued seagrass mortality results in increased sediment resuspension and increased nutrient (nitrogen and phosphorus) release from sediments, stimulating phytoplankton growth in the water column. The presence of phytoplankton and suspended sediment results in decreased light penetration to seagrass beds. This decreased light can limit seagrass growth and sustain the feedback loop. Dynamics of this feedback loop are probably not independent of the salinity regime. Seagrass wasting disease, caused by a slime mold (Labyrinthula sp.) infection, is more common at salinities close to or greater than seawater than at low salinities. High salinity may have played a role in the initial seagrass mass mortality event, but more likely has served to promote seagrass re-infection since that event. Incidence of this disease may therefore be directly affected by water management actions. (Rudnick et al. 2005 and references therein)

On a global scale, seagrasses--marine flowering plants that include the widely distributed genera Zostera, Thalassia, and Posidonia--in general appear to be in trouble (Waycott et al. 2009). Seagrasses form some of the most productive ecosystems on earth, rivaling even crops of corn and sugar cane. Seagrass meadows provide ecosystem services such as supporting commercial fisheries worth as much as $3500 per hectare per year, subsistence fisheries that support entire communities, nutrient cycling, sediment stabilization, and globally significant sequestration of carbon. Seagrasses and the services they provide are threatened by the immediate impacts of coastal development and growing human populations as well as by the impacts of climate change and ecological degradation. Seagrass losses also disrupt important linkages between seagrass meadows and other habitats, and their ongoing decline is likely producing much broader and long-lasting impacts than the loss of the meadows themselves. (Waycott et al. 2009 and references therein)

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Population

Population
Thalassia testudinum forms extensive dense seagrass beds throughout its range. This is the most important seagrass habitat-forming species in the Caribbean. There have been some declines in seagrass beds in developing areas, particularly in areas of nutrient enrichment and sedimentation. Generally, however, this species still very abundant throughout its range.

Out of 103 published studies on this species, 29 showed an increase in abundance (mix of biomass, aerial extent, or density), 17 decreased and 57 showed no change (Global Seagrass Trajectories Database, Carruthers pers. comm. 2007). The overall population trend is stable.

Caribbean regional biomass ranges from 3.2 g dw/m² (recorded at the latitude 17.8o in the Virgin Islands) to 820 g dw/m² (recorded along the 28.3o latitude on the Florida coast). These findings are summarized in Gacia (1999) and include data from publications ranging from 1959 until 1996.

The T. testudinum population in the Florida Bay covered 90% of mud bank and basins until a massive die off in 1987. Four thousand hectares were completely lost and 24,000 ha were affected due to primary die off caused by higher water temperatures, hypersalinity and increased biomass, leading to high respiration demands. This was followed by secondary die-off from increased turbidity due to increased phytoplankton beginning the fall of 1991. Between 1984 and 1994, mean short shoot density dropped by 22% throughout the Florida Bay and standing crop biomass dropped by 28%. While light attenuation may have been a factor, the patchy distribution of die-off suggests that primary die-off was the principle reason for the decrease in abundance (Hall et al. 1999). Between the spring of 1995 and spring of 1999, despite individual basins in the Florida Bay exhibiting changes in abundance by between 12-100%, the total abundance throughout the Bay exhibited little change due to declines in abundance in the western part of the Bay being offset by increases in abundance in the central and eastern basins (Durako and Hall 2000). Increases in seagrass coverage along with large occurrences of flowering plants were observed during the spring of 2000; however, primary die-off among high density stands was also observed north of Barneys Key with similar symptoms noted to the 1987 die-off (Nuttle et al. 2003).

Population Trend
Stable
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Threats

Major Threats
The biggest threats to Thalassia testudinum are coastal development, eutrophication, and sedimentation (T. Carruthers, F. Short, B. van Tussenbroek, J. Zieman pers. comm. 2007). Boat traffic, marina development, and sewage pollution from greatly expanded residential and hotel development is a particular problem in Florida. Trawling is also a threat in some parts of the range.
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Anthropogenic impacts on Thalassia testudinum Banks ex König

Many large-scales declines of seagrass meadows, affecting several different species in a wide geographical range, have been reported in recent decades (8); in more than 70% of the cases, human-induced disturbances have been held responsible for them (8, 21). Thalassia testudinum, in particular, is a species with a well-documented history of large die-offs and local declines (2, 12, 14, 17). Human-related changes in nutrient levels, salinity, composition of the biological community and climate have been identified as sources (or potential sources) of disturbance for this species.

Changes in nutrient levels: eutrophication.

Population growth and the massive use of fertilizers in agriculture have led to a generalized exponential increase in nutrient inputs to the coastal zones (8), which is most likely the main cause of seagrass decline worldwide (8). The most common mechanism for seagrass decline under eutrophic conditions is light reduction through stimulation of epiphytes, macroalgae and phytoplankton overgrowth (1, 7), although direct physiological responses such as ammonium toxicity may also contribute (1).

Specifically, seagrass leaves are often densely covered by epiphytic algae which can suppress seagrass productivity through the development of a thick coat as a result of nutrient enrichment (6, 7, 8); it has been proposed as a major mechanism for declines of seagrass meadows, and the importance of this factor has been experimentally proved for Th. testudinum (6, 7). However, it seems that the net effect of epiphytes on seagrass growth depends on a complex variety of ecological factors (7, 8). In a mesocosm experiment designed to test the effects of nutrient addition on the interactions among grazers, epiphytes, and Th. testudinum, it was shown that those effects depended critically on the intensity of grazing: in the presence of grazers, the turtle grass tended to produce a greater biomass in the tanks with higher nutrient load, but when grazers were absent, the direction of the effect was reversed, and plants with nutrients added grew less than the control plants (7).

Obviously, the control exerted by nutrient loads on this species’ productivity can led to shifts on the community composition and even to drastic alterations in seagrass beds dominance (1). The occurrence of those shifts has been experimentally demonstrated in Florida bay, where some areas previously dominated by Th. testudinum were intentionally fertilized with bird guano for eight years. For the first two years, T. testudinum production was positively affected by the nutrient enrichment, but gradually Halodule wrightii, normally an early-successional species in subtropical habitats, colonized the beds, and by the end of the study accounted for 97% of the aboveground seagrass biomass (1, 5). This shift in species dominance persisted at least eight years after nutrients were no longer added, suggesting that once an area has been fertilized, the system can be resistant to change (1).

The importance of considering watershed nutrient loads rather than water-column nutrient concentrations in assessing enriched conditions for Th. testudinum meadows (1) was shown in Sarasota Bay, Florida (24). There, the biomass and productivity of this species were negatively correlated with watershed N loads but not with water-column nutrient concentrations as assessed by routine monitoring programs (24). It demonstrates that the search for reliable early indicators of nutrient overenriched seagrass meadows is an urgent necessity in order to provide an effective management of these ecosystems (1); direct measures of water-column nutrients are generally ineffective, since in early phases of eutrophication the nutrients are rapidly taken up by plants, adsorbed to particulate sediments, or otherwise removed from the water (1).

Changes in salinity.

The role of human-induced changes in salinity as a possible cause of seagrass losses is not well understood (8). Some studies suggest that Th. testudinum meadows do not show an inmediate response to increased salinity; for example, a turtle grass meadow remained unaffected after more than six months exposure to the direct discharge of brine from a desalination plant (22). However, decreased freshwater inputs due to human activities have been proposed as a triggering factor for the most important die-off of Th. testudinum known to date, which occurred in Florida Bay in 1987 (17). About a third of the dense seagrass beds of western Florida were impacted over a period of just a year; 4,000 hectares were denuded and an additional 23,000 hectares were affected to a lesser degree (2, 17).

This die-off was probably the result of complex processes caused by an interaction of several factors (1), including eutrophication, enhanced pathogen virulence and elevated water temperatures (1, 12, 14, 16), but several authors indicate that increased salinity was a key factor in the initiation of this phenomenon (8, 12, 16, 27). Groundwater flow into the bay decreased as a result of extraction of water from the region’s aquifers (8, 27); canal construction, which diverted drainage water, further contributed to a diminished freshwater inflow (8, 27). This decrease in freshwater inflow supposedly caused a gradual transition in the original composition of the seagrass vegetation, resulting in dense monospecific meadows of Th. testudinum (27) that suffered a sudden decline, probably as a consequence of sulfide self-poisoning and other effects (1, 8, 12, 16).

Changes in composition of the biological community: invasive species.

A recent literature review has revealed that at least 56 non-native species, primarily invertebrates and seaweeds, have been introduced to seagrass meadows all around the world, largely through shipping/boating activities and aquaculture (26). There is, however, few specific data about the effects of these invasions, particularly for seagrass community structure and function (26).

Th. testudinum meadows have been locally colonized by the invasive green alga Caulerpa ollivieri Dostál, native from the Mediterranean Sea (26). To date, this alga has been reported from the Bahamas (11) and the Gulf of Mexico (9). A negative effect of C. ollivieri on Th. testudinum, by direct displacement, has been observed (11, 26). Eutrophication is considered a facilitating factor in the successful colonization of the meadows by C. ollivieri (11); in the locations were this invasive was recorded, Th. testudinum was covered by epiphytic algae, as typical under conditions of high nutrient loads, and C. ollivieri exhibited higher nutrient contents than native Caulerpa species, which was considered evidence for the role of sewage-derived nitrogen in its successful introduction (26). This proposition is consistent with the results of recent experiments in several different seagrass ecosystems, which confirmed that disturbance contributes to the invasibility of seagrass beds (26). Given the reduced knowledge about the role that introduced species, combined with nutrient pollution may play in global seagrass decline, more definitive studies and comprehensive surveys are required in this field (26).

Climate change

The global change in Earth’s systems and functioning which is currently taking place, mainly due to anthropogenic release of greenhouse gases into the atmosphere, is likely to alter to some degree most of the terrestrial and marine ecosystems. Seagrass meadows in general, and Th. testudinum populations in particular, may potentially be impacted by many factors related to climate change, including higher atmospheric concentration of CO2, higher temperatures, rising sea level, increase in the frequency and intensity of storms and increase in UV-B radiation (20).

Rising levels of atmospheric CO2, and the consequent alterations in the CO2/HCO-3 equilibrium in marine water, is expected to have significant effects on seagrasses. Seagrass photosynthesis is frequently limited by the availability of dissolved inorganic carbon under natural conditions (20); it has been experimentally shown for Th. testudinum (3). This carbon limitation has been attributed to the thickness of the diffusion boundary layer surrounding leaf surfaces or to a relatively inefficient HCO-3 uptake system (20), factors which vary according to each species morphology and physiology (8). Differences among species in the ability to compete for carbon would be expected to lead to shifts in species abundances and distributions, but long-term experimental studies of seagrass community responses to carbon enrichment are lacking (20).

In a similar way, increasing water temperature will directly affect seagrass metabolism, which may result in changes in patterns of species abundance and distribution (20). These direct effects will depend on the individual species’ thermal tolerances and their optimum temperatures for photosynthesis, respiration, and growth. Rising temperatures may also alter seagrasses through direct effects on flowering and seed germination (20); in Th. testudinum, flowering is apparently controlled by the water temperature (15), which implies that a phenological impact could be expected.

As a consequence of global higher temperatures, which produce a thermal expansion of ocean water and an acceleration of polar ice cap melting (10, 20), sea level has been rising steadily along the 20th century (10). According to the IPCC 4th Assessment Report, the global average sea level rose about 3.1 mm/year over 1993 to 2003 (10); by the end of 21th century the projected level rise will go from 0.18 m to 0.59 m, depending on the emission scenarios (10), but the used model excludes future rapid dynamical changes in ice flow, which are likely to occur, resulting in even larger figures. The greatest direct impact of an increase in sea level will be an increase in the depth of water and the consequent reduction in available light to the bottom (20). A primary effect of increased water depth will be to alter the location of the maximum depth limit of plant growth, directly affecting seagrass distribution (20).

Increased global temperatures are expected to increase the intensity and frequency of extreme weather events (4). A regime of increased storm disturbance may lead to a decrease in distribution of climax species like Th. testudinum and an increase in abundance of early colonizing and mid-successional species within the seagrass community (20). It is due to the fact that recolonization in seagrasses occurs primarily by means of horizontal rhizome growth and branching; in Th. testudinum the rates for both processes are slow [around 70 cm/year for horizontal rhizome growth, and a branching rate of 1 branch/1600 internodes (8)] compared with the growing rates showed by other smaller seagrasses (8).

There is a general agreement that increases in the amount of UV-B radiation reaching the surface of the Earth will produce both mutagenic and physiological damage in plants, including seagrasses (20). For three seagrass species occurring in the Caribbean (Th. testudinum was not included in the study), it was shown that increased UV-B radiation negatively impacted their photosynthetic capacity (25). Under ozone depletion, increased UV-B radiation is most likely to affect tropical seagrass systems such as Th. testudinum meadows, where the greatest amount of direct UV light reaches the Earth’s surface (20).

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Management

Conservation Actions

Conservation Actions
The range of Thalassia testudinum falls in some Marine Protected Areas (MPAs). In the Caribbean, T. testudinum is included in the 24 fully managed marine protected areas. This species is monitored by the CARICOMP network (Caribbean Coastal Marine Productivity network), including coral reefs and mangroves (Green and Short 2003). Currently, a seagrass management plan is being developed in Bermuda (Sarkis pers. comm. 2007). This species is also listed as Vulnerable (A2a) under the Bermuda Protected Species Act (Sarkis pers. comm. 2007).

This is a major habitat-forming species in the Greater Caribbean and should be monitored (Van Tussenbroek et al. 2006).
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Rudnick et al. (2005) emphasize that if the state of the seagrass community is to be used as a criterion to guide and assess the success of environmental restoration efforts, scientists and managers must specify the desirability of alternative states. Based on studies of historic changes of seagrass communities in Florida Bay and anecdotal information. it is likely that the Florida Bay of the 1970s and early 1980s, with lush T. testudinum and clear water, was probably a temporary and atypical condition. From an ecological perspective, restoration should probably strive for a more diverse seagrass community with lower T. testudinum density and biomass than during that anomalous period. (Rudnick et al. 2005 and references therein)

If efforts to restore the Everglades are successful, patterns of freshwater flow toward more natural patterns will drive Florida Bay’s seagrass community and trophic web toward its pre-drainage condition. Decreased salinity caused by increasing freshwater flow would likely have a direct effect on seagrass communities through physiological mechanisms, resulting in greater spatial heterogeneity of seagrass beds, a decrease in the dominance of T. testudinum, and an increase in coverage by other seagrass species. Decreased salinity would also likely decrease the infection of T. testudinum by the slime mold Labyrinthula. Light availability depends on phytoplankton growth and sediment resuspension, which in turn depend on nutrient availability, grazing, and stabilization of sediments by seagrass beds. (Rudnick et al. 2005)

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Relevance to Humans and Ecosystems

Benefits

Habitat structureBroad-scale Cost/Benefit: Virnstein (1995) stressed the importance of considering both geographic scale and pattern (landscape) in devising appropriate management strategies to maintain seagrass habitat diversity in the Indian River Lagoon. It was suggested that goals be established to maintain seagrass diversity and that these goals should consider not only the preservation of seagrass acreage but more importantly, the number of species of seagrass within an appropriate area. By maintaining seagrass habitat diversity, the maintenance of the diverse assemblage of amphipods, mollusks, isopods and fish, associated with seagrass beds, will be accomplished (Virnstein 1995).
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Wikipedia

Thalassia testudinum

Thalassia testudinum, commonly known as turtle grass, is a species of marine seagrass. It forms meadows in shallow sandy or muddy locations in the Caribbean Sea and the Gulf of Mexico.[3]

Description[edit]

Thalassia testudinum is a perennial grass growing from a long, jointed rhizome that may be buried 25 centimetres (9.8 in) deep in the substrate but is more usually found 5 to 10 cm (2 to 4 in) below the surface. Some nodes are leafless but others bear a tuft of several erect, linear leaf blades.[4][5] These are up to 30 centimetres (12 in) long and 2 cm (0.8 in) wide and have rounded tips. The flowers grow on short stalks in the axils of the leaves and are greenish-white, sometimes tinged pink, and are followed by seed pods.[6]

Distribution and habitat[edit]

Turtle grass is found growing in meadows in calm shallow waters throughout the Caribbean Sea and the Gulf of Mexico and as far north as Cape Canaveral in Florida. Extensive meadows can be formed on muddy sand, coarse sandy and clayey seabeds, especially those with a calcareous content. This grass favours high-salinity waters with low turbidity such as calm lagoons. It cannot grow in fresh water but some growth is possible at a salinity of 10 parts per thousand. The plant's preferred range is 25 to 38.5 parts per thousand with a temperature range of 20 to 30 °C (68 to 86 °F). It is found from low-tide mark down to depths of 30 metres (98 ft) depending on water clarity. It often grows in meadows with other seagrasses where it is the climax species.[5]

Reproduction[edit]

Turtle grass can reproduce both vegetatively and by sexual reproduction. The main propagative method is by increase in length of the rhizomes. This mainly takes place in spring and early summer but can happen at any time of year and results in an increase in the size of the turtle grass bed.[5] It has been found that where plants have been damaged mechanically, such as by the propellers of boats, the cut ends of rhizomes are unable to grow and holes may develop in the turtle grass meadow.[4]

Unusually for the marine environment, turtle grass is a flowering plant. In the spring and early summer, many turtle grass plants produce small flowers at the base of the leaves. Male and female flowers grow on separate plants. Fruits develop in two to four weeks and became detached and float away after about eight weeks.[5] The seeds are viviparous,[7] and can start new patches of seagrass, but it is thought that the main method of reproduction is vegetative.[5]

Seagrass bed with dense Thalassia testudinum and an immature queen conch (Eustrombus gigas), Rice Bay, San Salvador Island, Bahamas

Ecology[edit]

Turtle grass and other seagrasses form meadows which are important habitats and feeding grounds. Associated seagrass species include Halophila engelmannii and Syringodium filiforme. Many epiphytes grow on the grasses, and algae, diatoms and bacterial films cover the surface of the leaf blades. The grass is eaten by turtles, herbivorous parrotfish, surgeonfish and sea urchins while the leaf surface films are a food source for many small invertebrates.[5] Decaying turtle grass leaves are responsible for the majority of detritus in meadow areas. This grass is subject to periodic dieback episodes in the Florida Bay area. One such episode in 1987 killed off a large proportion of the plants and the resulting increased sedimentation and greater growth of epiphytes on the remaining plants caused a secondary dieback event. The areas affected have since been reseeded and planted with rhizomes and have recovered. In general, the population of this grass is stable.[7]

Rhizomatous green algae in the genus Caulerpa often live among the grasses and many animals make seagrass meadows their home. These include bivalves and other molluscs, polychaete worms, amphipods and juvenile fish which hide among the leaf blades, sea urchins, crabs and caridean shrimps.[5]

References[edit]

  1. ^ den Hartog, C. (2005). "Taxonomy and biogeorgraphy of seagrasses". In Larkum, Anthony W.D.; Duarte, Carlos; Orth, Robert J. Seagrasses: Biology, Ecology and Conservation. Springer-Verlag New York, LLC. ISBN 978-1-4020-2942-4. 
  2. ^ Guiry, Michael D. (2012). "Thalassia testudinum Banks ex König, 1805". In M. D. Guiry & G. M. Guiry. AlgaeBase. National University of Ireland, Galway. World Register of Marine Species. Retrieved 2012-11-07. 
  3. ^ Guiry, M. D.; Guiry, G. M. (2012). "Thalassia testudinum Banks ex König". AlgaeBase. Retrieved 2012-11-08. 
  4. ^ a b Colin, Patrick L. (1978). Marine Invertebrates and Plants of the Living Reef. T.F.H. Publications. p. 474–475. ISBN 0-86622-875-6. 
  5. ^ a b c d e f g Dineen, J. (2001-07-25). "Thalassia testudinum (Turtle grass)". Smithsonian Marine Station at Fort Pierce. Retrieved 2012-11-07. 
  6. ^ "Turtle-grass (Thalassia testudinum)". Interactive Guide to Caribbean Diving. Marine Species Identification Portal. Retrieved 2012-11-08. 
  7. ^ a b Short, F. T.; Carruthers, T. J. R.; van Tussenbroek, B.; Zieman, J. (2010). "Thalassia testudinum". IUCN Red List of Threatened Species. Version 2012.2. Retrieved 2012-11-08. 
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Notes

Comments

Thalassia testudinum is possibly the most important marine spermatophyte along the coasts of the Caribbean and Gulf of Mexico (C. den Hartog 1970). The species grows from the low-water mark to nearly 10 m depth in very clear water. Establishment occurs on a wide variety of substrates, including organic matter, rocky matter, coral sand, and dead reef-platforms. Once the species is established, the substrate type becomes less important, especially in areas of low current. Dead leaves and rhizomes accumulate among the erect living leaves for considerable periods of time. The beds are important not only in substrate development but also in substrate stabilization. Massive amounts of substrate are lost in areas without turtle-grass colonies during hurricanes, but only minimal loss occurs in turtle-grass beds. Substrate loss is lessened by roots and rhizomes binding the substrate, as well as by the leaves lowering water velocity. 

 Posidonia oceanica (Linnaeus) Delile was included in the Texas flora (D. S. Correll and M. C. Johnston 1970; D. S. Correll and H. B. Correll 1972) because of specimens washed ashore along the Gulf of Mexico. The specimens were later determined to be Thalassia testudinum, based upon comparative growth studies and upon flavonoid chemistry profiles (C. McMillan et al. 1975).

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Names and Taxonomy

Taxonomy

Comments: Kartesz (1994) spells the name 'testudina.' Kartesz (1999), FNA (1998 draft) and the International Plant Name Index as of January 19, 2006 spells it 'testudinum.'

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