The daggerblade grass shrimp, Palaemonetes pugio, is a small transparent to shrimp with a well-developed rostrum bearing several dorsal as well as three distinct ventral teeth, a smooth carapace and abdomen, and two pairs of chelate (claw-bearing) walking legs, the second pair more robust than the first. It has well-developed eyes with globular pigmented corneas and some slight yellow pigmentation in the eyestalks. The back is straight and the telson has two pairs of well-developed dorsal spines and also two pairs of posterior spines (Anderson 1985, Kaplan 1988, Rupert and Fox 1988).

Palaemonetes pugio can be found on the eastern shore of the United States. Its range extends from Maine to Texas. (Collins, 1981)

Biogeographic Regions: nearctic (Native )

United States

Origin: Unknown/Undetermined

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

Palaemonetes pugio is a widely distributed western Atlantic and Gulf of Mexico species occurring from Canada to Texas (Kaplan 1988). Palaemonetes pugio occurs in seagrass beds and other suitable habitats throughout the India River Lagoon system.

Palaemonetes pugio has a smooth carapace and abdomen. Grass shrimp also have three pairs of legs. The second pair is the strongest, while the third pair lacks chelae (claws). A well-developed rostrum (horn-shaped structure between the eyes) possessing dorsal and ventral teeth is present in this species, located between its compound eyes on its head. P. pugio has a tail with two pairs of spines, on pair located dorsally, the other located posteriorly (Anderson, 1996). The pleura (side plates) of the second abdominal somite (section) overlap the pleura of the first and third abdominal somite (Zimmerman, 2000). The abdominal pleura are rounded in this species (Anderson, 1996).

Male adults reach an average length of 23.5 mm. Female adults bearing eggs are usually about 30.0 mm while those without eggs are usually about 26.2 mm. The females undergo a period of growth just before their breeding season in the summer. (Daiber, 1982) The adults rarely grow beyond 50 mm. P. pugio are transparent with yellow coloring and brown spots. (Anderson, 1996)

Adult Palaemonetes pugio reach a length of around 5 cm (Kaplan 1988). The life span of P. pugio is 6 to 13 months (Alon and Stancyk 1982).

Holotype + Paratype for Palaemonetes pugio Holthuis, 1949
Collection: Smithsonian Institution, National Museum of Natural History, Department of Invertebrate Zoology
Preparation: Alcohol (Ethanol)
Collector(s): Schmitt
Year Collected: 1934
Locality: Chesapeake Bay, Lagoon Near Cove Point Light, Maryland, United States, North Atlantic Ocean

Palaemonetes pugio may be confused with several co-occuring shrimp species in the IRL. The relatively small size and the lack of chelae (claws) on the third pair of walking legs is sufficient to distinguish the caridean shrimps (including P. pugio) from the familiar penaeid shrimp species.Members of the genus Palaemonetes can be differentiated from other palaemonid shrimp (e.g., Palaemon, Macrobrachium) by the absence of mandibular palps, although this usually requires detailed examination beyond the scope of amateur naturalists (Anderson 1985). Likewise positive differentiation of P. pugio from co-occurring congeners requires close examination of subtle differences in the chelae, rostra, antennules, and other external features (see included figure, adapted from Anderson 1985).Other members of genus Palaemonetes from the IRL are P. intermedius, P. paludosus, and P. vulgaris.

Palaemonetes pugio normally inhabits the areas where fresh and saltwater combine. Their basic habitat is the salt marshes and connecting streams. These areas can include small tidal creeks, tidal guts, and ditches with low salinity. (Daiber, 1982) With the approach of cold weather, P. pugio has been known to move to deeper waters. (Anderson, 1996)

Aquatic Biomes: rivers and streams; coastal

Depth range based on 27 specimens in 1 taxon.
Water temperature and chemistry ranges based on 1 sample.

Environmental ranges
  Depth range (m): 0.3 - 11.3
  Temperature range (°C): 23.660 - 23.660
  Nitrate (umol/L): 0.457 - 0.457
  Salinity (PPS): 35.580 - 35.580
  Oxygen (ml/l): 4.845 - 4.845
  Phosphate (umol/l): 0.110 - 0.110
  Silicate (umol/l): 0.868 - 0.868

Graphical representation

Depth range (m): 0.3 - 11.3
 
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.

P. pugio adults prey on oligochaetes, polychaetes, and harpacticoid copepods. (Baxter, 1999)

P. pugio larvae feed upon zooplankton, algae, and detritus. (Anderson, 1996)

Like most grass shrimp, Palaemonetes pugio is a generalist forager that can consume a variety of dietary items depending on availability. They may forage as primary consumers, secondary consumers, and detritivores (Morgan 1980, Anderson 1985).Despite their association with seagrasses and other benthic aquatic vegetation, grass shrimp consume little to no actual macrophyte biomass. Rather, they primarily consume the epiphytic microalgae growing on the surfaces of seagrasses and other aquatic macrophytes (Morgan 1980).Grass shrimp also function as predators on meiofauna and small infauna including ostracods, nematodes, polychaetes, and oligochaetes (Bell and Coul 1978, Chambers 1981). Morgan (1980) also indicates that grass shrimp are capable of preying on motile fauna such as mysids. Epibenthic predation along with associated disturbance of sediments by grass shrimp is capable of altering infaunal community structure (Bell and Coul 1978, Knieb and Stiven 1982).Grass shrimp function as detritivores by aiding in the mechanical breakdown of seagrasses and other refractory plant materials. They also assimilate the microfloral and fungal biomass that colonizes and enriches the detritus and cycles that energy through the estuarine food web (Adams and Angelovic 1970, Anderson 1985).Grass shrimp also derive a substantial fraction of their nutrition from dissolved organic matter adsorbed onto fine (clay-sized) particles (Odum and Heald 1972). Competitors: Evidence for interspecific and intraspecific competitive interactions involving Palaemonetes pugio comes from published field and laboratory studies.Studies reveal that Palaemonetes vulgaris can displace P. pugio from preferred habitats (e.g., oyster reefs). Lab experiments by Chambers (1981) with P. pugio and P. vulgaris showed females were dominant over males and large shrimp were dominant over smaller ones. P. vulgaris also generally dominated P. pugio. Predators: Grass shrimp are an important item in the diets of a large number of estuarine species including economically important commercial and recreational fishery species (Overstreet and Heard 1982, Heard 1982, Anderson 1985). Grass shrimp are also consumed by killifishes and other forage fish that are themselves important prey items for larger piscivores (Harrington and Harrington 1972, Kneib and Stiven 1982). Abundant as they are, grass shrimp are an extremely important conduit in the transfer of energy from the producer and decomposer levels up to the higher consumer levels of the trophic pyramid (Anderson 1985).Grass shrimp minimize their exposure to predators through their association with benthic macrophytes such as seagrasses and marsh grass or other protective physical structure (e.g., oyster reefs). Displacement from these preferred refugia increases the predation rates (Thorp 1976, Coen et al. 1981, Heck and Thoman 1981). Parasites: The published literature indicates that grass shrimp are hosts for a number of parasites, including coccidians, microsporidians, trematodes, isopods and leeches (Overstreet and Weidner 1974, Anderson 1977, Overstreet 1978, Solangi and Overstreet 1980). Grass shrimp are also frequently parasitized by mated pairs of the bopyrid isopod Probopyrus pandalicola which form a conspicuous blister on the carapace of the shrimp (Rupert and Fox 1988). Nevertheless, parasites are not considered to be limiting to grass shrimp abundance and overall population health (Anderson 1985). Habitats: Palaemonetes pugio is a common inhabitant of seagrass beds and oyster reefs (Thorp 1967, Anderson 1985). Grass shrimp typically inhabit shallow coastal and estuarine environments, but they have been collected from depths exceeding 14 m (Williams 1965).Field surveys in Florida showed that P. pugio is most abundant in habitats characterized by aquatic macrophyes, relatively high turbidities, and low salinities (Livingston et al 1976). Weaver and Holloway (1974) report that densely vegetated habitats supported an abundance of grass shrimp far higher than adjacent habitats where macrophyte cover was lese dense.Grass shrimp are tolerant of relatively high turbidities, such as in tidal creeks and marshes, and this may afford the animals a degree of protection from predators in areas where submerged macrophyte cover is lacking (Livingston et al. 1976, Anderson 1985).Siroka (1977) observed that grass shrimp in tidal creeks migrate seaward or drift with the current during ebb tides and migrate upstream into tidal creeks during incoming tides. Activity Time: Movement and distribution in Palaemonetes pugio may be influenced by photoperiod, although the effect of tidal cycles is possibly of greater importance. Shenker and Dean (1978) report that some P. pugio are buried in the sediments during daylight, but active individuals are also encountered by day.

Palamonetes spp. shrimp are among the most abundant and widely distributed benthic macroinvertebrates of Atlantic and Gulf coast estuaries (Sikora 1977, Anderson 1985).

Palaemonetes pugio reproduces annually when the water warms up. Females bearing eggs have been demonstrated to be in much greater supply at 30 degrees Celsius and above. The females continuously produce eggs during the breeding season. Fertilized eggs take between 15-20 days to mature and new eggs are produced 1-2 days after the hatching of a previous group. Larger females tend to produce bigger eggs and larger quantities of eggs. (Daiber, 1982) Females can produce anywhere between 250-450 eggs in a season. (Anderson, 1996)

Before spawning, the ovaries in the female become more dense and visible. Additional setae are developed that will later hold the fertilized eggs. The female molts before mating can occur and the males do not stay with or guard females. Instead, the male must make contact with the female's exoskeleton to realize that mating is possible. Mating must occur within seven hours of the molt. (Anderson, 1996)

During copulation, the male and female move their genital regions to close proximity. The male inserts a spermatophore into the female's genitals where it remains until the eggs are ready for fertilization, usually within seven hours after the spermatophore has been inserted. Then through enzyme action, part of the spermatophore dissolves, allowing the sperm to fertilize the eggs. The fertilized eggs are then secured to the body by the setae. The female holds the eggs until the larvae are ready to hatch. The larvae break free from the egg membrane through their struggling, osmotic swelling of the inner membrane, and ventilatory movements by the mother. (Anderson, 1996)

Palaemonetes pugio mature at 1.5 to 2 months of age and 15-18 mm length (Anderson 1985). The duration of the spawning season varies with geographical location. In the Gulf of Mexico, the season extends from approximately March through October and animals may spawn more than once in a season. Overwintering older P. pugio typically spawn early in the year and die before the next winter (Alon and Stanyck 1982). In contrast, early arriving young-of-the-year can spawn late in the year as adults (Anderson 1985). In the colder northern waters of Rhode Island, P. pugio spawns only once during a shorter season lasting only from May-July (Welsh 1975).Fecundity in P. pugio is reportedly variable depending on geographic location. Females collected from Rhode Island in June averaged 486 eggs per female, while in Texas the average was only 372 eggs/female and only up to 247 in South Carolina (Welsh 1975, Wood 1967, Sikora 1977). A significant positive correlation exists between the length of ovigerous females and egg number (Wood 1967).Burkenroad (1947) describes the female premating condition, mating and spawning for P. vulgaris. Just prior to mating, the female undergoes molting. Copulation occurs within 7 hours of female molting and involves the paired shrimp positioning themselves so that their genital apertures are close to each other. The male transfers a spermatphore onto the genital sternites of the female and remains there until oviposition occurs around 7 hours later. Prior to oviposition, spermatozoa are released through a weakened (probably by female enzymatic secretions) spermatophore. Eggs undergo external fertilization as they are extruded from the female genital aperature. The fertilized eggs are then manually transferred and adhered to the pleopods and ventral setae of the female's abdomen where they are incubated.

Eggs hatch 12-60 days after fertilization, depending on the species and location. Free-swimming larvae are released from eggs with the aid of osmotic swelling of the inner membrane, undulation of the ventilating appendages of the female, and struggling of the larvae within the eggs (Davis 1965).Larval P. pugio are approximately 2.6 mm at hatching and around 6.3 mm at metamorphosis (Broad 1957, Anderson 1985). The duration of multi-stage (7-11 larval stages) larval development in P. pugio ranges from 11 days to several months, depending on the environment (Floyd 1977). The final larval stage metamorphoses into a postlarval form closely resembling the adult shrimp.The planktonic larvae feed on zooplankton, phytoplankton, and detrital material (Anderson 1985).

US Federal List: no special status

CITES: no special status

Rounded Global Status Rank: GNR - Not Yet Ranked

United States

Rounded National Status Rank: NNR - Unranked

P. pugio serves as a vital food source to many valuable commercial and sport fishes (Anderson, 1996). It has also been recognized as a useful bioassay test organism. This has led to much research involving grass shrimp and toxic chemicals (Anderson, 1996).

While the value of Palaemonetes pugio and other caridean shrimp as food or bait species is minimal, their importance in terms of the higher trophic levels which they support is difficult to overstate. Grass shrimp are an important trophic link, transferring energy and nutrients through estuarine food webs among several trophic levels including primary producers, grazers, decomposers, carnivores, and detritivores (Welsh 1975, Morgan 1980, Anderson 1985).P. pugio has been utilized as a bioassay organism in studies measuring the toxicity of petroleum hydrocarbons, cadmium, antifouling biocides, and a variety of pesticides including DDT, parathion, kepone, heptachlor, toxaphene, and others (Anderson 1985). They are also of potential value as an indicator of sediment quality in coastal areas impacted by pollution (Lewis and Foss 2000).