Sargassum natans, unlike the many benthic Sargassum species (i.e., those that live attached to the sea bottom), is a pelagic (free-floating) brown seaweed that occurs mainly far out in the North Central Atlantic Ocean, but washes ashore regularly. It is often associated with the Sargasso Sea, a very large region situated offshore from the southeastern United States, approximately south and east of Bermuda and seaward from the Gulf Stream. The Sargasso Sea, which accumulates large masses of Sargassum, results from a ring of prevailing ocean currents that enclose an enormous eddy (about 5.2 million square kilometers) which rotates clockwise as a result of the Earth's eastward rotation. Sargassum natans apparently reproduces only asexually, by fragmentation. (Gosner 1978; Kaplan 1988) Gower and King (2008) used satellite imagery to track the origin, distribution, and fate of floating Sargassum. Using this approach, they analyzed data from 2002 to 2008 and were able to present the first mapping of the full distribution and movement of pelagic Sargassum in the Gulf of Mexico and western Atlantic. Their results revealed a seasonal pattern in which Sargassum typically shows strong growth in the northwest Gulf of Mexico in the spring of each year, is transported into the Atlantic in about July, appearing east of Cape Hatteras as a “Sargassum jet", and ends up northeast of the Bahamas in February of the following year. Gower and King note that the idea that pelagic Sargassum originates in the Gulf of Mexico, as they propose, was fairly common in the 19th century (based on ship observations), but became less widespread in the following century. Based on their findings, Gower and King conclude that that most pelagic Sargassum has a life span of one year or less, with the major “nursery area” being in the northwest Gulf of Mexico. Given their estimated average flow of about one million tons of Sargassum out of the Gulf of Mexico each year, the authors suggest the implied carbon flux should be accounted for in productivity and carbon models.
Sargassum natans has a broad distribution offshore in the North Atlantic and may be found washed ashore on beaches from Canada to Brazil and Europe, especially in summer (Schneider and Searles 1991). Outside the Sargasso Sea, there is more Sargassum floating in the Gulf of Mexico than anywhere else in the world (Hinman 2007).
Sargassum species are generally bushy, with narrow blades that have toothed edges and a midrib. Small (6 mm or less) pea-like gas bladders are present on short stalks. Sargassum is golden-brown when fresh, but dries dark brown. It may reach a length of 60 cm or more. (Gosner 1978)
Sargassum fluitans, a somewhat less common pelagic Sargassum species, is similar to S. natans. The gas bladders of S. fluitans lack the needle-like spine present in S. natans and the bladder stalks of S. fluitans are often winged with a ridge of tissue (Lee 1986; Gosner 1978; Schneider and Searles 1991). (Note that Kaplan 1988 states, apparently incorrectly, that it is S. fluitans that has gas bladders with spines and S. natans that lacks them.)
Casazza and Ross (2008) sampled fishes from areas of open water with and without Sargassum in the Gulf Stream off North Carolina (U.S.A.). They found consistently more individuals (18, 799 versus 2,706) and more species (80 versus 60) in areas with Sargassum. In both habitats, nearly all fish collected were juveniles. The authors concluded that Sargassum provides an important nursery habitat for juvenile fishes off the coast of the southeastern United States, a conclusion consistent with several previous studies (e.g., Moser et al. 1998; Wells and Rooker 2004).
Hacker and Madin (1991) studied mimicry and camouflage in two species of shrimps living in mats of pelagic Sargassum natans. Latreutes fucorum resembles Sargassum fronds; Hippolyte coerulescens resembles the gas-filled bladders of Sargassum. When offered modified artificial Sargassum, Latreutes preferred a "fronds only" to a "bladders only" version while the reverse was true for Hippolyte. The authors found that as Hippolyte individuals get larger and outgrow the size of single bladders, they use disruptive coloration to break up the outline of their body so they continue to resemble a pair of bladders. In contrast, as Latreutes grow too large to match single fronds, diverse patterns of coloration to disrupt their body shape and resemble the random mosaic of different-colored Sargassum fronds and the open spaces between them become more important.
Pelagic Sargassum provides a scaffolding for an unusual community in the Sargasso Sea. Associated with these floating mats of seaweed are a diversity of epiphytic algae, encrusting hydroids, bryozoans, tube worms, shrimps, crabs, and other invertebrates, as well as several fish species, many of them not found elsewhere. When Sargassum is found washed ashore, careful inspection will often reveal some of these associated organisms as well. (Gosner 1978)
Molecular Biology and Genetics
Statistics of barcoding coverage: Sargassum natans
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