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This report should be cited as follows:

Coston-Clements, L., L.R. Settle, D.E. Hoss and F.A. Cross. 1991. Utilization of the Sargassum habitat by marine invertebrates and vertebrates - a review. NOAA Technical Memorandum NMFS-SEFSC-296, 32 p.

Copies may be obtained by writing:

National Technical Information Service 5285 Port Royal Road Springfield, VA 22161

or

National Marine Fisheries Service, NOAA

Southeast Fisheries Science Center

Beaufort Laboratory

101 Pivers Island Road

Beautort, NC 28516-9722

Cover illustration from Teal and Teal, 1975.

Numerous species of brown algae (Class Cyclosporeae: Order Fucales: Family Fucaceae) of the genus Sargassum occur throughout the world's tropical and temperate oceans. The pelagic complex in the western North Atlantic is comprised primarily of Sargassum natans and S. fluitans. Both species are hyponeustonic and fully adapted to a pelagic existence (Parr, 1939). Known commonly as gulf-weed, sea holly, or sargassum, they are characterized by a brushy, highly branched thallus (stem) with numerous leaf-like blades and berrylike pneumatocysts (floats). These floating plants may be up to several meters in length but are typically much smaller. See Hoyt (1918), Winge (1923), Parr (1939), Taylor (1960), Prescott (1968), and Humm (1979) for detailed descriptions of the various species.

Sargassum floats contain mostly oxygen with some nitrogen and carbon dioxide, and are responsible for buoyancy. Oxygen content is dependent on the oxygen partial pressure of the surrounding medium and independent of photosynthetic activity (Hurka, 1971). Woodcock (1950) also found diurnal fluctuations in gas pressure within the floats, and attributed it to similar changes in oxygen partial pressure during daylight and darkness.

There is some debate as to whether sargassum found in the Gulf Stream, Gulf of Mexico, Caribbean Sea, and the North Atlantic Central Gyre is material detached from littoral plants in the Antilles and other tropical regions during storms (Peres, 1982) or whether the oceanic components are a separate species group independent of their coastal littoral relatives. Most researchers

consider the pelagic forms a separate and distinct species group having evolved from benthic species (Winge, 1923; Parr, 1939; Friedrich, 1969; Butler et al., 1983; Stoner and Greening, 1984). Supporting evidence includes the lack of sexual reproduction characteristic of benthic forms, the loss of a basal holdfast, and the number of endemic organisms in the associated community (10 invertebrates and 2 vertebrates). Benthic forms, such as S. filipendula, occur in the open ocean in small quantities, but should be considered flotsam (Hoyt, 1918; Winge, 1923; Parr, 1939; Dooley, 1972).

Sargassum circulates between 20 and 40 N latituded and 30 W longitude and the western edge of the Florida Current/Gulf Stream, with an apparent center of distribution within the North Atlantic Central Gyre between 28 and 34 N latitude (Fig. 1) (Winge 1923; Ryther, 1956; Dooley, 1972; Butler et al. 1983). Large quantities also occur on the continental shelf. Some of this material is cast upon beaches along the eastern seaboard (Hoyt, 1918; Humm, 1979; Winston, 1982), while much of it remains on the shelf or is entrained into the Gulf Stream. Throughout the region, sargassum frequently aggregates into large windrows in response to wind forcing (Winge, 1923; Langmuir, 1938; Faller and Woodcock, 1964) or shear currents along frontal boundaries (Stommel, 1965). Woodcock (1950) demonstrated that sargassum can be downwelled along such convergence zones; the depth of descent being dependent on the buoyancy of individual algal clumps and the magnitude of the Langnuir circulation cell. Clumps that do not sink below a

critical depth (» 100 m) can withstand the hydrostatic pressure and will ultimately rise to the surface. There is also, however, a time-at-depth relationship that can influence the true critical depth at which buoyancy is lost and the algae sink to the bottom (Johnson and Richardson, 1977). Peres (1982) questioned the return to the surface after being downwelled. When buoyancy is lost, sargassum sinks slowly to the bottom (about 2 days to reach 5000 m) and provides a resource for bottom dwelling consumers (Schoener and Rowe, 1970).

The contribution of sargassum to total primary production in the western North Atlantic is variable and dependent on the region examined and on accumulated biomass. Carpenter and Cox (1974) estimated average production across the western Sargasso Sea at =1.0 mg C m-2d-1, with higher values frequently occuring at localities on the continental shelf and in the northern Sargasso Sea (above 30 degrees N). Howard and Menzies (1969) found production highest in the Gulf Stream, lowest on the shelf, and the intermediate in the Sargasso Sea. These authors estimate sargassum probably contributes about 0.5% of the total primary production in the region, but nearly 60% of the total in the upper 1 m of the water column (Howard and Menzies, 1969; Carpenter and Cox, 1974; Hanson, 1977; Peres, 1982). In addition, epiphytic cyanobacteria (Dichothrix and Oscillatoria) contribute significantly to overall production and nitrogen fixation within the immediate sargassum community (Carpenter, 1972; Carpenter and Cox, 1974; Phlips and Zeman, 1990). Both sargassum and its associated blue-green algae

epiphytes are adapted to conditions of strong sunlight, that is, photosynthesis is not inhibited at high light intensities (Phlips, et al., 1986).

A survey conducted in the Sargasso Sea from 1933 to 1935 (Parr, 1939) estimated sargassum biomass at 524 to 1642 mg m-2. Stoner's (1983) estimates in the Sargasso Sea from the years 1977 to 1981 were less than 6% of those earlier estimates. He suggested that the decline may have been anthropogenically influenced. Subsequent analysis of Stoner's data, however, concluded that there had been no significant change in the biomass of sargassum from 1933 to 1981, except in an area northeast of the Antilles. This apparent decline was attributed to seasonal variation in sargassum abundance or long-term current shifts, and apparently not to pollution (Butler et al. 1983; Butler and Stoner, 1984). Reliable estimates of total biomass are not available, however, as no directed statistical study encompassing the vast range of these species has been conducted.

Pelagic sargassum supports a diverse community of marine organisms including micro- and macro-epiphytes (Carpenter, 1970; Carpenter and Cox, 1974; Mogelberg et al., 1983), fungi (Winge, 1923; Kohlmeyer, 1971), more than 100 species of invertebrates (Table 1), over 100 species of fishes (Table 2), and four species of sea turtles (Carr, 1987a; Manzella and Williams, 1991). Some inhabitants, unique to the sargassum habitat, have evolved unusual

shapes and coloration affording them the additional advantage of camouflage among the floating plants. Others are less specialized and utilize the habitat for foraging or protection from predators. Community structure is variable; influenced by season, geographic location, and algal "age" (Weis, 1968; Fine, 1970; Butler et al. 1983; Stoner and Greening, 1984). Weis (1968) also noted differences in epibiont diversity between species of sargassum. An important factor in the structure of the community is related to compounds occurring in the exudate released by the algae during growth. Tannins produced on the distal growing tips of sargassum have an inhibitory effect on colonizing epibionts (Conover and Sieburth, 1964; Sieburth and Conover, 1965). This antifouling effect lasts a short time and a succession of bacteria, hydroids, bryozoans, and blue-green algae rapidly follow (Winge, 1923; Conover and Sieburth, 1964; Ryland, 1974). Carpenter and Cox (1974) also suggest that low epibiont density within some areas of the Sargasso Sea may be nutrient limited rather than limited by the antibiotic activity of sargassum exudates. Natural chemical compounds, including phenolic compounds, produced by algae may also serve as a deterrent to herbivores (Paul, 1987; Hay and Fenical, 1988; Hay et al., 1988; Steinberg, 1988).

For details of community metabolism, respiration, trophic web and chemistry, we refer the reader to the works of Culliney (1970), Burns and Teal (1973), Smith et al. (1973), Johnson and Braman (1975), Blake and Johnson (1976), Hanson, (1977), Geiselman (1983), Morris et al. (1976), and Trapnell et al. (1983).

Invertebrates

The invertebrate fauna consists of both sessile and motile forms (Table 1). Epizoans include colonial hydroids, encrusting bryozoans, the polychaete Spirorbis, barnacles, pycnogonids, and the tunicate Diplosoma. Older plants gradually become heavily encrusted with these organisms and ultimately sink. This biomass then gradually disintegrates, providing valuable nourishment for animals in deeper water (Parr, 1939; Weis, 1968; Schoener and Rowe, 1970; Butler et al. 1983). Conspicuous among the motile fauna are decapod crustaceans, particularly the Portunus crabs, and shrimps Latreutes and Leander, various molluscs, including the sargassum snail Litiopa melanostoma, polychaetes, flatworms, and nudibranchs. Fine (1970) found very high numbers of portunids in his late summer samples in the Gulf Stream and Sargasso Sea. Only Portunus sayi is commonly considered a resident of the community; the remaining megalopa and juveniles are transitory and utilize the habitat as a nursery.

Dooley (1972) examined stomach contents of the eight most abundant fish species yielding further insight into the invertebrate component of the sargassum community. These included hydroids, copepods, phylosoma larvae, shrimp zoea and postlarvae, crabs, pycnogonids, barnacles, tunicates, polychaetes, bivalves, gastropods, and platyhelminthes. The presence of two rather enigmatic members of the sargassum fauna were revealed by Morgan

et al. (1985) through stomach content analysis of several large epipelagic predatory fishes. They found the large mysis of the penaeoids, Cerataspis monstrosa and C.petit), frequently co-occurred with sargassum in the stomachs of surface feeding tuna (Scombridae) and dolphin Coryphaena hippurus. Nothing is known about the adult stage or life history of these rare crustaceans.

Vertebrates - Fishes

There is a well known assemblage of small fishes associated with sargassum rafts, many of which serve as forage for commercially or recreationally exploited species (Table 2). Dooley (1972) described 54 species from 23 families in the sargassum community of the Florida Current, while Bortone et al. (1977) reported 40 species from 15 families in the eastern Gulf of Mexico. Only 14 species from 11 families are known from the Sargasso Sea (Fedoryako, 1980; 1989).

Young jacks (Carangidae) live among the protective branches of sargassum and feed heavily on copepods and larval decapods. Apparently sargassum carries along a resident plankton population capable of sustaining these voracious predators (Yeatman, 1962). Sub-adult jacks range further from the rafts but dart in to feed on shrimp and young fishes living in the sargassum. The filefishes and triggerfishes (Balistidae) are also abundant and feed primarily on hydroids, encrusting bryozoans, and other invertebrates. Another major predator is the voracious sargassumfish, Histrio histrio, which selectively preys upon shrimp and young fish (Adams, 1960).

Large predatory species associated with the sargassum habitat include jacks, dolphins (Coryphaenidae), barracudas (Sphyraenidae), mackerels and tunas (Scombridae), swordfish (Xiphiidae), and billfishes (Istiophoridae) (Gibbs and Collette, 1959; Stephens, 1965; Dooley, 1972; Fedoryako, 1980; Carr, 1986; C. Manooch, pers. comm.). It is believed that dolphin, a much sought after game and food fish, takes shelter under flatoam (including sargassum) because of the enhanced availability of prey (Dooley, 1972). Filefish, triggerfish, jacks, flyingfish (Exocoetidae), and puffers (Tetradontidae) are among the species identified in dolphin stomachs (Gibbs and Collette, 1959; Dooley, 1972; Manooch et al., 1984). Fragments of sargassum were also commonly found. Manooch et al. (1984) stated "The close association (=dependence) of dolphin with fish and invertebrates that form the sargassum community is unmistakable". Manooch and Mason (1983) also reported finding sargassum fragments in 26% of yellowfin tuna, Thunnus albacaras, stomachs they examined as well as in 12% of blackfin tuna, T. atlanticus. They believed the material was ingested incidentally with normal prey.

There is less known about the ichthyoplankton associated with the habitat, but it seems likely that the same hydrodynamic mechanisms that drive the formation of sargassum rafts, i.e., convergence of surface water within shear zones or Langmuir cells, will also aggregate surface oriented organisms (Kingsford, 1990). There is some evidence that this is the case for swordfish, Xiphias

gladius, larvae and cobia, Rachycentron canadum, eggs near the Gulf Stream frontal zone (Hassler and Rainville, 1975; J.J. Govoni, pers. comm.). In addition to feeding and shelter, adults of some oceanic pelagic fishes use sargassum as a spawning substrate (Dooley, 1972: Peres, 1982) or as a nursery area for larvae and juveniles. Most notable among these are the flyingfishes (Winge, 1923; Breder, 1938) which are a major component of the diet of large oceanic fishes.

Vertebrates-Sea Turtles

During the pelagic stage, hatchling loggerhead, Caretta caretta, green, Chelonia mydas, Kemp's ridley, Lepidochelys kempi, and hawksbill, Eretmochelys imbricata, sea turtles have been observed in sargassum off Florida, Georgia, North Carolina, and Texas (Smith, 1968; Fletemeyer, 1978; Carr and Meylan, 1980; Carr, 1986; 1987a; Schwartz, 1988; 1989; Manzella and Williams, 1991; Schwartz, pers. comm.). Hundreds of loggerhead hatchlings, both dead and alive, were found in the wrack of sargassum deposited on the shore at Cocoa Beach, Florida following a hurricane in September, 1979 (Carr and Meylan, 1980). Stomach contents of the dead hatchlings showed that almost all contained sargassum floats and leafy parts. Schwartz (1988) reported numerous loggerhead hatchlings captured during commercial trawling for sargassum. This observation constitutes the largest known aggregation of loggerhead hatchlings encountered off the North Carolina coast.

Hatchling turtles are thought to actively seek out frontal zones and hence sargassum rafts. These areas are then utilized for forage and protection during the "lost year" (Carr 1986; 1987a,b). Witham (1988) suggested an alternative hypothesis for this association. He noted that it remains untested as to whether sea turtles actually benefit from their association with sargassum or whether they are at increased risk from predation, entanglement, and stranding.

The pelagic sargassum habitat of the northwestern Atlantic consists of both truly pelagic forms and flotsom detached from coastal regions. While within the neuston it provides numerous species of invertebrates and vertebrates a source of food, shelter, and substrate. Community structure varies with season, location, and algal age.

While the relationship of many species within this habitat is well understood, others remain less well known. This is particularly true for egg and larval stages of fishes, some crustaceans, and juvenile sea turtles. Functional relationships between the animals and the habitat have not been elucidated (e.g., we do not know the effect of the loss of sargassum on fish or sea turtle populations). Because estimates of oceanic biomass of sargassum are variable and inadequate, we also do not have a clear understanding the population dynamics of the sargassum habitat,

i.e., what is the standing crop, the productivity and the effect of harvesting on living marine resources?

We would like to express our thanks to Drs. J. J. Govoni and F. J. Schwartz for providing unpublished data, Drs. W. W. Kirby Smith and R. E. Robbins for assistance with the invertebrate taxonomy, and to Dr. C. S. Manooch, III for review of the report.

Personal Communications

Dr. J. J. Govoni, National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Southeast Fisheries Science Center, Beaufort Laboratory, Beaufort, North Carolina, 28516.

Dr. C. S. Manooch, III, National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Southeast Fisheries Science Center, Beaufort Laboratory, Beaufort, North Carolina, 28516.

Dr. F. J. Schwartz, University of North Carolina, Institute of Marine Science, 3407 Arendell Street, Morehead City, North Carolina, 28557.

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