Population Dynamics of the Sponge-Dwelling Alpheid Synalpheus Longicarpus, with Observations on S. Brooksi and S. Pectiniger, in Shallow-Water Assemblages of the Eastern Gulf of Mexico

Investigations were conducted on species composition, population dynamics, and ecological interactions of alpheid shrimps associated with the loggerhead sponge Spheciospongia vesparia in shallow-water assemblages of the eastern Gulf of Mexico. Three species, Synalpheus brooksi, S. pectiniger, and the numerically dominant S. longicarpus were found in coexistence. Biomass and numbers of all three species found within the sponge were directly proportional to increasing sponge volume. Dominance of S. longicarpus, a eurythermal tropical species, is attributed to successful year-round reproduction. S. brooksi and S. pectiniger are fugitive species in this particular association. The latter species are able to colonize the habitat, but seasonal temperature minima by depressing egg production limit their ability to coexist in greater abundance. Coexistence of the three shrimp species appears to coincide with a wide degree of niche overlap. Potential competition for limited available space within the host may be intense and, combined with the differing reproductive success of each species, may be of considerable importance in determining population size and species composition of the associates. Sponges provide a unique habitat for a wide array of cryptic marine organisms. Previous studies have revealed the species composition of associates of various sponge species (Pearse, 1932, 1950; Fishelson, 1962; Dauer, 1973; Riitzler, 1976; Westinga and Hoetjes, 1981) and have examined the importance of sponge size, structure, and location (Long, 1968; Riitzler, 1976; Uebelacker, 1977; Westinga and Hoetjes, 1981) as factors affecting associated species composition. Many sponge species harbor abundant and consistently similar associations, yet little is known of the life history and dynamic role of many of these cryptic species within their particular host sponge. Loggerhead sponges, Spheciospongia vesparia (Lamarck) have been reported to harbor large populations of snapping shrimps of the genus Synalpheus (Decapoda: Alpheidae) (Coutiere, 1909, 1910; Pearse, 1932, 1950; Hoetjes et al., 1977; Westinga and Hoetjes, 1981; Dardeau, 1984). With the exception of Dobkin's (1965) description of the larvae of S. brooksi, the life history of most species of Synalpheus is poorly known. We report the population dynamics of three species of snapping shrimps: Synalpheus brooksi Coutiere, 1909, S. longicarpus (Herrick, 1891), and S. pectiniger Coutiere, 1907, which are dominant decapod crustacean inhabitants of the loggerhead sponge Spheciospongia vesparia in the eastern Gulf of Mexico. Relationships among associates and between associates and host were examined to obtain additional information regarding the life histories of these cryptic shrimps. DESCRIPTION OF STUDY AREA The coastal fauna of the Gulf of Mexico exhibits affinities of both warm temperate and tropical origin. Northern gulf fauna is Carolinian warm temperate, whereas the fauna of the southern Gulf is primarily Caribbean tropical in nature (Briggs, 1974; Lyons and Collard, 1973). Establishment of a distinct zoogeographic boundary is hampered by wide ranging temperatures and seasonally changing