- Factors Affecting the Distribution of Atyid Shrimps in Two Tropical Insular Rivers
We investigated factors affecting distribution of atyid shrimps, common inhabitants of insular freshwater ecosystems. Several abiotic and biotic variables were measured to determine their influence on atyid shrimp densities in two streams on the western Pacific island of Guam. Randomly selected sites, composed of three habitat types (riffles, runs, and pools), were surveyed in the rainy and dry seasons. We made visual counts of instream fauna in 2-m2 quadrats within each site. Various statistical analyses suggested that habitat type is a major factor affecting atyid distribution on Guam. However, results of a transplant experiment, conducted to test the effect of predators on atyid distribution directly, were noteworthy: no atyids remained in pools containing the transplanted jungle perch Kuhlia rupestris in the field. Our data indicate that both environmental factors and faunal interactions may be important influences on atyid distribution.
Atyid shrimps are common in the freshwater habitats of oceanic islands. There are seven recorded species of stream atyids on the western Pacific island of Guam: Atyoida pilipes (Newport, 1847), Atyopsis spinipes (Newport, 1847), Caridina longirostris (Milne-Edwards, 1837), Caridina nilotica (Roux, 1833), Caridina serratirostris de Man, 1892, Caridina typus Milne-Edwards, 1837, and Caridina weberi de Man, 1892. All except Atyopsis spinipes were seen during this study. Caridina longirostris and C. weberi are new records for Guam (this study). All of these species are native to Guam, but none is endemic. Like most other atyids (Shokita 1976, Benzie 1982), all those occurring in the rivers of Guam are amphidromous, with their larvae drifting downstream to the ocean, developing in the plankton, and migrating back upstream as juveniles. Although some previous studies have addressed the postlarval ecology of atyids in various geographical locations (Hart 1981, Carpenter 1983, Dudgeon 1985, de Silva 1988, Crowl and Covich 1994), less information is available from the tropical Pacific Region. Couret (1976) provided a comprehensive summary of the biology of the Hawaiian atyid Atyoida bisulcata Randall, 1840, including aspects of its ecology. Bright (1979) included some ecological information in his study of freshwater ecosystems in Palau. Ellis-Neill (1987) concluded that current velocity and a greater abundance of leaf litter are important influences of invertebrate distribution (including atyids) on Guam. The ecology of atyids on Yap was briefly mentioned in Bright (1989).
In this study, we addressed the following question: Which environmental and faunal factors affect the distribution of atyids on Guam? We analyzed several abiotic and biotic variables, including season, reach, habitat, pH, water temperature, maximum depth, canopy cover, substrate composition, abundance of aquatic vegetation, current velocity, and densities of other riverine fauna. Preliminary observations suggested that predation was an important faunal factor influencing atyid distribution. We examined predation in [End Page 389] two ways. First, we compared densities of atyid species with potential predator densities in a correlation analysis. In the streams of Guam, potential predators include the jungle perch Kuhlia rupestris (Lacépède, 1802); the freshwater eel Anguilla marmorata Quoy & Gaimard, 1824; the sleeper goby Eleotris fusca (Bloch & Schneider, 1801); the Mariana goby Awaous guamensis (Cuvier & Valenciennes, 1837); and the Tahitian prawn Macrohrachium lar (Fabricius, 1798). Stomach contents of these species (except M. lar) have been found to contain atyids (R. B. Tibbatts, pers. comm.). Second, to test the effect of predators on atyid distribution directly, we conducted a transplant experiment in the field, involving K. rupestris. Kuhlia are free-swimming, visual predators, occurring only in the lower reaches of rivers, below barrier waterfalls. If predation by Kuhlia is an important influence on the distribution of atyids in the streams of Guam, then densities of atyid species would be expected to be lower in pools containing transplanted K. rupestris than in control pools containing no K rupestris.
Materials And Methods
We collected data from two rivers (Figure 1). The Asmafines River (coordinates at the mouth: lat. 13° 19' 37" N, long. 144° 39' 03" E) is a high-gradient stream located in the south on Guam's steeper western slope, with a perennial channel length of 1341 m and an elevation of 134 m at the headwaters (Best and...