Diel and seasonal variation in species composition and abundance of caridean shrimps (Crustacea, Decapoda) from seagrass meadows on the north coast of Puerto Rico

Seagrass meadows dominated by Thalassia testudinum near Dorado, on the north coast of Puerto Rico, were sampled for 13 months by pushnet with a 1-mm mesh bag. Ten random samples were taken monthly both day and night at each of two replicate sites. Eighteen species of caridean shrimp were collected but 99.9% of the total abundance (N = 74,816) was accounted for by nine species: Latreutes fucorum (57.0%), Periclimenes americanus (10.9%), Thor manningi (9.5%), Alpheus normanni (8.3%), Hippolyte curacaoensis (6.0%), Latreutes parvulus (3.1%), Processa bermudensis (2.3%), Processa riveroi (1.4%), and Leander tenuicornis (1.4%). Night samples were similar to day samples in overall species composition except that the day-burrowing processids were conspicuously absent from daytime collections. However, with the possible exception of Hippolyte curacaoensis, mean abundances of all species were consistently higher at night. Greatest night-day differences in frequency of occurrence in samples were found in the two Processa species, the least in leaf-inhabiting species such as Hippolyte curacaoensis and Latreutes fucorum. Laboratory observations strongly indicate that nocturnal emergence onto the surface of the seagrass beds from daytime burrows below the sediment accounts for the increased night collectability of Alpheus normanni and the two Processa species. Laboratory observations also indicate increased nocturnal activity (in the form of increased swimming) for the other species, especially Latreutes fucorum and Hippolyte curacaoensis, which may make them more susceptible to net capture at night.

Abundances of the principal (most numerous) nine caridean species were markedly seasonal, with highest abundances in late spring and summer and again in December and January, with population lows in October and November and again in February and March.

Caridean densities estimated in this study (overall, 16 shrimps/m² of sampling effort in day samples, 49/m² in night samples) are much higher than those of most previous studies in seagrasses. The small mesh net used (1 mm) probably accounts, in part, for the high abundances measured for these very small carideans; much lower densities are reported in studies using nets with larger mesh (e.g., 6–7 mm).