Floods, Habitat Hydraulics and Upstream Migration of Neritina virginea (Gastropoda:Neritodae) in Northeastern Puerto Rico

Massive upstream migrations of neritid snails (Neritidae: Gastropoda) occur in tropical and subtropical streams worldwide, but their seasonality and proximate causes are unknown. We monitored massive upstream migrations of Neritina virginea for 99 weeks, and conducted a detailed study of snail density, size, and hydraulic descriptors in lower Río Mameyes, northeastern Puerto Rico. The study assessed the 1) timing and seasonality of upstream migration, 2) size composition of migratory aggregations, 3) patterns of habitat use, and 4) role of floods on upstream migration. Massive upstream migrations (500–3000ind/m2) were observed in 44 of 99 weeks of observation. While N. virginea aggregations occurred at random time intervals, they were clumped during rainy periods. Migratory aggregations consisted mostly of small individuals (5–7 mm). Greater mean density was consistently observed in a stable riffle than in an unstable run (115.7 and 17.8 ind/m2, respectively), but mean density increased and mean size reduced in both reaches during the first 7 upstream migratory events. N. virginea density and size dynamics differed between reaches as a function of habitat hydraulics. While juveniles used the stable riffle as a permanent habitat and preferred passageway, they also used an adjacent, unstable reach after storm events. Density variation was correlated with days postflood (>3.5 m3/s) in both reaches. Our observations indicated that massive upstream migrations of N. virginea juveniles occur at least once a month, presumably as habitat-dependent responses to floods.