Physical and biological factors affect the vertical distribution of larvae of benthic gastropods in a shallow embayment

Abstract

Marine gastropods form a diverse taxonomic group, yet little is known about the factors that affect their larval distribution and abundance. We investigated the larval vertical dis- tribution and abundance of 9 meroplanktonic gastropod taxa (Margarites spp., Crepidula spp., Astyris lunata, Diaphana minuta, Littorinimorpha, Arrhoges occidentalis, Ilyanassa spp., Bittiolum alternatum and Nudibranchia), with similar morphology and swimming abilities, but different adult habitats and life-history strategies. We explored the role of physical (temperature, salinity, density, current velocities) and biological (fluorescence) factors, as well as periodic cycles (lunar phase, tidal state, diel period) in regulating larval vertical distribution. Using a pump, we collected plankton samples at 6 depths (3, 6, 9, 12, 18 and 24 m) at each tidal state, every 2 h over a 36 and a 26 h period, during a spring and neap tide, respectively, in St. George's Bay, Nova Scotia. Con- currently, we measured temperature, salinity, density, fluorescence (as a proxy for chlorophyll, i.e. phytoplankton density), and current velocity. Larval abundance was most strongly related to tem- perature, except for Littorinimorpha and Crepidula spp., for which it was most strongly related to fluorescence. Margarites spp., A. lunata, Ilyanassa spp. and B. alternatum exhibited either diel or reverse-diel vertical migration during 1 or both lunar phases. For Crepidula spp., Littorinimorpha, A. occidentalis and Nudibranchia, larval vertical distribution differed between lunar phases. Only the larval vertical distribution of Margarites spp., D. minuta and Ilyanassa spp. varied with tidal state during 1 or both lunar phases. The key factors determining the vertical distribution of gastro- pod larvae were temperature, fluorescence, and light, although the importance of each factor var- ied among taxa. Differences in vertical distribution may enable these larvae to partition over a wide range of potential habitats for settlement.