Revisiting the meiofauna paradox: dispersal and colonization of nematodes and other meiofaunal organisms in low- and high-energy environments

How do small, benthic meiofaunal organisms become cosmopolitan over large geographic ranges? Abiotic forces including oceanic currents are believed to be of key importance in aiding marine meiofaunal dispersal. We investigated the effect of distance from substrate and site exposure on meiofaunal colonization and transport in the water column. First, we tested how distance from substrate and sediment grain size affected colonization of azoic sediments by meiofauna in a sheltered inlet. Nematodes, crustacean nauplii and small amphipods colonized distant sediment cages (3 m above bottom) as quickly and abundantly as cages closer to the ocean floor. Some of the 30 recorded genera of nematodes predominated in one height treatment whereas abundance of others was not related to distance from the substrate. Polychaetes and harpacticoid copepods colonized near-benthic cages more rapidly and abundantly than those farther away suggesting active dispersal. Nematodes, harpacticoids and polychaetes were more abundant in fine than in coarse sediments, while nauplii and amphipods did not differ in abundance between sediment types. In part two of this study, we surveyed occurrence of meiofauna in the water column at several sheltered and exposed sites using plankton nets towed at fixed distances from 0.5 to 6.5 m above the ocean floor. Because oceanic currents increase sediment suspension and transport, we expected to see more meiofauna in samples collected from exposed than from sheltered sites. However, with the exception of polychaetes, which were more abundant in the water column of sheltered sites, there was no difference in meiofaunal abundances between the two exposure classes. Meiofauna, including the 14 identified nematode genera, were collected in greatest numbers nearer to the ocean floor and dwindled further up in the water column. The presence of meiofauna high in the water column of even the most sheltered sites combined with the quick and abundant colonization of distant, sheltered sediment cages suggests that even very weak currents are sufficient to suspend and transport these animals or that many meiofaunal taxa are capable of active dispersal into the water column.