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

Abstract

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.

A new look into the small-scale dispersal of free-living marine nematodes

“…This may be attributed to the tidal current of 9 cm/s, able to resuspend and transport the most abundant genera which dwell in the uppermost sediment layers in muddy areas (COMMITO & TITA 2002). Although nematodes are considered poor swimmers (PALMER 1984, FEGLEY 1985, they are commonly found in plankton samples (BOECKNER et al 2009) and at the sediment-water interface. However, their abundance in the water column is always much lower than in the sediment (PALMER 1984).…”

“…The rapid dispersal of meiofauna has been ascribed primarily to water column processes, including passive erosion and active emergence, which exert a major influence on recruitment and colonisation in larger spatial scales (CHANDLER & FLEEGER 1993, COMMITO & TITA 2002. Passive largescale dispersal modes may include transportation through the water column, erosion/suspension, emersion/suspension and rafting on algal mats and floating debris (BOECKNER et al 2009, DERYCKE et al 2008.…”

“…The organisms most commonly found in the water column dwell in the upper layers of sediment and so are more susceptible to passive suspension (PALMER & GUST 1985, COMMITO & TITA 2002. This is especially true in high-energy areas, in which meiofaunal taxa are resuspended by waves and tidal currents (BOECKNER et al 2009). The nematode species that are most susceptible to passive dispersal are probably the epigrowth feeders, such as Ptycholaimellus ponticus (Filipjev, 1922), which depend primarily on seaweed or bacteria found on the sediment surface (COMMITO & TITA 2002).…”

Diplolaimella dievengatensis (Nematoda: Monhysteridae) as model organism in ecotoxicity assay

“…If conspecific, the ubiquity of this species along the Atlantic Ocean to support the "meiofauna paradox", in which small benthic organisms with little mobility and without planktonic larval stages can be dispersed over large geographic ranges (Giere, 1993). It was experimentally demonstrated that even weak currents are able to suspend and transport meiofaunal organisms and moreover these are capable of active dispersal into the water column (BOECKNER et al, 2009). In this way, further studies also are necessary in order to investigate possible patterns of population structuration along the species distributional ranges.…”

“…Since our data support the hypothesis of an amphi-oceanic distribution of D. dievengatensis, this strengthened the paradox. Such long distance dispersal may be human mediated, through ballast water for example (Radziejewska et al, 2006), or may be related to current-driven sediment transport (Boeckner et al, 2009).…”