The cosmopolitan distribution of many meiofaunal organisms raises questions about the dispersal mechanisms of these animals. Their small size and the lack of a planktonic life stage in many taxa suggest that passive dispersal is the main factor determining distribution and colonization patterns. This study investigates the settling behaviour of meiofauna while descending in a water column. Two ex-situ settling experiments were conducted with (1) macrophyte associated meiofauna from an intertidal flat and (2) meiofauna of microbial mat sediments from the deep-sea Håkon Mosby mud volcano (HMMV). Containers (126L) filled with sieved seawater were used as settling chambers and five different substrates, placed on the bottom of the vessels, were offered to the descending meiofaunal assemblage. The substrates used Fucus spiralis, sulfidic agar medium, bare agar and an empty Petri dish. For the deepsea experiment azoic sediment with algae, azoic sediment with bacteria, a sulfidic medium, azoic sediment and an empty Petri dish were used. The experiments were run for 12 hours. Significant differences in meiofaunal densities between the treatments were found in both experiments. Nematode and nauplius larvae showed fourfold higher densities in the Fucus treatment compared to the controls in the case of the intertidal experiment. Deep-sea nematodes and harpacticoid copepods in the sulfide treatment differed by a factor of five from all other treatments in terms of densities. Proportions of harpacticoid copepods were increased in the treatments compared to the reference sample. Nematode assemblages in the treatments showed no significant differences to the reference samples. These findings suggest that meiofauna can selectively settle once they are suspended in the water column and therefore actively contribute to their dispersal. In both experiments meiofauna was increased in substrates similar to their original habitat and proportional differences in harpacticoid copepods suggests a more efficient settlement compared to nematodes. The observed active settling behaviour may be very advantageous for the organisms in finding suitable habitat patches in heterogeneous environments. This study confirms previous research and adds important information to the understanding of meiofaunal dispersal mechanisms. |
