Should we redefine meiofaunal organisms? The impact of mesh size on collection of meiofauna with special regard to nematodes

C Ptatscheck,W Traunspurger,S Gehner

doi:10.1007/s10452-020-09798-2

Abstract

Meiofaunal organisms are the predominant metazoans in benthic systems and important members of the benthic food web. They are defined by mesh size and specifically by their retention on a sieve with a 44-µm mesh size. In this study, we examined the accuracy of this standard collection method by counting the number of meiobenthic individuals, life stages and nematode species in a sample. A filter cascade consisting of five different mesh sizes (41 µm, 30 µm, 20 µm, 10 µm and 1 µm) was used to fractionate a natural freshwater meiobenthic collection, and the individuals in each fraction were then counted. In line with the current definition of meiofauna, all tardigrades, microcrustaceans, chironomids and oligochaetes were retained by the largest mesh size, whereas 9% of the rotifers were first retained on the 30-µm meshes. For nematodes, 23% were not retained on the 41-µm meshes and individuals were collected even from the 1-µm fraction. With declining mesh size, the yield of retained nematodes increased, the age structure shifted to juveniles, evenness declined, and the species composition changed. As all of these findings were significant, this study therefore shows that the current definition of meiofauna is not sufficient to encompass the entire spectrum of meiofauna present in a sample and may result in misleading assessments of the diversity and composition of these organisms. We therefore propose that, especially for nematodes, a definition based on a smaller mesh size (at least 20 µm) is more appropriate.

Highlights

The term ‘‘meiobenthos’’ or ‘‘meiofauna’’ is well established in aquatic biology and was first used by Mare (1942) to describe the smallest metazoans that colonize sediments and other substrates of aquatic habitats
We focused on nematodes, the most abundant meiobenthic group, and hypothesized that (H3) the current definition of meiofauna is insufficient to reflect the real composition of nematode communities in a sample because it results in the omission of juvenile stages and possibly even some species
Tardigrades, microcrustaceans, chironomids and oligochaetes were completely retained by the 41-lm meshes

Summary

Introduction

The term ‘‘meiobenthos’’ or ‘‘meiofauna’’ is well established in aquatic biology and was first used by Mare (1942) to describe the smallest metazoans that colonize sediments and other substrates of aquatic habitats (e.g., microcrustaceans, rotifers and nematodes). Through a literature search on the databases Google Scholar (last search: 17th February 2020), we obtained 100 studies, all of them published within the last 40 years (1980–2020), that investigated meiofaunal structure in permanent freshwater habitats while noting the mesh size used to collect the organisms In 60% of these studies, meiofauna was collected with a minimum mesh size in line with that used by Giere (2009) or even smaller (B 44 lm), whereas many of the other studies relied on much larger (up to 100-lm) meshes (Fig. 1). Studies in which meiofauna was mentioned only casually, for example as bycatch during plankton sampling, were not taken into account, but we noted that the mesh sizes used by the respective authors were up to 250 lm

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