Introduction to Meiobenthology

The terms “macrobenthos” and “microbenthos” were already well established when in 1942 Molly F. Mare coined the term “meiobenthos” to define an assemblage of benthic metazoans that can be distinguished from macrobenthos by their small sizes (note that the Greek “μειος“ means “smaller”). Therefore, the study of meiobenthos per se is a relatively new component of benthic research, despite the fact that meiobenthic animals have been known about since the early days of microscopy. This book will mainly focus on metazoan meiofauna, which mirrors the author’s field of expertise. Hence, the term “meiobenthos” is used here synonymously to “meiofauna.” However, an ecological picture cannot be drawn without also considering relevant benthic protists (e.g., ciliates, foraminiferans, amoebozoans), and microalgae (e.g., diatoms).

Today, members of the meiofauna are considered mobile and sometimes also haptosessile benthic animals, smaller than macrofauna but larger than microfauna (the latter term is now restricted mostly to Protozoa). The formal size boundaries of meiofauna are operationally defined, based on the standardized mesh width of sieves with 500µm (1,000µm) as upper and 44µm (63µm) as lower limits: all fauna that pass through the coarse sieve but are retained by the finer sieve during sieving are considered meiofauna. In a recent move, a lower size limit of 31µm has been suggested by deep-sea meiobenthologists in order to quantitatively retain even the smallest meiofaunal organisms (mainly nematodes). Using biomass as a measure, meiofauna (in freshwater) have been defined to include all mobile benthic organisms with masses of between 2 and 20µg (Hakenkamp et al. 2002). What began as an arbitrarily defined size-range of benthic invertebrates has since been supported by studies on the size spectra of marine benthic fauna. Quantitative sizetaxon studies (Schwinghamer 1981a; Warwick 1984; Warwick et al. 1986a; Duplisea and Hargrave 1996—see Sect. 9.2) infer that the (marine) meiofauna represent a separate biologically and ecologically defined group of animals, a concept well known in the case of the (interstitial) meiofauna of sands (Remane 1933, see Sect. 1.2). In addition to the “permanent” meiofauna, members of the “temporary” meiofauna belong to the meiofaunal size category only as newly settled larvae that later grow to become macrofauna. An exact upper size limit that will be passed by these temporarily small organisms (often juvenile molluscs and annelids) is difficult to define.