Abstract
Meiofauna includes an astonishing diversity of organisms, whose census is far from being complete. Most classic ecological studies have focused on hard-bodied Ecdysozoan taxa (notably Copepoda and Nematoda), whose cuticle allows determination at species-level after fixation, rather than soft-bodied, Spiralian taxa, which most often lose any diagnostic feature in fixed samples. Yet, metabarcoding studies have recently revealed a species-richness of soft-bodied taxa comparable, and in cases superior, to that of Copepoda and Nematoda together. However, given objective difficulties inherent to their study, which necessarily has to be performed on living individuals, and their limited utilisation for ecological and applicative research, taxonomic expertise on soft-bodied organisms has declined over the years, and diversity of these phyla in most areas of the world is presently completely unknown. Here we present an expert-based survey of current knowledge on the composition and distribution of soft-bodied meiofaunal taxa in Italy, with special references to the predominantly or exclusively meiobenthic phyla Gastrotricha, Gnathostomulida, Platyhelminthes, Rotifera, Xenacoelomorpha, and macrofaunal taxa with conspicuous meiofaunal representatives (Annelida, Mollusca and Nemertea). A total of 638 described species have been reported from Italian coasts; furthermore, the existence of a large number of undescribed species is mentioned. Knowledge of Annelida, Gastrotricha, and Rotifera appears particularly detailed, placing Italy among the best-known country worldwide. In contrast, knowledge of Platyhelminthes and Xenacoelomorpha appears patchy, and limited to few areas. Sampling effort has been uneven, with most species recorded from the Tyrrhenian Sea, while large sections of the Adriatic and Ionian seas have been poorly explored. Results highlight the role that Marine Biological Stations, notably the Zoological Station “Anton Dohrn” in Naples, have had in promoting the study of soft-bodied taxa in Italy.
Highlights
Out of 34 metazoan phyla, at least 23 have some representatives in the meiofauna (Cerca et al 2018), operationally defined as the fraction of fauna passing through sieves with 500 μm mesh width and being retained by sieves with 31 μm mesh width
The taxonomic bias affecting meiofaunal studies in general is felt in soft-bodied taxa, whose diagnostic features need to be observed on living material (Higgins & Thiel 1988), making quick examination of samples necessary, ideally on facilities close to the coast, such as Marine Biological Stations
Given these operative difficul ties, and the limited utilisation of soft-bodied taxa for ecological studies and applicative research, which nowadays attract most of the funds, it is not surprising that taxonomic expertise on soft-bodied organisms has declined over the years, leading to the current situation where there are few taxonomic experts worldwide, if any
Summary
Out of 34 metazoan phyla, at least 23 have some representatives in the meiofauna (Cerca et al 2018), operationally defined as the fraction of fauna passing through sieves with 500 (or 1000, depending on the definition) μm mesh width and being retained by sieves with 31 (or 63) μm mesh width (see, i.e., Giere 2009). The taxonomic bias affecting meiofaunal studies in general is felt in soft-bodied taxa, whose diagnostic features need to be observed on living material (Higgins & Thiel 1988), making quick examination of samples necessary, ideally on facilities close to the coast, such as Marine Biological Stations Given these operative difficul ties, and the limited utilisation of soft-bodied taxa for ecological studies and applicative research, which nowadays attract most of the funds, it is not surprising that taxonomic expertise on soft-bodied organisms has declined over the years, leading to the current situation where there are few taxonomic experts worldwide, if any.
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