Marine nematode deep‐sea biodiversity – hyperdiverse or hype?

TLDR

Nematodes and the deep sea have long been considered potentially hyperdiverse, with estimates exceeding one million species. Using a large‐scale equatorial Pacific dataset, the authors estimated regional diversity, finding coastal waters exhibit higher regional diversity than the deep sea despite lower sample diversity, and highlighting how disturbance and patch dynamics shape local diversity and the pitfalls of extrapolating local to global patterns. The study finds the deep sea has modest regional diversity, nematodes are as diverse as other benthic taxa such as Polychaeta, and overall the data contradicts hyperdiversity hypotheses for marine nematodes or deep‑sea benthos, raising the question of whether any taxon exceeds a million species.

Abstract

Abstract Nematodes have been identified as a potentially hyperdiverse group and the deep sea as a potentially hyperdiverse environment (i.e. > 1 million species). A large‐scale data set from the equatorial central Pacific is used to estimate regional diversity with results that challenge this view; regional diversity is higher in some coastal waters despite lower sample diversity in coastal waters than in the deep sea. The data suggests a paradigm where the deep sea has modest regional diversity, despite high local diversity through patch dynamics, because similar patches in a similar habitat are repeated for considerable distances. Disturbance in shallow water dominates over patch‐dynamic mechanisms reducing local diversity but regional diversity is high because of the close packing of multiple habitats within a single region. The Pacific data are also used to demonstrate the pitfalls of extrapolating from local to global diversity. There is no reason to conclude that nematodes are less diverse than other benthic groups, indeed where direct comparison is possible the Nematoda appear to be as diverse as the Polychaeta, the most diverse macrofaunal taxon. This analysis is not consistent with the hypothesis that either marine nematodes or the deep‐sea benthos are hyperdiverse raising the question whether any environment or metazoan taxon has more than a million species.