Deep-sea meiofauna and macrofauna diversity and functional diversity: are they related?

Meiofauna and macrofauna are closely linked by numerous types of interactions, but comparative studies are rare. It has been postulated that meiofauna and macrofauna diversities are related, and that a higher functional diversity of meiobenthic and/or macrobenthic organisms can enhance a higher functional diversity. A simultaneous study of the deep Mediterranean sea meiobenthic and macrobenthic communities was conducted, from the west to central-eastern basin and from 1,200 to 2,700 m depth. We posed the following questions: 1) are meiofauna and macrofauna diversities related? 2) Does a higher diversity enhance a higher functional diversity? and 3) Does the relationship between diversity and functional diversity change considering different benthic size classes and taxa? In order to answer to our questions, the biodiversity and functional diversity of meiofauna and macrofauna were analysed, with a focus on three target groups: meiobenthic nematodes, macrobenthic nematodes and polychaetes. Meiobenthic nematodes and macrofaunal diversity appeared to be related only in terms of turnover diversity. A cross-taxon analysis on macrobenthic nematode and polychaete diversity and meiofauna diversity revealed a positive relationship in all cases that were studied. Considering functional diversity, many relationships emerged between the two benthic components. A higher meiobenthic nematode diversity did not enhance a higher functional diversity. A higher macrofauna diversity enhanced a higher functional diversity, but not for all macrobenthic taxa (i.e., nematodes). We can conclude that the diversity-functional diversity relationship changed across different size classes, even when the same taxon was considered. The diversity-functional diversity relationship changed across taxa even if they belonged to the same size class.