Demersal amphipod migrations: spatial patterns in marine shallow waters

Abstract

AbstractThe nocturnal displacement to the water column of species hiding in the substrate during the day (so-called demersal migration) is a widespread phenomenon in coastal habitats. However, despite the relevance of these migrations to coastal ecosystem functioning, most of their ecological aspects are poorly understood. We aim to characterize spatial distribution patterns of demersal amphipods by comparing samples collected by light traps deployed both at rocky and nearby sandy areas. Dispersal abilities of such species were explored by considering two spatial scales of separation between rocky and sandy substrates: (1) <100 m and (2) >1 km. A total of >45,000 amphipod specimens were collected in the water column during the whole study, belonging to 50 species. The numerically dominant species were Guernea coalita, Bathyporeia cf. elegans, Perioculodes longimanus and Nototropis swammerdamei. The present study highlights the relevance of substrate type and study location on coastal demersal amphipod migrations. Amphipod assemblages were significantly different in the water column above rocky and sandy bottoms, even at a small spatial scale. Species emerging from rocky bottoms tend to remain in the proximity of this habitat; they showed lower abundances in sandy areas located nearby and almost none of them reached distant sandy areas. In contrast, migrant species commonly found on sediment during the day were collected at higher abundances in light traps deployed above rocky bottoms. Although several factors (e.g. food availability, predation pressure) are proposed to explain these patterns, the purpose of this horizontal migration remains unknown.