Suprabenthic crustacean fauna of a dense Ampelisca community from the English Channel

Ninety-six species (97,677 individuals) were collected over the course of 6 h in five suprabenthic sledge hauls from a very dense Ampelisca fine sand community from the Bay of Morlaix (western English Channel). All the species migrated into the water column at night (98% of the specimens collected in the suprabenthos were found in the night hauls). The 23 most abundant species collected were classified into five groups based on their height within the water column, but two groups predominated: the upper suprabenthic species, abundant at 0.80-1.45 m above the sea-bed; and the lower suprabenthic species which were abundant only near the sea bottom (similar to 0.1-0.75 m high). Three different patterns of nocturnal vertical migration were distinguished based on the timing of maximum swimming activity: at dusk; at the beginning of the night; or later in the night. Sexually dimorphic patterns of free-swimming behaviour was observed in Ampelisca and some other species of Amphipoda (Bathyporeia tenuipes, Metaphoxus fultoni), and Cumacea (Bodotria pulchella, Pseudocuma longicornis), with many more males than females migrating into the water column at night. Finally, the density of suprabenthic crustaceans in nocturnal hauls was amongst the highest reported from infralittoral or circalittoral suprabenthic studies on other parts of the Atlantic Ocean sampled during spring.