Suprabenthic pycnogonids from Creixell beach (Tarragona, west Mediterranean)

Five stations from the Creixell sublittoral zone were sampled monthly in 1991-1992, parallel to the coastline, with a suprabenthic sledge. On these soft bottoms 211 pycnogonids belonging to the following ten species were recorded between 0 and 3.5 m depth: Ammothella longipes, Tanystylum conirostre, Achelia echinata, Achelia vulgaris, Nymphonella tapetis, Endeis spinosa, Callipallene emaciata, Callipallene tiberi, Anoplodactylus petiolatus and Anoplodactylus virescens. Ammothella longipes was the most abundant species (82.0% of the taxocoenosis) and the only resident throughout the year; 2/3 of their specimens were postlarvae and juveniles and 1/3 adults. The mean density of this species was 5.3 specimens 100 m-2 and its biomass 2 mg 100 m-2. The data of A. longipes obtained on this beach are compared with the corresponding ones from a photophilous algal hard substratum on the Costa Brava (Gerona) at the same depth. A Posidonia oceanica meadow, located 500 m off the coastline, is probably the source of the majority of pycnogonids and the passive migration of specimens (mainly subadults) regulates their presence on the sandy bottoms. Moreover, hydro-dynamics appears to be the main environmental factor influencing the bathymetric pycnogonid distribution in the sublittoral zone of the beach.