Unusual capabilities for surface movement in a normally deep-burrowed Antarctic bivalve

Many infaunal marine bivalve molluscs have the capacity to move over the sediment surface by 'creeping' or 'leaping' movements, either spontaneously or in response to predators or physical disturbance.1 The evolution of a deep-burrowing habit in some bivalve families has been accompanied by loss of the capacity for surface movement and for reburial,2-3 since burrowing to greater depths provides effective protection from both natural disturbance and most predators. It is unusual, therefore, to discover that one normally deeply burrowing species, living in the cold waters of Antarctica, has a capacity for surface movement and by a different method to any previously described for an infaunal bivalve. Laternula elliptica (King and Broderip) (Anomalodesmata, Laternulidae) is among the most abundant and ubiquitous infaunal bivalves in shallow waters around the Antarctic continent, reaching population densities of up to 86 m2 in a variety of soft sediment types.4 Adult individuals of up to 90 mm shell length may be 12-13 years old.5 L. elliptica show many features in common with other deepburrowing genera such as Mya, Lutraria or Xirfaea including long, muscular, fused and periostracumcovered siphons that are capable of considerable extension; extensively-fused mantle edges that firmly seal the mantle cavity except for siphonal and pedal apertures; a relatively small foot; and a shell that is emarginated posteriorly and cannot contain all of the soft tissues. Unlike such well-studied temperate bivalves and other species of Laternula from lower latitudes,*-7 L. elliptica retain throughout their life the ability to re-burrow if uncovered from the sediment, and also exhibit a hitherto undescribed mode of movement on the sediment surface, following dislodgement, powered by contractions of the muscles of the siphons and mantle. The observations reported here were made at the Dunstaffnage Marine Laboratory (DML) on a group of 14 L. elliptica (shell lengths ranging 44.6-78.0 mm). These were the survivors of a larger group collected near the British Antarctic Survey (BAS) base at Signy Island, Antarctica by one of us (MCR) during the austral summer of 1994-95, and subsequently retained at c 0°C, without sediment, in the marine aquarium of BAS, Cambridge for observations on their energy metabolism. At DML, small groups (<6) of the bivalves were placed in a glasssided aquarium containing c 16 cm depth of sand, with recirculating seawater at -1 to 0°C and their activity recorded for c 24 h by time-lapse video. For logistical reasons the period available for observation was limited, but over a 6-day period of recording 10 individuals (71.4%) reburied at least once and 2 (14.3%) (shell lengths 65.2 and 71.0 mm) showed surface movement as described here, before subsequently reburrowing.