OBSERVATIONS ON THE SWIMMING AND BUOYANCY OF SOME THECOSOMATOUS PTEROPOD GASTROPODS

Cymbulia peroni produces forward propulsion only on the upstrokeof its ‘wingbeat’ the downstroke opposing slightnegative buoyancy. The species' torpedo-shaped pseudoconchaprovides stability as well as buoyancy. Corolla ovata producespropulsive power on the downstroke of its ’wings‘and shows periods of active upward swimming alternating withpassive downward gliding/parachuting. Both cymbuliid speciesare gelatinous slow movers, close to neutral buoyancy, and areincapable of effective diurnal vertical migration. Two cavoliniidspecies were also studied; Cilo pyramida and Cuvierina columnella.Both were dense and sank quickly if they ceased active swimming.Both had postural responses to slow sinking, but vigorous activitywas required to hover in the water column. Clio could not climbrapidly enough to carry out effective diurnal vertical migration,but was capable of fast horizontal swimming during which thetriangular shell appeared to act as a hydrofoil. The smallerCuvierina could not swim quickly in a horizontal direction,but was capable of swimming vertically at rates sufficient tomaintain station with moving isolumes and may therefore carryout significant diurnal vertical migrations. (Received 28 September 1989; accepted 12 December 1989)