Swimming behaviour of the larvae of pecten maximus (l.) (bivalvia)

Laboratory observations indicate that larvae of Pecten maximus (L.) pass through three phases of swimming behaviour before metamorphosis. Trochophores and early veligers generally swim upwards and, in shallow containers, tend to accumulate at the surface. Their swimming cilia beat without interruption so swimming is continuous. After 3 days of development, pauses in ciliary beating occur intermittently and the veliger becomes capable of withdrawing into its shell. It alternately swims up a vertically orientated spiral and sinks. This type of swimming behaviour is also displayed by the pediveliger, but this larva also periodically swims close to, or crawls on, the substratum. The tendency to accumulate at the surface becomes less pronounced as larvae grow older. By the pediveliger stage it is replaced by an increasing tendency to accumulate close to the substratum. A hypothesis is advanced which suggests that the spiral path of ascending swimming is due to forces generated by ciliary beating and to the effect of gravity on the larva. Veligers 33 days old had a mean vertical velocity of 1-4 mm/s while older and younger larvae swam at 1-o mm/s or less. The mean vertical velocity of 10-, 22- and 31-day-old veligers and 41-day-old pediveligers was significantly higher when they were subjected to a 2-bar pressure increase than when they were at atmospheric pressure, but 3-day-old veligers showed no such difference. Veligers in a closed vessel rose from the bottom in greater numbers in response to a 2-bar increase in pressure: immediately after the pressure was returned to normal, significantly more larvae sank. For 16- and 17-day-old veligers a 1 -2-bar increase was sufficient to affect the number of rising larvae, while a 1.o-bar increase was not.