GROWTH OF JUVENILE ARCTICA-ISLANDICA UNDER EXPERIMENTAL CONDITIONS

In two laboratory experiments, the effects of temperature and food ava ilability on the growth of 10- to 23-mm high specimens of the bivalve Arctica islandica were estimated. Each experimental set-up consisted o f 5 treatments in which either the food supply or the temperature diff ered. It was demonstrated that Arctica is able to grow at temperatures as low as 1 degrees C. A tenfold increase of shell growth was observe d at temperatures between 1 degrees and 12 degrees C. The greatest cha nge in growth rate took place between 1 degrees and 6 degrees C. Avera ge instantaneous shell growth varies between 0.0003 at 1 degrees C to 0.0032/day at 12 degrees C. The results suggest that temperature hardl y affects the time spent in filtration, whereas particle density stron gly influences that response. Starved animals at 9 degrees C have thei r siphons open during only 12% of the time, whereas the siphons of opt imally fed animals were open on average during 76% of the observations . Increased siphon activity corresponded to high shell and tissue grow th. At 9 degrees C, average shell growth at the optimum cell density o f 20x10(5) cell/l was 3.1 mm corresponding to an instantaneous rate of 0.0026/day. An algal cell density (Isochrysis galbana, Dunaliella mar ina) ranging between 5 and 7x10(6) cell/l is just enough to keep shell s alive at 9 degrees C. Carbon conversion efficiency at 9 degrees C is estimated to vary between 11 and 14%.