Growth and the delay of metamorphosis of the larvae ofMytilus edulis(L.)

Abstract

Techniques for the induction of spawning of Mytilus edulis, the delay of spawning and the rearing of the larvae are described. The rate of growth of Mytilus larvae increased with increased temperature from 10–18° C, but from 13–18° C the growth was relatively temperature-independent. At temperatures higher than 18° C larvae from a littoral area grew at a slightly increased rate, but larvae from a sub-littoral area showed decline of the growth rate. There was a decline in the rate of growth during the life of a larvae at all temperatures. Cleavage and early development occurred from 8–18° C and increased with increased temperature. Isochrysis galbana and Monochrysis lutheri were good foods for Mytilus larvae. Growth rate increased with mcreased cell concentration to 100 cells Isochrvsis/µ litre and 2.0 µ litre packed cell volume of Monochrysis/litre. A mixture of these 2 species supported more rapid growth than either species indivldually. Larvae fed with Nannochloris atomus and Chlorella sp. grew slowly. This was probably due to a substance secreted by the algae and present in greatest concentration during the stationary phase of growth of the algae. Larvae from an area of full salinity had a narrow salinity optimum for growth at 30–33 ‰. Larvae from an area of reduced salinity had a lower salinity optimum and greater tolerance of low salinity. The optimum salinity range varied with temperature. There was a large size-variation of larvae from the same parents and reared under similar conditions. The range of size-variation increased with increased growth rate. The rate of feeding increased with the age of the larvae and with increased temperature but did not differ in different food-cell concentrations. Full-grown larvae cleared cells from suspension at an average rate of 0.62 rnl/larva/day. The larvae first became capable of metamorphosis at the pediveliger stage. This stage was reached at a constant size of 260 fl regardless of very different conditions of culture. The behaviour of the larvae at the time of settlement is described. If suitable attachment substrates were withheld from the larvae metamorphosis was delayed. During the delay of metamorphosis the velum slowly degenerated and the foot grew in size. These changes were used to identify 3 stages of the delay of metamorphosis. These are described. During the delay of metamorphosis there was a decline in the feeding rate as the feeding currents on the velum became disrupted. Eventually the larvae became unable to feed. Attachment and metamorphosis could occur for the time for which the larvae could delay metamorphosis, from ca. 40 days at 10° C to only 2 days at 20° C. Differences in the quality or the quantity of the food did not effect the duration of the delay. Larvae were able to delay metamorphosis for longer times at optimum salinities. There was no correlation between the rate of growth to the pediveliger stage and the duration of the delay. During the delay of metamorphosis the rate of growth gradually declined to zero. Cessation of growth coincided with the degeneration of the velum. There was a correlation between maximum size and temperature, due to the longer time available for feeding and growth at the lower temperatures. A comparison of the pediveligers from two areas that differed in the availability of suitable natural substrates indicated that delay of metamorphosis can occur in the field. This delay was responsible for the different mean maximum sizes of the larvae from the two areas. The observations on the delay of metamorphosis are summarized and discussed. Delay of metamorphosis is the maintainance of a certain level of organization rather than a period of growth of new tissue. It is suggested that phenomena similar to those described may be widespread amongst marine lamellibranchs.