33 



movements. If the substrate is not congenial for settling, the larva abandons 

 it and begins to swim again. If the substrate is suitable for settling, the larva 

 cements itself to it and metamorphoses into a juvenile oyster. 



be 



mc 



scm 



Figure 28: Foot and byssus complex of the pediveliger of Mytilus edulis L. in transverse 



section (from Bayne, 1971). 



be — byssus complex; efm — external fold of mantle; f — foot; ifm — internal fold of mantle; 



mc — mantle cavity; scm — secretory cells of mantle. 



Rudiments of the definitive organs : The gill apparatus, renopericardiac 

 complex, and circulatory and reproductive systems are not fully developed and 

 are nonfunctional in the pediveliger. 



METAMORPHOSIS 



Metamorphosis, the most crucial period in the life cycle, is completed when 

 the pediveliger, while creeping, finds a substrate suitable for settling and, 

 discharging the byssus threads, attaches itself to the selected site. Larvae of 

 different species exhibit a well-defined substrate specificity. In the absence of 

 a desirable substrate the larva may postpone metamorphosis. According to 

 Bayne (1976), the larvae of M edulis are capable of delaying metamorphosis 

 by 40 days at a temperature of IO°C and by two days at 20°C; larvae of O. 

 edulis can delay metamorphosis by several days (Cranfield, 1973) and larvae 

 of Pecten megellanicus by a month (CuUiney, 1974). In some species, during 

 an overall delay of metamorphosis, the velum is discarded and the larva loses 

 its ability to swim; in other species the larva continues to swim by means of 

 the velum. In Brachiodontes glomerata, in the absence of a substrate not only 

 the velum may disappear, but a narrow band of the dissoconch may form 

 (Campos and Ramorini, 1980). 



