77 



often, it occurs from July to early August, when the water temperature is 

 16 -17°C. In Peter the Great Bay this species spawns twice, end of May to June 

 and mid- August to September inclusively (Dzyuba, 1972; Kasyanov et al., 

 1980). Larvae are found in the plankton of Vostok Bay (Peter the Great Bay) 

 throughout summer; there are two population peaks in July and in September, 

 and larvae develop in a temperature range of 13-22°C. Duration of the pelagic 

 stage of development of C. grayanus along the southern coast of Sakhalin (Sea 

 of Okhotsk) and in Peter the Great Bay is 5-6 weeks. 



Drozdov and Kulikova (1979) have described the embryonal and postem- 

 bryonal development of C grayanus. 



LONG-BRISTLED MODIOLUS, MODIOLUS DIFFICILIS 

 (KURODA AND HABE) 



(Mytilidae) 



Veliconcha 



The shell is oblong-ovate (see Figure 53,C). The anterior end is not raised 

 and is slightly longer than the posterior end. The umbones are large and broad. 

 The shell is compact. The prodissoconch II has noticeable concentric striation. 

 The valves are massive. The larva is dense yellowish-brown. The hinge con- 

 sists of eight-nine lateral teeth and 15-18 small central teeth. The length of 

 the hinge is greater than the width of the umbones. The ligament is rectangu- 

 lar. Large, dark-colored eye spots appear at a shell length of 270 |im or more. 

 The larva settles at a shell length of 280-300 |J.m. 



Ecology 



Modiolus larvae are found in August-September in the plankton of Vostok 

 Bay (Peter the Great Bay). 



Larvae of this species are not described in the literature. 



MUSCULISTA SENHOUSIA (BENSON) 



(Mytilidae) 



Veliger and Veliconcha 



M. senhousia forms a clutch in the shape of twisted cords which are 

 composed of large adhesive eggs. Larvae hatch with a straight hinge margin. 

 Their later development is pelagic (Matveeva, 1975). Morton (1974) stated that 

 in the South China Sea, this species releases gametes in the water. The first 

 and most minute larvae observed by us in the plankton had a shell length of 



