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Ultrastructural studies of ovarian follicle cells and mature eggs in four sibling species in the polychaete genus Capitella have revealed distinct and consistent morphological differences that parallel in some respects the differences between the species in egg size, and embryonic and larval development. Capitella spp. I and II are extremely similar in all respects: the follicle cells lack lipid and contain a modest amount of glycogen; the mature eggs are rich in lipid and glycogen and contain very similar proteid yolk granules. In both species mature eggs have a characteristic electron-dense band and a zone of mitochondria in the cortical ooplasm. These sympatric species have eggs that are similar in size and lecithotrophic larvae that are planktonic for only a short time. Capitella sp. III (Capitella jonesi) has ovarian follicle cells containing a small amount of lipid and no glycogen, while the mature eggs have a small amount of lipid, abundant glycogen, and large proteid yolk granules. These small eggs show no evidence of an electron-dense band or any concentration of mitochondria in the cortical ooplasm. This species has planktotrophic larvae that remain in the plankton for many weeks. Capitella sp. IIIa has ovarian follicle cells rich in both lipid and glycogen. The large mature eggs are rich in lipid, have relatively little glycogen, and have abundant proteid yolk granules. The cortical ooplasm contains electron-dense material similar to that observed in the eggs of species I and II but it is distributed in a discontinuous band. This species has direct development, and juvenile worms emerge from the parental brood tube after metamorphosis. The egg envelopes and microvilli of the eggs of all four sibling species undergo substantial morphological changes following release from the ovary into the coelom. The significance of these morphological and biochemical differences between the species is not known, but the lack of intraspecific variation in these characters suggests that their presence or absence reflects specific differences in the processes of yolk formation and utilization.

ULTRASTRUCTURAL DIFFERENCES IN THE EGGS AND OVARIAN FOLLICLE CELLS OF CAPITELLA (POLYCHAETA) SIBLING SPECIES