Gonadal morphology and gametogenesis in the sea pen Pennatula aculeata (Anthozoa : Pennatulacea) from the Gulf of Maine

The ultrastructural features of gametogenesis are described in male and fem ale colonies of the sea pen Pennatula aculeata. Specimens were collected fo r observation and fixation at 113 to 231 m depth in the Gulf of Maine, USA, in August 1993. The species is gonochoric, and all stages of gametogenesis are observed in both male and female colonies > 45 mm in height. Gametogen esis shows several features that differ from sea anemones. The developing o ocytes and sperm cysts are completely encompassed by gastrodermally derived follicle cells, and they are released from the mesenteries into the coelen teron before they are fully differentiated. Following maturation in the coe lenteron, the eggs and intact sperm cysts are expelled through the mouths o f the autozoids during spawning. The expulsion of sperm cysts suggests that they function as primitive spermatophores, perhaps as a way of reducing sp erm dilution. Vitellogenesis results in the biosynthesis of lipid droplets which are the sole nutrient reserves in the egg. Heterosynthetic vitellogen esis is characterized by the importation of lipid precursors into the oocyt e, and there is some indirect evidence that hypertrophic follicle cells pla y a role in production, transport, and/or mediation of these precursors. Sp ermatogenesis is similar to that of other anthozoans. The spermatozoon has a cone-shaped head, a posterior nuclear fossa, a ring of lipid-like bodies in the midpiece, a prominent cytoplasmic collar surrounding the proximal fl agellum, and a single mitochondrion, but the posterior region of the sperm also contains previously undescribed concentric rings of cisternae resembli ng smooth endoplasmic reticulum.