Gonad structure and gamete morphology of the eastern South Pacific chiton Acanthopleura echinata Barnes, 1824

We examined gonad structure and gamete morphology of Acatithopleura echinata, one of the largest and most abundant chitons along the coastline of the eastern South Pacific. Observations of sections of the gonads of adult A. echinata revealed that the wall of the testis and ovary is Constituted of a muscular-connective tissue layer and that this wall is folded toward the lumen, forming numerous tissue Plates. In the testis, the germinal epithelium, Which forms a continuous covering on the plates and on the lateral and ventral walls, gives rise to layers of spermatogonia, then spermatocytes, and finally spermatids. In the ovary, the germinal epithelium is found on the lateral and ventral walls of the ovary, but not on the tissue plates, and previtellogenic and vitellogenic oocytes are attached to the tissue plates. Mature gametes accumulate in the lumen of both the testis and ovary. Numerous simple and externally ciliated blood vessels run from the dorsal aorta to the tissue plates and, in addition to supplying blood to the gonad tissues, may aid with the explosion of gametes during spawning. We used light and scanning electronic microscopy to examine gametes that were obtained frorn a spontaneous spawning and by stripping the gonads. Spawned oocytes are in the first meiotic prophase and measure 345 mum in diameter. They are tilled with green yolk, and are Surrounded by a type III chorion, with numerous cylindrical projections that expand distally. The spermatozoa are primitive and possess an 8-mum-long triangular head, with an anterior filament.