Graduate Thesis Or Dissertation
 

Seasonal gametogenesis and egg laying in the prosobranch gastropods Nucella lamellosa, Nucella emarginata, Searlesia dira and Amphissa columbiana on the Oregon coast

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  • A 15-month field study was made on the reproductive cycles of the prosobranch gastropods Nucella lamellosa, Nucella emarginata, Searlesia dira and Amphissa columbiana on the Oregon coast. Amphissa columbiaria and Searlesia dira were collected from Boiler Bay, Nucella emarginata from Seal Rock and Nucella lamellosa from inside Yaquina Bay. All of these species are carnivores and produce relatively few large eggs which are deposited in capsules. Each species produces a non-pelagic crawling larva whose developmental time is highly temperature dependent. The seasonal gametogenic process in females was studied by measuring oocytes in histological sections. Since these oocytes are elliptical in cross section due to packing in the ovary, the long and short axis of each oocyte was measured and converted to the diameter of a circle of equivalent area. The time required to produce an egg is approximately 20 months in Nucella lamellosa, 16 months in Searlesia dira and Amphissa columbiana and 14 months in Nucella emarginata. Daily relative growth rates for oocytes are initially low in all four species, approximately 1.6% per day, increasing two or three months prior to spawning to 5.6% in Nucella emarginata, 4.9% in Searlesia dira and 5.4% in Amphissa columbiana, but only 2.0% in Nucella lamellosa. Pre-vitellogenic oocytes begin to appear when middle-sized oocytes begin to grow rapidly to larger sizes. This rapid increase in oocyte growth rates begins in July for Amphissa columbiana and Searlesia dira and in September for Nucella emarginata and Nucella lamellosa. In males of all four species spermatogenesis continues throughout the year except for a brief period immediately after spawning. Spermatogenesis is not well correlated with environmental temperature or salinity changes, but the percentage of sperm in tubules of the testis does decrease during the spawning season. Amphissa columbiana, Searlesia dira and Nucella emarginata begin spawning in October and November at the time coastal upwelling ceases. Nucella lamellosa, found inside Yaquina Bay, does not begin spawning until June, well past the period of extremely low salinities of December and January. The oogenic cycles of Nucella lamellosa, Searlesia dira and Amphissa columbiana are adapted to maximize fecundity through the use of a restricted breeding season and a fixed-sized juvenile emerging at the time of the year when food availability for juveniles is at a maximum. Fecundity is maximized by Nucella emarginata in response to an increased food supply by repeated spawnings over a long period of time. Juvenile size at hatching varies in this species due to a constant number of food eggs and a variable number of embryos placed in each capsule.
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