Reproductive biology of the brooding Antarctic lamellibranch Kidderia subquadratum Pelseneer Public Deposited


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  • The reproductive biology of Kidderia (Kidderia) subquadratum (Pelseneer, 1903) was studied from collections which were made at Palmer Station, Antarctica during 1970 and 1971. Kidderia subquadratum is a small lamellibranch (the largest observed was 6.8 mm long) found on rocky substrates in the intertidal and subtidal zones of the Antarctic Peninsula. The sexes are separate and the females comprised 54.1% of the population studied. The differential in the sex ratio is neither size nor age related. The females are ovoviviparous and retain their embryos in brood pouches of the demibranchs. The brood pouch is a modification of the form and function of the ancestral, molluscan ctenidia and allows the retention of embryos by the female. The eggs are large and rich in yolk. Embryogenesis is without traces of the typical marine, molluscan larvae, the trochophore and the veliger. Development of the embryo follows the normal indirect, marine lamellibranch pattern until gastrulation. This includes unequal cleavage, formation of a stereoblastula with a reduced blastocoel, and gastrulation by epiboly with slight invagination. Following gastrulation development is considerably modified and follows the normal freshwater lamellibranch pattern of direct development. This is partially attributed to the large amounts of yolk that the egg contains, to the absence of the free living trochophore and veliger larvae, and to the incidence of brood protection. No structures analogous to the trochophore and veliger larvae occur. The reproductive system is described and includes the male and female systems, and the ctenidial and siphonal systems. The female gonad is unusual in that unilaminar ovarian follicles are present. The follicles consist of simple squamous follicle epithelia and either developing oögonia or oöcytes. Oögenesis takes 15 to 19 months. Egg formation consists of the rounding-off of a single germ cell which includes a nucleolus, nucleus, and cytoplasm complex. This complex, called an oögonium, grows out from the germinal epithelium into the lumen of the ovocyst. The solitary egg formation of K. subquadratum is modified and includes a unilaminar follicle epithelium which surrounds the growing oögonium. The follicle epithelium does not appear to serve any nutritional role in egg growth. Oögonia are sloughed-off during the year and lost from the population. There is no consistent seasonal variation in the numbers of oögonia observed in the ovaries. Primary oöcytes are present in females of the population from December until July. Individual females spawn continuously for a minimum of five months. Spermiogenesis and spawning in the male precedes the development of primary oöcytes by one month. Males spawn only once or at discrete periods with periodic build-ups of spermatozoa. Spermatozoa are shed directly into the water. Fertilization takes place in the brood pouches, epibranchial chambers, or oviducts, or all three areas, of the adult female after the eggs are released from the ovary. Spermatozoa are drawn through the inhalant siphon of the female, into the mantle cavity. Entrance to the brood pouches is afforded by the gill ostia, openings between the gill filaments. The eggs may be fertilized as primary oöcytes during the germinal vesicle stage of maturation, or after the breakdown of the germinal vesicle, or during both stages. The follicular epithelium, which is persistent throughout development, may bind the embryos together and thus prevent their premature loss from the brood pouches. The brood size is dependent on the size of the female. The larger females contain the greater numbers of embryos. From one to 250 embryos were observed in individual females during the study. The brood potential, per female of the population, is 96 ± 63 individuals. There is almost a 90% loss of the initial brood potential during development. Development of the embryo can take place, at least for short periods, outside the maternal organism. The period of development of the embryos is five months. The juveniles have a mean size of 0.503 mm at liberation which occurs during the austral winter and spring. The byssus system of the embryo is active at liberation thus allowing immediate attachment to a substrate. The gill at liberation is a simple, paired structure consisting of the descending lamellae of the inner demibranchs. Sexual differentiation of the juvenile gonad occurs within approximately four months of liberation at a mean size of 0.916 mm. Gametogenesis begins in the females prior to a length of 1.333 mm or approximately at an age of nine months. Sexual maturity in the juveniles occurs between 2.00 and 2.66 mm and represents an age of approximately 18 to 22 months. There is no upper size or age limit of fertility in the males or females of the population. Active feeding in the juveniles begins two months after release from the maternal organism. The major food of K. subquadratum, a filter feeding bivalve, consists of diatoms, algal fragments, dinoflagellates, and organic detritus. The winter and spring periods of liberation are attributed to the synchronizing effect of the spring phytoplankton blooms which are light limited. Light is considered to be the primary controlling factor on reproduction of Kidderia subquadratum. Direct development is thought to have arisen in the Kidderia subquadratum population as a result of the loss of the trochophore and veliger larval stages of development which was brought about by the evolution of maternal brood protection in the population. Direct development is probably maintained in the population by the combined pressures of yolk-rich eggs, ovoviviparity, a five month period of development of the embryos, and a winter-spring liberation of the juveniles.
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