- 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
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.