Life-history strategy, with ctenidial and pallial larval brooding, of the troglodytic ‘living fossil’Congeria kusceri(Bivalvia: Dreissenidae) from the subterranean Dinaric Alpine karst of Croatia

Abstract

Among freshwater bivalves, the brooding of embryos and larvae within the maternal ctenidia is well known. Exceptions to this generalization are the non-brooding freshwater and estuarine species of Dreissena and Mytilopsis, respectively. It was reported that the freshwater troglodytic cousin, Congeria kusceri Bole, 1962, of these dreissenids does not brood either. It is herein demonstrated that C. kusceri undergoes one reproductive cycle each year. Sexes are separate, with an early male and later female bias. A small percentage (2.14%) of individuals is hermaphroditic. The gonads mature over summer from May to November. Spawning commences in September, when females release mature oocytes into their ctenidia and inhale sperm from mature males. Here the oocytes are fertilized, and develop within interfilamentary marsupia. Ctenidial tissues glandularize, and may provide a source of maternal nutrition for the embryos. At the late prodissoconch-1 or prodissoconch-2 stage (PR2, ~220 μm), larvae are released into the infrabranchial chamber via a birth channel along the outer edge of the ventral marginal food groove of both inner demibranchs. Here, they are brooded further in mantle pouches located beneath the inhalant siphon. Subsequently, after the PR2 stage (nepioconch/dissoconch), they are released from the inhalant siphon and assume an independent life as crawling juveniles. Such juveniles may be found amongst clusters of adults. Not only is C. kusceri unique amongst the Dreissenidae in possessing the capacity to brood internally fertilized ova, but it is also exceptional amongst the Bivalvia in possessing the described methods of brooding and birth. Explanations for both lie in its troglodytic lifestyle, decadal length longevity and habitat: that of byssal attachment to the hard surfaces of underground freshwater rivers, caves, pits, and sinkholes in the Tethyan arc of the Dinaric karst. Internal fertilization of a few large yolky eggs, lecithotrophic larvae, ctenidial brooding, and secondary pallial parental care represent relatively recent, Late Miocene, evolutionary adaptations from a Tethyan lentic ancestor.