Biology and anatomy of the living fossil Congeria kusceri (Bivalvia: Dreissenidae) from subterranean rivers and caves in the Dinaric karst of the former Jugoslavia

Abstract

The cave-dwelling Congeria kusceri is described and aspects of its unique ancestry and biology elucidated in the context of its, for a bivalve, similarly unique habitat. The species lives in subterranean waters of the Dinaric karst of the former Yugoslavia. C. kusceri is acutely heteromyarian in form commensurate upon its lifestyle of byssal attachment to the walls of either permanently or periodically inundated karst caves. Shells are sometimes covered by travertine, precipitated calcium carbonate, and the tubes of the, similarly unique, serpulid Marifugia cavatica. Neither species occurs outside such caves and, as a consequence, the shell and tissues of C. kusceri are colourless and there are few sense organs, i.e. no statocysts and light receptors, although the pallial papillae possess tiny sense cells and there is a supra-branchial osphradium.

Mantle fusions are extensive and involve the inner folds posteriorly to create the siphons, but also the inner surfaces of the middle folds ventrally so that here the mantle is largely covered by a thin (4–5 μm), fibrous, periostracum and there are, on the separated middle folds around the pedal gape, external ciliary cleansing tracts. There are internal cleansing tracts too, but the former have been identified, hitherto, only in representatives of the Pectinidae. In terms of general anatomy, Congeria kusceri has large ctenidia and tiny labial palps as an adaptation to the mineral waters of its habitat containing little food and, thus, that little is rejected when collected. There are few sorting currents in the stomach either and the intestine is short.

The aragonitic shell comprises two layers, outer and inner, both with a homogeneous crossed-lamellar structure. Each shell valve, however, contains many growth checks, represented by thin prismatic layers. Assuming that a growth check is formed once each year, possibly in late summer when water levels fall in the caves, making them accessible for investigation, an individual of, for example 12.7 mm shell length, was possibly 25 years of age. If so, an individual of 25 mm would be very much older. Because, with age, the species lays down growth checks at progressively reduced increments, a lifespan of many decades may be possible. Extreme reduction in shell growth causes marginal thickening and incurving. The species is dioecious, i.e. separate sexes, with females brooding lecithotrophic (70 mm) eggs in the ctenidia, the filament bases of which become highly glandular. Mature and brooding individuals occur when water levels are low in the caves, suggesting that internal fertilization takes place at this time too, sperm transfer being in the Ælm of surrounding water to prevent loss. Fertilized eggs probably hatch either as larvae with a short planktotrophic life or as crawl-away juveniles. Analysis of large samples suggests that recruitment is probably low, and there is evidence that the species is threatened by pollution of its habitat.