BIOLOGY AND ANATOMY OF THE LIVING FOSSIL CONGERIA-KUSCERI (BIVALVIA, DREISSENIDAE) FROM SUBTERRANEAN RIVERS AND CAVES IN THE DINARIC KARST OF THE FORMER YUGOSLAVIA

The cave-dwelling Congeria kusceri is described and aspects of its uni que ancestry and biology elucidated in the context of its, for a bival ve, 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 attach ment to the walls of either permanently or periodically inundated kars t caves. Shells are sometimes covered by travertine, precipitated calc ium carbonate, and the tubes of the, similarly unique, serpulid Marifu gia cavatica. Neither species occurs outside such caves and, as a cons equence, 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-br anchial 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 cover ed by a thin (4-5 mu m), fibrous, periostracum and there are, on the s eparated middle folds around the pedal gape, external ciliary cleansin g 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 habita t containing little food and, thus, that little is rejected when colle cted. There are few sorting currents in the stomach either and the int estine 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 prism atic layers. Assuming that a growth check is formed once each year, po ssibly in late summer when water levels fall in the caves, making them accessible for investigation, an individual of, for example 12.7 mm s hell 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 marg inal thickening and incurving. The species is dioecious, i.e. separate sexes, with females brooding lecithotrophic (70 mu m) eggs in the cte nidia, the filament bases of which become highly glandular. Mature and brooding individuals occur when water levels are low in the caves, su ggesting that internal fertilization takes place at this time too, spe rm transfer being in the film of surrounding water to prevent loss. Fe rtilized eggs probably hatch either as larvae with a short planktotrop hic life or as crawl-away juveniles. Analysis of large samples suggest s that recruitment is probably low, and there is evidence that the spe cies is threatened by pollution of its habitat.