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

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 mm), ®brous, 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 identi®ed, 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 ®lament 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 ®lm 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.

Diagnostic Genetic Markers and Evolutionary Relationships among Invasive Dreissenoid and Corbiculoid Bivalves in North America: Phylogenetic Signal from Mitochondrial 16S rDNA

“…The relationships of the two widespread extant genera are examined in this study, including the freshwater Dreissena and the estuarine and marine Mytilopsis (Rosenberg and Ludyanskiy, 1994). The genus Congeria was believed to be extinct since the Miocene (Nuttall, 1990) but C. kusceri recently was described living in underground caves in the former Yugoslavia (Morton et al, 1998). Stepien and Skidmore have obtained specimens from Morton and are sequencing them to compare with the results of the present study.…”

“…The origin of Corbicula in Asia has not been determinable from allozyme (Smith et al, 1979;McLeod and Sailstad, 1980;Hillis and Patton, 1982) or morphological (Morton, 1996) data. Sequence data hold promise for elucidating the relationships of the heteromyarian dreissenoid genus Congeria, which was believed extinct since the Miocene Epoch (Nuttall, 1990), with C. kusceri recently described as living in underground caves in Slovakia (Morton et al, 1998). Congeria and Mytilopsis are hypothesized to be sister taxa (Nuttall, 1990;Morton et al, 1998), which can be tested with DNA sequence data.…”

“…Sequence data hold promise for elucidating the relationships of the heteromyarian dreissenoid genus Congeria, which was believed extinct since the Miocene Epoch (Nuttall, 1990), with C. kusceri recently described as living in underground caves in Slovakia (Morton et al, 1998). Congeria and Mytilopsis are hypothesized to be sister taxa (Nuttall, 1990;Morton et al, 1998), which can be tested with DNA sequence data. Mitochondrial 16S rDNA also may be useful for clarifying the number of species and evolutionary relationships in the genus Mytilopsis, which have been enigmatic (Morton, 1993(Morton, , 1997.…”

Population density, habitat dynamic and aerial survival of relict cave bivalves from genus Congeria in the Dinaric karst

“…Previously reported temperatures are in the higher range; Morton et al (1998) and Puljas et al (2014) reported temperature of 13.5°C, whereas Jalžić (1998) by a water level rise meaning that an influx of cold water from the surface causes them. For instance, the lowest temperature in Žira and Jama u Predolcu caves was recorded at the same time, in February 2012, and was caused by the water level rise in the whole Trebišnjica river basin.…”

“…Congeria mulaomerovici is known from only 3 localities in the Sana River basin in Bosnia and Herzegovina. The most research has been done on C. kusceri (e.g., biology and anatomy, Morton et al, 1998;phylogeny, Stepien et al, 2001; life history, Morton & Puljas, 2013; growth and longevity, Puljas et al, 2014), as some of the caves it inhabits are easily accessible and available for research throughout the year.…”