Analyses of phylogeographic and population genetic structure of marine species with large population sizes, low fecundity, larval brooding, and limited dispersal such as the New Zealand flat oyster, Ostrea chilensis Küster, 1844, permit testing of the effects of historical and contemporary processes affecting patterns of coastal genetic differentiation and diversity. Analyses of DNA sequences from two mitochondrial DNA regions (COI, Cyt-b) compiled from O. chilensis individuals from 14 localities revealed a pronounced structure with three genetic clusters [Hauraki Gulf (HG); North; South] being resolved. These results suggest that historical processes have contributed to patterns of regional separation that have in turn contributed to regional genetic differences that are still observable today. Divergence time estimates of HG from North+South (1.97 Ma) and of North from South (0.36 Ma) indicate that O. chilensis from these regions represent evolutionarily significant units that evolved separately and rapidly in the early to middle Pleistocene. Analyses revealed no obvious periods of population expansion within any of the regions. These results contribute to a better understanding of how historical factors can result in pronounced mitochondrial DNA patterns of contemporary genetic structure and show how key life-history characteristics, e.g., larval brooding, can contribute to phylogeographic structure in coastal marine invertebrates.

Aneuploidy, or presence of an abnormal number of chromosomes, has been commonly observed in marine bivalves. This phenomenon has also been shown to be negatively correlated with growth and survival rates in several species of oysters and clams. The impact of pollutants on the aneuploidy level in bivalves has also been previously demonstrated. Over the past two decades, evidence of nonrandom chromosomal loss has been demonstrated, with the identification of the missing chromosomes in aneuploid karyotypes reported in several species. The results have shown that only a few pairs were preferentially affected by the loss of one homologue chromosome in different species of oysters and clams. Recently, during the genotoxicity assessment of the marine environment in Qatar using the pearl oyster Pinctada radiata (Leach, 1814) as model and surrogate species, we observed hypodiploid and hyperdiploid karyotypes. Hyperdiploidy was more frequent, which is highly uncommon in bivalves. The identification of the extra and missing chromosomes in aneuploid cells showed that pairs 4, 5, 12, and 13 were found to be more susceptible than others in having an extra individual chromosome. The same set of pairs showed a tendency to lose one homologue chromosome. Several hypotheses on the preferential chromosomal susceptibility in bivalves, in general, are discussed.

In this paper, a taxonomic review of the bivalves of the genus Cuspidaria Nardo, 1840 from the southern southwestern Atlantic is conducted. Specimens deposited in malacological collections and samples collected onboard the R/V Puerto Deseado off Mar del Plata (∼36°S) and MPA Namuncurá/Burdwood Bank area (∼54°S), between 200 and 3,000 m depth, are the focus of this revision. The specimens were analyzed through conchological and anatomical features. The geographic and bathymetric distributions for each species are provided and possible factors determining biogeographic patterns are discussed. As a result, Cuspidaria infirma n. sp., Cuspidaria cancellata n. sp., Cuspidaria namuncura n. sp., and Cuspidaria cf. kerguelensis (Smith, 1885), are described. In addition, Cuspidaria exigua (Jeffreys, 1876), Cuspidaria bicarinata Jeffreys, 1882, Cuspidaria platensis (Smith, 1885), Cuspidaria tenella Smith, 1907, Cuspidaria infelix Thiele, 1912, and Cuspidaria barnardi Knudsen, 1970 are redescribed after the study of new specimens. Elliptic Fourier analyses were performed for each side of the shell to delimit species objectively using their shape. Results showed a clear differentiation on both valves among species. Cuspidaria bicarinata and Cuspidaria exigua, both North Atlantic species, were recorded for the first time in the southwestern Atlantic, and Cuspidaria infelix and Cuspidaria tenella, both Antarctic/sub-Antarctic species, expanded their distribution northwards. Two cluster analyses, for species and areas respectively, revealed a vertical zonation, separating species into two different groups highly corresponding to deep-sea water mass distributions.