World Congress of Malacology Antwerp ... - Unitas Malacologica

Bahamian Islands were submerged as recently as the Eemian interglacial era (125,000 years before
present). Thus, much of the present fauna is the product of subsequent recolonizations.
The Cerion fauna of Long Island, situated along the southeastern margin of the Great Bahamas Bank,
is the most morphologically diverse within the Bahamian Archipelago. A significant portion of this
morphological diversity is concentrated along a section of the central east coast of the island, where
the entire, non-overlapping ranges of three endemic species, flanked by a broader ranging species (4
transition zones on the order of 20-100 m) are contained within a 12 km region of coastline.
We have analyzed the morphological and genetic variability of the Recent fauna of Long Island and
compared it with representative samples from throughout much of the range of this genus.
Phylogenetic hypotheses based on mitochondrial and nuclear genes sequences from multiple
individuals from each population were used to evaluate morphology-based hypotheses, and to assess
the genetic distinctiveness of neighboring populations.
In addition, we sampled Holocene representatives of Cerion throughout this 12 km region of
coastline by excavating pits and sampling from sand quarry walls and erosion cuts. These Cerion
shells were individually dated by amino acid racemization analysis, using the D-alloisoleucine/Lisoleucine
(A/I) ratio calibrated against a series of radiocarbon-dated shells. We were able to
reconstruct complete temporal sequences from about 4000 yr BP to the present from some of our
excavation sites. Very substantial changes in shell morphology are documented in the temporal
sequences constructed for most of the sites excavated. At every site, shells older than ca. 3000 yr are
substantially different from living forms. More recent changes are also evident at most sites, with
living individuals significantly larger than all but the most recent (160 years and this
longevity is similar to those of live- caught clams from the North Sea and Atlantic Ocean. Analysis of
18 O/ 16 O ratios in shell material showed clear seasonality of growth line deposition during the lowest
seawater temperatures. Reconstruction of the seawater temperatures from the oxygen isotopes at the
time of shell deposition suggest an annual temperature range of between 3.6 and 12.8°C.
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