Shell size measurements confirmed that clams from the cold Benguela were significantly
rounder, flatter and less wedge-shaped than clams from the warm Agulhas. A
genetic comparison of four D. serra populations inhabiting both regimes aimed to clarify
if populations are separate (sub-) species reflected in allelic variation. Genetic analysis
of twenty-two protein-coding loci was carried out by starch-gel electrophoresis. Populations
studied are conspecific and possess genetic variation in the range of most other
marine bivalves, which allows for potential adaptation to environmental changes. Little
to moderate genetic differentiation among sub-populations relative to the maximal differentiation
under complete fixation (FsT = 0.016 - 0.089), moderate differentiation of
individuals relative to their sub-population (Fis = 0.265 - 0.452), and comparably high
differentiation of individuals relative to the compound population (FIT = 0.300 - 0.473)
were found. The effective number of individuals exchanged between populations in
each generation is high enough (1.44 - 8.65) to counteract genetic drift. Therefore it is
proposed that observed morphological differences represent phenotypic plasticity enabling
this species to inhabit different biogeographic regions. Gene flow, balanced selective
pressure and evolutionary inertia are proposed as explanations for similarities of
the geographically most distant populations. The substantial differentiation of the two
Namibian populations indicates a potential biotic barrier and requires separate studies
of the population dynamics.