|Beschreibung:||The cockle Cerastoderma edule is a common bivalve that inhabits the marine soft-bottom intertidal along European shores. This invertebrate plays a key role in coastal food webs of the Northeast Atlantic coasts due of its high abundances. I studied cockles from 19 sites along the distribution range with the aim to describe the variation of geographic population structures on different analytical levels. Cockles from the Barents Sea to the African Atlantic coast were analysed with respect to their geographical differentiation in shell morphology, parasite load and population genetics.
Morphological characteristics were analysed for a potential geographic pattern within the European cockle C. edule. Spatial differences of eight shell characteristics within samples from 18 sites were studied. However, biogeographic groups could not be discovered due to strong local variability in shell form parameters. Environmental conditions and/or co-existing species were suspected to shape the plasticity of cockle shells at each site. At sites where the sister species C. glaucum occurred together with C. edule, high variability of shell characters was found in both species. The lack of morphological character displacement in mixed populations confined reliable species identification to the DNA level.
Since parasites often strongly influence host morphology and its life history traits, the biogeography of the trematode assemblage was studied for C. edule as first and second intermediate host. At ten European locations ranging from Norway to Portugal, 30 cockle specimens per site were sampled to detect trematode diversity and abundance. Cockles from the Scandinavian sites shared dominant trematode species (Himasthla elongata, Meiogymnophallus minutus and Renicola roscovita) with the south-western European populations. However, two geographical groups could be revealed, based on the occurrence of rare species and changes in the abundances of common trematodes. The geographic distribution of suitable hosts for closing parasite life cycles is most likely responsible for the separation of a southern and a northern community. Thus, the scatter of shell forms was not reflected in the parasite assemblages, but a similarity to warm and cold temperate biogeographic provinces could be found.
The biogeographic groups in the parasite fauna raise the question whether the intermediate host may share the same pattern in terms of subpopulations. Hence, another hidden diversity was under survey comparing the common barcoding sequence (5’ part of mitochondrial cytochrome c oxidase (COI)) for intraspecific variation of C. edule populations. Analyses revealed significant biogeographic clustering of cockle populations along the European shores. Among the 383 C. edule individuals studied, the occurrence of 79 sequence variants divided the distribution range in two large-scale geographic groups. These geographic clusters are similar to marine biogeographic provinces. The south-western group includes all cockles from Morocco to the British Isles while one Scottish population showed particular differences. The northern group comprises the Wadden Sea, Shetland Islands as well as the Norwegian Sea. Within this northern group, Skagerrak/Kattegat populations showed both regional sequence characters indicating a secondary contact zone or a glacial refuge. The Arctic population near Murmansk deviates from all others but shows some genetic connection to Norwegian populations. In contrast to studies on other marine species, higher genetic diversity was found in populations at high latitudes than in the southern populations. Northern cockle populations displayed a complex network of genetic structure, and analyses indicate a surprisingly old demographic age.
This study revealed that European cockle populations exhibit a well structured pattern along the East Atlantic shore on two different levels, bivalve population genetics and the associated trematode assemblage. Morphological plasticity was not constant throughout the range, but varied without a clear large-scale geographical pattern. With this thesis study, I contribute to the knowledge of the large scale patterns of marine organisms endowed with pelagic larvae and corroborate the upcoming hypothesis that against expection, coastal invertebrates are not panmictic on continental scale.
The extraordinary genetic pattern with indication for a northern refuge may provide a basis for further studies to clarify the influence of Pleistocene ice ages on coastal marine fauna and flora. Studies on physiological characteristics of cockles may reveal an adaptation of warm and cold temperate populations. Co-speciation of host and parasite with the possible development of subspecies may be discovered by a phylogeographic approach. A shift of biogeographic patterns is expected with global warming whereas local extinctions may reduce plasticity and diversity of coastal biota.