PHOTOSYMBIOSIS IN HEART COCKLES: DO MICROSTRUCTURAL WINDOWS VARY ACROSS GENUS CORCULUM?

Studying the shells of a genus for which there are limited in situ photographs and few first-hand accounts of field observations is reminiscent of working with fossil taxa. A prime example is the heart cockle (Corculum cardissa), a photosymbiotic bivalve in family Cardiidae. C. cardissa has microstructural windows in its shell that condense and direct light to dinoflagellate algae living deep in its protected mantle tissues and providing significant nutrition. These windows are identified as regions of prismatic crystals spanning the thickness of the shell, interrupting the main crossed-lamellar structure. Recently, this genus was split from one species to seven based on phenotype, including shell translucency. Tested here, is the hypothesis that translucency is correlated with the presence of microstructural windows across the genus. Representatives of each species were cut perpendicular to their ribs in order to analyze intercostal space by scanning electron microscopy. Five species and one subspecies are found to have microstructural windows and corresponding translucent shells. Two species and a previously unnamed species were found to have neither microstructural windows nor translucent shells. Results suggest that these species either have adaptions for alternative mechanisms to allow light to reach algae or they do not have a symbiotic relationship. Furthermore, windowless C. roseum and the unnamed species are noted to live at depths of 7 m and 52 m, respectively, where sunlight may be more difficult to locate. Despite recorded observations being rare, windowless C. impressum and all of the translucent species have been collected in less than 1 m of water at low tide. Although there has not yet been genetic assessment of the relatedness of these species, this may suggest that there is a phylogenetic divergence between those with and without windows. While extensive field work, dissection and genetic sequencing are required to confirm these findings, implications of this study may be applied to fossil taxa. Microstructural assessment of well-preserved fossil shells could assist in the interpretation of potential photosymbiotic relationships in extinct mollusks.