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Flashing in the 'Disco' Clam Ctenoides ales (Finlay, 1927): Mechanisms and Behavioral Function


This dissertation investigated the ‘disco’ clam Ctenoides ales (Limidae), which is the only bivalve to have a behaviorally-mediated flashing display. Topics covered include (i) mechanisms, ultrastructure and movement that produce the flashing, (ii) the fitness value (function) of the flashing, (iii) the clams’ sensory abilities and vision, and (iv) the clams’ ecology, distribution and habitat. The flashing occurs on the clams’ mantle lip. Electron microscopy revealed two distinct tissue sides; one highly scattering side that contains dense aggregations of spheres composed of silica (white), and one highly absorbing side that does not (red). High-speed video confirmed that the two sides alternate rapidly, creating the appearance of flashing. Optical modeling suggested that the sphere’s diameter is nearly optimal for scattering light, especially at shorter wavelengths, which predominate in the ocean. This simple mechanism produces a striking optical effect. Three potential hypotheses for the fitness value of the flashing were investigated; conspecific attraction, prey luring, and/or predator deterrence. The lack of movement toward other C. ales when given visual cues in behavioral trials, as well as the clams’ inability to resolve flashing in other C. ales suggested conspecific attraction was not the function of the flashing. The lack of significant differences in prey abundance in experiments testing flashing versus non-flashing clams suggested prey luring was also not the function of the flashing. Predator deterrence is considered a possible function of the flashing due to (i) sulfur presence in the clam’s tissues, (ii) behavioral responses by predators during feeding trials as well as tissue preferences that suggest potential distastefulness, and (iii) the clams’ increase in flash rate when exposed to predators in the lab and the field. The presence of photosensitive pigments (rhodopsin, tubulin and retinochrome) was suggested from immunohistochemistry results in the ~40 eyes of C. ales in collaboration with A. Nahm-Kingston (University of Maryland Baltimore County). Transmission electron microscopy done in collaboration with R. Dubielzig, L. Teixeira, and C. Schobert (University of Iowa Veterinary School) confirmed that the morphology of the eye was inconsistent with image formation or the ability to resolve flashing in conspecifics. Therefore, the visual capability of C. ales is most likely used for predator detection. The clams’ distribution, depth, main habitats, projected sex ratios, movement based on size (sex) and sphere properties with depth were identified through SCUBA research at four field sites. The clams are found throughout the Indo-Pacific, from depths 3m to ≥50m. Their movement doesn’t vary with sex, and they exhibit a clumped distribution seemingly skewed towards males, which compounds their vulnerability in aquaria collections and highlights the need to determine their conservation status.

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