Comparative and functional anatomy of the mantle margin in ark clams and their relatives (Bivalvia: Arcoidea) supports association between morphology and life habits

Form and function of the mantle edge in Protobranchia (Mollusca: Bivalvia)

We analyzed, by optical and transmission electron microscopy, the morphology and function of the mantle edge, including the formation of the periostracum, of ten species of protobranchs. Five species from the order Nuculida, four species from the order Nuculanida and one species from the order Solemyida were studied. A second outer fold, which seems to function as a template for the internal marginal crenulations of the valves, is present in the crenulated species of Nucula. The minute non-crenulated Ennucula aegeensis shows the glandular basal cells displaced toward the periostracal groove, resembling a minute additional fold between the outer and middle folds. Intense secretion of glycocalyx, together with active uptake of particles, have been observed in the inner epithelium of the middle mantle fold and the whole epithelium of the inner mantle fold in all the studied species. Contrary to the rest of the bivalves, all the protobranchs analyzed have two basal cells involved in the formation of the external nanometric pellicle of the periostracum, a character that would support the monophyly of protobranchs. A three-layered pattern is the general rule for the periostracum in protobranchs, like for other bivalves. The presence of pouches of translucent layer inside the tanned dark layer under periostracal folds is characteristic of the species with a folded periostracum; its function is unclear but could give flexibility to the periostracum. The non-nacreous internal shell layer and the presence of translucent pouches under periostracal folds in Sarepta speciosa resemble those found in nuculanids. However, the free periostracum is rather similar to those of N. hanleyi and E. aegeensis, with a continuous vesicular layer. All the latter supports the inclusion of Sarepta in the order Nuculanida but could indicate either a basal lineage or that the translucent vesicular layer is an adaptive trait.

Hard to get, easy to lose: Evolution of mantle photoreceptor organs in bivalves (Bivalvia, Pteriomorphia)

Morphologically diverse eyes have evolved numerous times, yet little is known about how eye gain and loss is related to photic environment. The pteriomorphian bivalves (e.g., oysters, scallops, and ark clams), with a remarkable range of photoreceptor organs and ecologies, are a suitable system to investigate the association between eye evolution and ecological shifts. The present phylogenetic framework was based on amino acid sequences from transcriptome datasets and nucleotide sequences of five additional genes. In total, 197 species comprising 22 families from all five pteriomorphian orders were examined, representing the greatest taxonomic sampling to date. Morphological data were acquired for 162 species and lifestyles were compiled from the literature for all 197 species. Photoreceptor organs occur in 11 families and have arisen exclusively in epifaunal lineages, that is, living above the substrate, at least five times independently. Models for trait evolution consistently recovered higher rates of loss over gain. Transitions to crevice-dwelling habit appear associated with convergent gains of eyespots in epifaunal lineages. Once photoreceptor organs have arisen, multiple losses occurred in lineages that shift to burrowing lifestyles and deep-sea habitats. The observed patterns suggest that eye evolution in pteriomorphians might have evolved in association with light-guided behaviors, such as phototaxis, body posture, and alarm responses.

Phylogeny and anatomy of marine mussels (Bivalvia: Mytilidae) reveal convergent evolution of siphon traits

Convergent morphology is a strong indication of an adaptive trait. Marine mussels (Mytilidae) have long been studied for their ecology and economic importance. However, variation in lifestyle and phenotype also make them suitable models for studies focused on ecomorphological correlation and adaptation. The present study investigates mantle margin diversity and ecological transitions in the Mytilidae to identify macroevolutionary patterns and test for convergent evolution. A fossil-calibrated phylogenetic hypothesis of Mytilidae is inferred based on five genes for 33 species (19 genera). Morphological variation in the mantle margin is examined in 43 preserved species (25 genera) and four focal species are examined for detailed anatomy. Trait evolution is investigated by ancestral state estimation and correlation tests. Our phylogeny recovers two main clades derived from an epifaunal ancestor. Subsequently, different lineages convergently shifted to other lifestyles: semi-infaunal or boring into hard substrate. Such transitions are correlated with the development of long siphons in the posterior mantle region. Two independent origins are reconstructed for the posterior lobules on the inner fold, which are associated with intense mucociliary transport, suggesting an important cleansing role in epifaunal mussels. Our results reveal new examples of convergent morphological evolution associated with lifestyle transitions in marine mussels.

Form and function of tentacles in pteriomorphian bivalves

Tentacles are remarkable anatomical structures in invertebrates for their diversity of form and function. In bivalves, tentacular organs are commonly associated with protective, secretory, and sensory roles. However, anatomical details are available for only a few species, rendering the diversity and evolution of bivalve tentacles still obscure. In Pteriomorphia, a clade including oysters, scallops, pearl oysters, and relatives, tentacles are abundant and diverse. We investigated tentacle anatomy in the group to understand variation, infer functions, and investigate patterns in tentacle diversity. Six species from four pteriomorphian families (Ostreidae, Pinnidae, Pteriidae, and Spondylidae) were collected and thoroughly investigated with integrative microscopy techniques, including histology, scanning electron microscopy, and confocal microscopy. Tentacles can be classified as middle fold tentacles (MFT) and inner fold tentacles (IFT) according to their position with respect to the folds of the mantle margin. While MFT morphology indicates intense secretion of mucosubstances, no evidence for secretory activity was found for IFT. However, both tentacle types have appropriate ciliary distribution and length to promote mucus transportation for cleaning and lubrication. Protective and sensory functions are discussed based on different lines of evidence, including secretion, cilia distribution, musculature, and innervation. Our results support the homology of MFT and IFT only for Pterioidea and Ostreoidea, considering their morphology, the presence of ciliated receptors at the tips, and branched innervation pattern. This is in accordance with recent phylogenetic hypotheses that support the close relationship between these superfamilies. In contrast, major structural differences indicate that MFT and IFT are probably not homologous across all pteriomorphians. By applying integrative microscopy, we were able to reveal anatomical elements that are essential for the understanding of homology and function when dealing with such superficially similar structures.