Comparative anatomy of siphons in tellinoidean clams (Bivalvia, Tellinoidea)

Siphons are tubular organs formed by fusion and posterior extension of the marginal mantle folds. They are supposed to have performed key roles in the evolution of bivalves by enabling these animals to occupy several ecological niches. However, anatomical details of these organs are scarce for one of the most diverse lineages of tropical bivalves, the superfamily Tellinoidea. We investigated the siphonal morphology of 15 species, sampling five tellinoidean families, by integrating scanning electron microscopy, confocal microscopy, and histology. The siphons revealed variations in length, pigmentation, tentacles, papillae, and number of nerve cords. Due to the presence of sensorial structures, such as papillae and tentacles, we reclassify the siphons of Tellinoidea from type A to A+. Additional anatomical patterns were identified at family and genus levels. For example, the incurrent siphon shorter than the excurrent and 24 tentacles are putative synapomorphies of Donacidae. We also highlight shared siphonal traits between Donacidae and Solecurtidae as well as between Semelidae and Tellinidae. In addition, our data support the idea of Psammobiidae as a paraphyletic lineage. Overall, we provide an extensive comparative data set on siphonal traits with significant relevance for bivalve taxonomy, functional anatomy, and evolutionary investigations.