Discovery and anatomy of the arenophilic system of cuspidariid clams (Bivalvia: Anomalodesmata)

Abstract

The arenophilic system of anomalodesmatan clams comprises multicellular glands, muscular papillae, and radial lines of adhesive secretion, which glue sand grains and other extraneous particles to the external surface of the shell. The presence and morphology of these organs have been established in an increasing number of taxa, but arenophilic glands were not previously known to occur in the deep-sea, carnivorous family Cuspidariidae. Dissections, histological sections and ultrastructural examination of members of the cuspidariid genera Cuspidaria and Cardiomya revealed a functional arenophilic system positioned around the siphonal apparatus. Glands are contained in muscular papillae at the posterior tip of the siphonal sheath, from where they deposit their adhesive secretion onto the shell rostrum. Discovery of an arenophilic system in cuspidariid clams adds support to the idea that the glands are a synapomorphy of Anomalodesmata, secondarily absent in only one of the major component branches (the clade joining Thraciidae, Cleidothaeridae, and Myochamidae). Moreover, similarity between the arenophilic papillae we observed on the siphonal sheath of Cuspidaria, Cardiomya, and Myonera, and literature reports on the crown of tentacles of Multitentacula is puzzling and highlights the need for further studies of the latter genus.