The infaunal Clavagellacea (Anomalodesmata) are among the weirdest bivalves, and include the “watering pot” shells, with a minute true shell embedded in an adventitious tube. This paper describes the formation and function of the tube of Brechites vaginiferus and is correlated with an investigation of shell structure and anatomy.
Because of extensive mantle fusion, the body is entirely covered by periostracum. The only apertures to the exterior are the pedal gape and the siphonal orifices (a small fourth pallial aperture progressively closes up). Glands lining the pedal gape secrete the basal plate of the tube. Perforations in it (the plate tubules and lateral tubules) are caused by papillae locally inhibiting the laying down of secretion. The siphonal part of the tube is secreted by glands discharging from the siphonal orifices (middle folds). The secretion presumably pours down the maximally expanded siphons, which compact the burrow wall and against which the tube is laid down, sand grains adhering to it. Fusion with the concave basal plate is at the margin of the pedal disc forming a fringe of fused tubules around it. Radial mantle glands in the siphonal orifices (again middle fold) produce a secretion that glues sand grains to the periostracum around the siphonal tips, for camouflage.
The tube is secreted when an otherwise naked, infaunal juvenile reaches a mature size. The tube is produced only once so that further enlargement is impossible, except by addition of “plaited ruffles” at the siphonal end, as the siphons elongate and as the tube sink's into the sand. New tube material is laid down inside and beyond the existing tube opening. Similarly, the basal plate is internally strengthened by further secretion from the pedal gape. Progressively, lateral tubules and the orifice in the plate corresponding to the pedal gape are occluded; other disc tubules remain open.
A juvenile clavagellid, possibly that of Humphreyia strangei but more likely a species of Brechites, has been examined. In most respects it is a typical bivalve. The true shell has an external ligament with a lithodesma and both adductor muscles are present. Visceral mass dimensions are typical, with a digging foot. The ctenidia are greatly enlarged within the expanded mantle cavity, mostly accounted for by long siphons, with a fourth pallial aperture mid-ventrally.
The adult Brechites vaginiferus is functionally “amyarian” with only minute anterior pedal retractor muscles. The foot is reduced and strap-like. The body is minute in relation to the tube, with only the enormously elongated ctenidia extending to the base of the siphonal orifices. Most modifications concern the mantle (as noted above). In nearly all other respects the animal is simplified in recognition of its aberrant mode of life.
The function of the tube of Brechites is discussed. A protective role is self-evident but burrowing can be achieved by water being expelled from the basal plate. The same action may be used to take in nutrients from the subsurface deposits. The relationships of Brechites and the Clavagellacea are discussed. Affinity is with the passive burrowing Laternulidae and origins lie in the Pholadomyacea.