The watering pot shell Dianadema minima (Bivalvia, Anomalodesmata, Clavagellidae): re‐description and an interpretation of adventitious crypt formation

. Two small cemented, cryptic, species of Dianadema (Bivalvia: Anomalodesmata: Clavagellidae) have been recorded from shallow waters of the Indian Ocean. These are Dianadema minima and Dianadema mascarensis . A study of their shells and adventitious crypt morphologies, however, suggests that the two species are conspecific, the former name taking precedence. This re‐description of D. minima suggests that on settlement, the larva develops a juvenile shell that continues to grow until a length of ∼9.0 mm is reached, when it secretes around itself a primary calcareous crypt and cements itself ventrally into a concavity in coral rubble. Subsequently, a crown of tubules is secreted from the dorsal surface of the exposed mantle and an adventitious siphonal tube is formed, which is characteristically oriented at ∼45° from the horizontal. In the absence of preserved internal tissues and pre‐adult life history stages, there is no possibility of fully comprehending exactly how such a remarkable adventitious structure is produced. Dianadema minima does, however, provide us with another example of the remarkable adaptive radiation of the Clavagelloidea and, in particular, the pattern of convergent evolution expressed not just between D. minima (Clavagellidae) and Humphreyia strangei (Penicillidae), but also between the two families that have independently evolved “watering pot” shells and tubes.