Pseudomelatoma‘s mysterious teeth
Published 27 December 2023
In many older texts, you will find members of the gastropod superfamily Conoidea referred to as the ‘Toxoglossa’. This name, meaning ‘poison tongue’, was inspired by the modification of their radula to deliver a powerful hit of venom to knock out prey. In its most derived form, the radular membrane itself is all but gone, with the enlarged, hollow marginal teeth produced only a few at a time. A single detached marginal tooth is passed forward to the end of the proboscis where it is wielded like a venomous shiv, allowing the prey to be rendered immobile before being swallowed whole. However, not all conoids use this system, and among the notable exceptions are members of the genus Pseudomelatoma (Kantor 1990).
Pseudomelatoma is a genus of medium-sized conoids, growing up to 45 mm in length, found along the coasts of California and Baja California. Shells are an elongate-fusiform shape, with a tall spire and long, narrow body whorl, ending in a short anterior canal. The whorls are sculpted with weak, broadly rounded nodes, with relatively strong axial ribs on the lower section. The shell as a whole is generally dark brown and covered by a brownish periostracum (Powell 1966).
Pseudomelatoma is one of a small group of conoid genera, together with Hormospira and Tiariturris, united by their distinctive radular structure and restricted to the central eastern Pacific coast. In these three genera, the radula possesses a broad central tooth, with a single central cusp, flanked by scythe-like marginal teeth. The shape of the marginal teeth, and the absence of an appropriate muscular sphincter at the tip of the proboscis, indicates that they are not passed forward as in other conoids. Nevertheless, the pseudomelatomines have a large venom gland, sometimes up to half as long as the entire shell. The preferred prey of Pseudomelatoma and related genera has not yet been identified but is presumably grasped by the muscular proboscis alone. Envenomation probably occurs in the anterior part of the proboscis via direct release of the venom in the predator’s saliva, and the radula is used to rip and tear the prey once it reaches the buccal cavity. Also notable is an elongation of the anterior part of the oesophagus, suggesting that the prey may be partially digested in the anterior section of the digestive tract before it reaches the stomach (Kantor 1990).
Not unreasonably, Kantor (1990) and other authors presumed that this mode of prey capture and processing was primitive for conoids. However, molecular phylogenetic studies have not supported a basal position in the conoid tree for the Pseudomelatoma group (Kantor & Puillandre 2012). This could mean that the more ‘derived’ conoid system has evolved on multiple occasions within the broader clade, or it could mean that the pseudomelatomine arrangement is actually derived. Perhaps Pseudomelatoma and its associates are specialised for a prey animal that would elude the standard toxoglossan system. Answering this question might require identifying just what that prey animal is.
Kantor, Y. I. 1990. Anatomical basis for the origin and evolution of the toxoglossan mode of feeding. Malacologia 32 (1): 3–18.
Kantor, Y. I., & N. Puillandre. 2012. Evolution of the radular apparatus in Conoidea (Gastropoda: Neogastropoda) as inferred from a molecular phylogeny. Malacologia 55 (1): 55–90.
Powell, A. W. B. 1966. The molluscan families Speightiidae and Turridae. An evaluation of the valid taxa, both recent and fossil, with lists of characteristic species. Bulletin of the Auckland Institute and Museum 5: 1–184.