Cerithium caeruleum

Shell of Cerithium caeruleum from Tanzania

This species is widespread in the tropical Indo-Pacific (Red Sea, Kenya, Madagascar, Mozambique, Tanzania and Western India).[1][4][5] The shell of Cerithium caeruleum is large, solid, stocky, knobby, reaching 41.5 mm length and 19.2 mm width, and comprising 8-12 angulate, nodulose whorls. Early teleoconch whorls sculptured with 2 or 3 spiral cords, numerous fine spiral lirae, and weak axial ribs. Adult teleoconch whorls angulate, sculptured with numerous spiral lirae separated by incised spiral lines, fine, colabral axial lines, and with two dominant spiral cords— small, beaded, belt-like, subsutural cord, and very large, nodulose, median cord. A median spiral cord of penultimate whorl has 8-12 large nodes and a wide sutural ramp. Nodes are sometimes spiny and axially drawn out. The suture is slightly impressed. Body whorl large, wide, elongate, with weak siphonal constriction. Body whorl microsculpture same as on adult whorls, but with 4 or 5 major nodulose, beaded, or nearly smooth, spiral cords. The aperture is ovate, a little more than one-third the shell length. Columella concave with moderate thick callus. Anterior siphonal canal is short and strongly reflected left of shell axis. Anal canal is deep and folded into weak inner denticles. Shell color grayish-blue flecked with white; nodes black, early whorls white.[2]

Radular ribbon is robust, long, about one-fifth the shell length. Rachidian tooth is squarish; basal plate is with pair of posterior lunule-shaped ridges and weak, central, posterior projection. Cutting edge of rachidian has large, triangular, pointed main cusp, flanked on either side by two, sometimes three, blunt, very small denticles. The Lateral tooth has basal plate and large central ridge with pustule on central-basal portion of ridge and long, narrow outer posterior extension. Cutting edge of lateral tooth with large triangular major cusp, one inner flanking denticle and 2 or 3 outer flanking denticles. Marginal teeth is with wide shafts, narrow bases, and curved, hooked, blade-like apices bearing long pointed tips. Inner marginal tooth with 2 or 3 inner flanking denticles and 1 or 2 outer, flanking denticles; outer marginal tooth same, but lacking outer flanking denticles.[2]

The overall shape and sculpture of C. caeruleum are much like those seen in members of the genus Clypeomorus Jousseaume, and several authors have assigned this species to Clypeomorus,[6] but the resemblance is superficial, as pointed out earlier in this monograph. Cerithium caeruleum is generally much larger than most Clypeomorus species. In size and overall morphology, it most closely resembles the sympatric species, Clypeomorus petrosa gennesi, which is also an intertidal, hard-substrate species.[2] However, Cerithium caeruleum is larger, less pupiform, lacks the whorl sculpture of three tightly beaded spiral cords, and is distinctly more angulate in outline than Clypeomorus petrosa gennesi. There are also significant differences in the radulae of these two species. There is a remarkable convergence in shell morphology between Cerithium caeruleum and the batallariid, Batillaria sordidia family Potamididae. The overall color and shell sculpture of Cerithium caeruleum are very close to those observed in Batillaria sordida.

Intraspecific morphological variation occurs in shell size and strength of sculpture. Due to the harsh intertidal environment, protoconchs are normally eroded, as is much of the shell sculpture. Some morphs are quite knobby while others may be smoother, especially on the body whorl. Immature specimens are fusiform with large, wide body whorls and resemble immature specimens of Cerithium echinatum Lamarck.[2]

This species is confined to the continental margins of the western Indian Ocean where it is found as far south as Natal and Madagascar. It also occurs on Aldabra Atoll and in the Red Sea and Persian Gulf, extending eastward to the coasts of Pakistan and western India.[2]