Maria’s Tegula Shell, Agathistoma mariana

Maria’s Tegula Shell, Agathistoma mariana. Shell provided by the commercial fishermen of the greater Los Cabos area, Baja California Sur, April 2018. Size: 1.3 cm (0.5 inches) in diameter.

Phylogeny: Maria’s Tegula Shell, Agathistoma mariana (Dall, 1919), is a gastropod mollusk that is a member of the Tegulidae Family of Top Shells. The genus Agathistoma is one of eight genera in this family, and there are twenty-five species in this genus. They are also known as the Marianum Tegula Shell and in Mexico as Tegula de Maria.

Description: Maria’s Tegula Shells have a low profile, a circular outline, and consist of  four or five whorls. The shell’s exterior is sculpted by fine lines following the spiral with some of these lines being marked with fine nodes. The base has a waxy appearance and the umbilicus is deep. The exterior of the shell is gray to light brown in color, with reddish-brown mottling. The area around the umbilicus is green, and the interior is whitish. Maria’s Tegula Shells reach a maximum of 1.6 cm (0.6 inches) in diameter.

Habitat and Distribution: Maria’s Tegula Shells are found attached to, and under, rocks. They live in the intertidal zone and to depths up to  20 m (65 feet). They are a subtropical to tropical Eastern Pacific species that are found in all Mexican waters of the Pacific Ocean with the exception that they are absent from the west coast of the Baja Peninsula.

Ecology and Behavior:  Maria’s Tegula Shells are a poorly studied and understood species. There is very limited documentation of their diet, predation, reproduction or ecosystem interactions. Other species in Family Tegulidae are grazers, feeding on algae and detritus. Sea stars, crabs, octopuses, and shore birds are common predators for these shells. They are gonochoric and reproduce sexually, through broadcast spawning, with external fertilization.  There is no mention in the available literature of them engaging in any types of parasitic, commensal, or symbiotic relationships, however their empty shells are used by several species of hermit crabs for housing. From a conservation perspective they have not been formally evaluated however they are fairly common with a relatively wide distribution and should be consider to be of Least Concern.

Synonyms: Omphalius marianus and Tegula mariana.