MITRA MITRA: AN EYE-CATCHING SEASHELL WITH ITS ORANGE AND WHITE COLORS

Mitra mitra (family Mitridae) is the Latinized name for a gastropod seashell having brilliant orange spots on a white background. Some books refer to it as the “Giant Miter” shell. This heavy/solid shell, up to 18 cm in length, was named by Linnaeus in 1758. This gastropod is common today and lives mainly in the tropical (warm) shallow waters of the Indo-Pacific. It can be found also in southeast Africa (but not the Red Sea) and less commonly found in Colombia, Costa Rica, the Galapagos Islands, and Clipperton Island.

It lives predominantly on sand and rocky bottoms in intertidal to subtidal depths (about 80 m depth), but it has been also found in bathyal depths.

Apertural (front) views of a juvenile (6.2 cm length, 1.9 cm width) and an adult specimen (12.3 cm length, 3.5 cm width) of Mitra mitra [the “Episcopal Miter”]. Both most likely from the Philippines.

Abapertural (back) views of the previously shown specimens of M. Mitra. 

The shape of the M. mitra shell is elongate. It resembles the miter headgear worn by Episcopal bishops, hence, the name “Episcopal Miter” was given to this shell by early workers. The shell is smooth, except for three “folds” on the anterior inner lip side of the aperture. The anterior end of the shell is twisted (i.e., it has a siphonal fasciole).

During life, M. mitra ploughs the front of its heavy shell into sand in search of Sipuncula worms (also called bristleworms). This gastropod is characterized by having a very long tube, called the epiproboscis, that can be extended from its mouth into the sand. This tube can extend for nearly the length of the Mitra shell! The mouth of M. mitra has distinctive radula teeth (tiny teeth arranged like a zipper) used for eating. 

The systematics (classification) of marine gastropods have been revised drastically  in recent years by molecular phylogenetic studies that have now become the standard way for studying all living mollusks. In addition, information about their radula has also been used. Fedosov et al. (2018) published the first comprehensive molecular phylogenetic study combined with the radulae data of mitrids. They found that of the 402 so-called species of extant (living) mitrids reported in all the published literature before 2018, only 103 are actually valid! Furthermore, they discovered that of the 32 extent genera previously reported, only 16 are valid. They concluded that contrary to previous reports that mitrids have a geologic record back to Late Cretaceous time, this record actually only began in Miocene time.

Also, these modern studies proved that the genus name Mitra can be applied only to four similar looking species: Mitra mitra, M. stictica (figured below), M. papilis, and M. deprofundis. These four species share the same kind of radula, as well as very similar shell morphology and shell-color pattern. Previous studies had lumped the shell of Mitra mitra into a very large group with somewhat similar looking shells.

Apertural and abapertural views of Mitra stictica (4.3 cm height, 1.4 cm width), the “Pontifical Miter”]. Specimen most likely from the Philippines.

Reference Cited:

Fedosov, A, et al. 2018. The collapse of Mitra: molecular systematics and morphology of the Mitridae (Gastropoda: Neogastropoda). Zoological Journal of the Linnean Society, vol. 183 (issue 2): pp. 253–337. The pdf is free.