Vidler’s Simnia Mollusk and Shell, Simnia arcuata

Vidler’s Simnia, Simnia arcuata. An exceptional portfolio of underwater photographs taken in Zihuantanejo Bay, Guerrero, March 2022, courtesy of Ron Woheau, Zihuantanejo.

Vidler’s Simnia, Simnia arcuata. An exceptional portfolio of underwater photographs taken in Zihuantanejo Bay, Guerrero, March 2022, courtesy of Ron Woheau, Zihuantanejo.

Phylogeny:  Vidler’s Simnia, Simnia arcuata (Reeve, 1865), is a species of sea snail that is a member of the Ovulidae Family of False Cowrie Shells. The genus Simnia is one of forty-four genera in the Ovulidae Family, and there are thirteen species in the Simnia Genus. They are known in Mexico as Ovúliudo Aruata.

Morphology: Vidler’s Simnias are soft-bodied gastropods. As  gastropods they have a  muscular foot, two upper, and two lower tentacles, eyes located at the tips of the upper tentacles, and a mouth with special rasp-like feeding organ called the radula. The radula is like a file, composed of many tiny teeth. Their colorful mantle completely covers their shell. The mantle is papillated and varies in color from red to orange to white, with contrasting red, orange or white papillae. This color pattern closely resembles the color pattern of the host species that they feed on, the Red Gorgonian, Leptogorgia chilensis. This phenomenon is known as “alimentary homochromy”,  whereby they obtain their color via feeding on the host. This affords them with a high level of camouflage that helps them avoid predation. The shell has an elongated spindle shape with an elongated aperture. The aperture has thick lips, which are lighter in color than the rest of the shell. It has an anterior and posterior siphon notch, and it lacks an operculum. The exterior is smooth and shiny. The shell may be whitish, rose, or gray-brown in color. Vidler’s Simnias reach a maximum of 2.6 cm (1.0 inches) in length.

Habitat and Distribution: Vidler’s Simnia is a benthic species that is found under rocks, in crevices or on rock walls, from the lower intertidal zone to depths up to 18 m (60 feet). They are a temperate to tropical Eastern Pacific species with their range in Mexican waters not well documented and they may have a discontinuous distribution. They are found in all Mexican waters of the Pacific Ocean with the excpetion that they are absent from north of Isla San Martin, Baja California, along the northwest coast of the Baja Peninsula.

Diet: Vidler’s Simnias live on the Red Gorgonian, Leptogorgia chilensis and feed on its  polyps. They anchor to the gorgonian by a long and narrow foot. They are considered to be ectoparasites.

Reproduction: Vidler’s Simnias are gonochoric (male or female for life). Reproduction is sexual, through broadcast spawning. The eggs are laid at the base of their host gorgonian. The embryos develop into planktonic trocophore larvae and later into veliger larvae before becoming fully grown adults.

Ecosystem Interactions: Vidler’s Simnias are poorly studied and very little has been documented about their behavioral patterns and ecology.

Human Interactions: Vidler’s Simnias are occasionally collected by shell collectors, otherwise they have very limited impact on human activities. From a conservation perspective the Vidler’s Simnia has not been formally evaluated.

Synonyms: Delonovolva aequalis vidleri, Neosimnia arcuata, Neosimnia quaylei, Neosimnia tyrianthina, Neosimnia vidleri, Neosimnia vidleri tyrianthina, Ovulum arcuatum, Ovulum neglectum, Ovulum vidleri, Pseudocyphoma gibbulum, Simnia vidleri, and Spiculata advena.