Vidler’s Simnia Shell, Simnia arcuata

Vidler Simnia Shell, Simnia arcuata. Size: 2.2 cm (0.9 inches) x  0.8 cm (0.3 inches). Shell collected from Islas de los Coronados, Baja California. October 1998. Collection, photograph and identification courtesy of Bob Hillis, Ivins, Utah.

Phylogeny: Vidler’s Simnia Shell, Simnia arcuata (Reeve, 1865), is a member of the Ovulidae Family of False Cowry Shells, and the subfamily Simniinae. The genus Simnia is one of forty-four genera in this family, and there are thirteen species in this genus. They are also known as the Gorgonian Spindle Shell and in Mexico as Ovúlido Arcuata. The genus name Simnia comes from the Greek word meaning “curl”. The species name arcuata is Latin for “bowed”, “arched”, or “curved”. This refers to the curvature of the shell’s outline. The common name is in honor of the 19th-century Australian shell collector William M. Vidler.

Description: Vidler’s Simnia Shells are spindle-shaped and thin in construction. The exterior of the shell is sculpted with spiral striae at the anterior and posterior ends. The aperture runs the length of the shell and is curved. The lips and siphon ends are thickened. These shells vary in color including: whitish with pink blush, rose, and gray-brown. Most specimens have yellow or orange tips. The living animal varies in color. The mantle is usually orange with red spots,  tan with reddish-brown or dark brown spots, or whitish with reddish-brown or dark brown spots. The foot varies in a similar manner. The mantle has numerous, translucent papillae. Vidler’s Simnia Shells reach a maximum of 2.6 cm (1.0 inches) in length.

Habitat and Distribution: Vidler’s Simnia Shells are found attached to red gorgonians such as Leptogorgia chilensis and various Muricea species. They live at depths between  at depths between 9 m (30 feet) and 30 m (100 feet). They are a subtropical to tropical Eastern Pacific species.  Vidler’s Simnia Shells are found in all Mexican waters of the Pacific Ocean, though they may have a discontinuous distribution.

Ecology and Behavior: Vidler’s Simnia Shells are ectoparasites that feed on the polyps and flesh of their gorgonian hosts, including Leptogorgia chilensis. Their predators are poorly documented but may include fish and crabs. They are gonochoric and reproduce sexually with internal fertilization. The eggs are laid in capsules, surrounded by a jelly-like matrix, on their host gorgonian. Their engagement in any type of commensal or symbiotic relationship has not been formally documented. From a conservation perspective the Vidler’s Simnia Shell has not been formally evaluated. However, they are fairly common with a relatively wide distribution and should be considered to be of Least Concern.

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.