Ribbed False Limpet Shell, Siphonaria costata
Ribbed False Limpet Shell, Siphonaria costata. Shell collected off the beach at Punta Chivato, Baja California Sur, May 2025. Size: 1.7 cm (0.7 inches) x 1.1 cm (0.4 inches) x 0.4 cm (0.2 inches). Collection and photographs courtesy of Colin Campbell, DVM, Punta Chivato, Baja California Sur. Identification reconfirmed by Dr. Tracy R. White.
Phylogeny: The Ribbed False Limpet, Siphonaria costata (G.B. Sowerby I, 1835), is a gastropod mollusk that is a member of the Siphonariidae Family of False Limpet Shells. The Siphonaria Genus is one of four genera in the Siphonariidae Family, and there are one hundred forty-five species in the Siphonaria Genus. Unlike True Limpets, Ribbed False Limpets are pulmonate mollusks, indicative that they have traded their gills for primitive lung-type organs. The genus name Siphonaria comes from a Greek word that refers to the characteristic siphon (or tube) located on the right side of the animal, which connects the vascular cavity to the environment. The species name costata comes from Latin and means “ribbed” or “having ribs”. Some sources treat this species as a subspecies of the Maura False Limpet, Siphhonaria maura.
Description: The Ribbed False Limpet Shell is irregularly circular to ovate, and relatively low in profile. The apex is located just below the shell’s center, points posteriorly, and is often eroded. The exterior of the shell is sculpted with coarse, rounded, radial ribs. The ribs may continue to the margin giving some specimens a very irregular border. The exterior of the shell may be whitish to light gray, often with brown rays or blotches. The interior is brown to dark brown, with white around the margin. Ribbed False Limpet Shells reach a maximum of 2.0 cm (0.8 inches) in length.
Habitat and Distribution: Ribbed False Limpets are found attached to rocks in the intertidal zone. 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 north of Todo Santos, Baja California Sur, along the central and northwest coasts of the Baja Peninsula, and north of Guaymas, Sonora, in the Sea of Cortez.
Ecology and Behavior: Ribbed False Limpets are a poorly researched and documented species. Other similar species in this genus are grazers that feed primarily on micro algae. They are prey for gastropods, crabs, and shore birds. Ribbed False Limpets are simultaneous hermaphrodites and reproduce sexually, with indirect fertilization. The eggs are laid as a mass on the substrate. Their engagement in any type of commensal, parasitic, or symbiotic relationship has not been formally documented. From a conservation perspective they have not been formally evaluated. However, they are fairly common with a relatively wide distribution and should be considered to be of Least Concern.
Synonyms: None.