Onchidiidae Family of False Chitons

One False Chiton of the Onchidiidae Family can be found in this website:

Mexican Chiton, Onchidella binneyi. A representative of the Onchidiidae Family of False Chitons.

Phylogeny: False Chitons of the Onchidiidae Family are gastropod mollusks in the Heterobranchia subclass of “different-gilled snails”. They are in the Euthyneura infraclass,  which means they have two pairs of sensory head tentacles, with different shapes and functions. False Chitons are in the Tectipleura subterclass, Eupulmonata superorder, and the Systellommatophora order. The Onchidioidea superfamily is the only superfamily in this order and the Onchidiidae Family is the only family in this superfamily. The Onchidiidae Family contains two subfamilies, thirteen genera and around sixty-six species. The name Onchidiidae comes from the Greek word onkos, meaning swelling. This refers to the swollen callus at the lower lip of the original type species. Species in the Onchidiidae Family are also commonly called Onch Slugs, Onchidiid Slugs, and Tectipleuran Sea Slugs.

Morphology: False Chitons in this family are irregularly ovate in outline and dome-like or conical in profile. They are shell-less marine pulmonate gastropods. They are more closely related to air-breathing land and freshwater snails and slugs than they are to most other sea snails and sea slugs. They have a leathery exterior that may be smooth, bumpy, or spikey. They often have sand or other sediment attached to their backs for camouflage. They have a broad foot, similar to a limpet, but unlike a limpet, they can’t hold tightly to the substrate. Onchidiids have small, well-developed eyes on retractable stalks. They have a pneumostome at the posterior end of their body which allows them to breath air into their lungs. The lungs are a modified portion of their mantle cavity. Some species also have gill-like papillae on their backs to breath under water. Onchidiids are small to medium in size, ranging between 1.0 cm (0.4 inches ) and 7.0 cm (2.8 inches) in length.

Habitat and Distribution: Onchidiids live in marine, brackish, and terrestrial environments.  Marine and brackish water species are usually found on rocks, mangroves or mud. To avoid drying out during daylight they hide in the shade or under the mud. They form an air bubbly under the mud so they can breath while buried. They live throughout the intertidal zone. Onchidiids are found worldwide in temperate to tropical seas. Four species from the Onchidiidae Family are found in Mexican waters.

Reproduction: Onchidiids are simultaneous hermaphrodites (having both male and female reproductive organs). They reproduce sexually with internal fertilization. To mate they line up side by side, facing opposite directions, and inseminate each other. The eggs are laid in ribbon-like masses, which hatch into veliger larvae. The larvae metamorphose into juvenile slugs.

Ecosystem Roles: Onchidiids are omnivorous grazers that feed on algae, bacteria, and diatoms. Some also eat zooplankton.  While grazing they ingest sand, detritus, bits of sponge, and foraminifera. These may help grind food and aid digestion. Onchidiids have several defensive mechanisms to avoid predation. They often have cryptic coloration for camouflage and are adept at hiding in small spaces. They have a very slippery outer coating that makes them hard to grab. Lastly, some species secret a milky acidic fluid that makes them foul tasting. However they are preyed upon by shore brids, crabs, fish, flat worms and nemertean worms.