Mexican Chiton, Onchidella binneyi
Mexican Chiton, Onchidella binneyi. Chiton collected off a rock above the water line at Km 21, Cabo Real, Baja California Sur, April 2007. A unique collection made mid-morning as they were out in plain sight, moving fairly rapidly back to shelter, which was about 10 meters away. Size: 2.5 cm (1.0 inch). Identification courtesy of Dr. Richard Brusca, Tucson, Arizona.
Phylogeny: The Mexican Chiton, Onchidella binneyi (Stearns, 1893), of the Onchidiidae Family of False Chitons, is a shell-less marine pulmonate gastropod mollusk. 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. The Onchiidae Family contains two subfamilies- Onchidellinae and Onchidiinae. The genus Onchidella is the only genus in the subfamily Onchidellinae, and it contains thirteen species. The genus name Onchidella comes from the Latinized Greek words meaning “little lump”. This species was named in honor of the American malacologist William Binney, who specialized in freshwater and terrestrial mollusks. This species is also known as the Leather Limpet.
Morphology: The Mexican Chiton is a small air breathing sea slug that has an ovate outline and a dome-like profile. They lack a shell and are pulmonate (having lungs versus gills). They have a long narrow foot, which is about 1/3 the width of the animal. Other species in the area have a wider foot. Mexican Chitons have small, well-developed eyes on retractable stalks. They are charcoal gray in color and usually covered with warts and papillae. Mexican Chitons reach a maximum of just over 2.5 cm (1.0 inches) in length.
Habitat and Distribution: During the day, Mexican Chitons are found under rocks and in cracks and crevasses where they are protected from drying sunlight. The live in the upper limits of the intertidal zone, and only venture lower at night during low tide. They lack any adaptation that allows them to clamp tightly to rocks in the surf zone. Therefore, when wave surges happen they are easily dislodged and swept into deeper water where they risk drowning. Mexican Chitons are a subtropical to tropical Eastern Pacific species. In Mexican waters they range from Bahía de los Ángeles, Baja California Sur to Guatemala. They are absent from the west coast of the Baja Peninsula.
Diet: Mexican Chitons are herbivorous grazers that feed primarily on diatoms and macro algae from the genera Porphyra and Enteromorpha.
Predation: Very little has been documented regarding the predation of Mexican Chitons. They possess a toxin, Onchidal, that provides them with a defense secretion, which seem to repel the fish and crabs that would normally prey on sea slugs.
Reproduction: Mexican Chitons 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 Interactions: Mexican Chitons are a poorly studied and understood species and there is very limited documentation of commensal, parasitic, or mutualistic relationships for this species.
Human Interactions: Mexican Chitons are of interest to medical scientists because of their toxin- Onchidal. Onchidal has been demonstrated to inhibit Staph a, indicative that it can inhibit Gram-positive bacteria. It has also been shown to be effective on lymphocytic leukemia cell growth in humans. It is used in various cancer treatments, as an anti-inflammatory, and for its cholesterol lowering effects. Otherwise, Mexican Chitons have no direct impact on human activities. From a conservation perspective they have not been formally evaluated however they are fairly common with a relatively wide distribution and should be consider to be of Least Concern.
Synonyms: Onchidella carpenteri, Onchidium carpenteri, and Oncidiella hildae.