Tethys fimbria is a species of predatory sea slug, a nudibranch, a marine gastropod mollusk which occurs in the Mediterranean Sea and the eastern Atlantic Ocean. ICZN opinion 200 ruled that Tethys fimbria is a valid name and Tethys leporina Linnaeus, 1758 is a synonym.
Tethys fimbria is the only species in the genus Tethys, which belongs to the family Tethydidae, superfamily Tritonioidea, order Nudibranchia, class Gastropoda, phylum Mollusca and kingdom Animalia.
Tethys, and the closely related genus Melibe, have a large oral hood, used in the capture of food, and a set of prominent cerata down each side of the body. In Tethys the cerata are smooth, with a pair of small basal gills, while in Melibe the cerata are usually papillate or tuberculate, and there are no basal gills. Like all members of the family Tethydidae they lack a radula. Internally, Tethys has no stomach plates, the posterior digestive gland forms a solid mass, surrounded by the gonad, while in Melibe there are a ring of stomach plates, the digestive gland is usually diffuse, branching or broken up some way, and the gonad lies in masses beneath the digestive gland.
The length of the body of Tethys fimbria can reach up to 30 cm. Its large and broad oral hood in the frontal part of its body is fringed with sensitive papillae. The rhinophores are small, mainly hidden in the expanded collar of the sheath that surrounds each one. The large flattened cerata are easily autotomised. The animal is a translucent white except for the blackish brown spots on the cerata, and sometimes on the edge of the oral hood and the foot.
The cerata can be self-amputated as a defence mechanism when the slug is in danger. Tethys fimbria is also a good swimmer.
Within the mantle large amounts of prostaglandins are produced. Subsequently, these prostoglandins are moved to the cerata.
The habitat of Tethys fimbria is soft sandy-mud, at depths varying from 20 to 150 metres. They feed on small crustaceans, using their broad hood to capture their prey.
The photo of this pair of Tethys fimbria was taken at a depth of 30m at Il-Kantra, on Gozo’s south coast.
Photo taken by Brian Azzopardi