Cratena peregrina

The Cratena peregrina is a nudibranch belonging to the class Gasteropoda, order Nudibranchia and family Facelinidae. It is a very abundant species in the Mediterranean Sea and the eastern coasts of the Atlantic Ocean. It lives on rocky bottoms in crystalline and well oxygenated waters, at depths between 5 and 50 meters.

Its slender and elongated body can reach up to 5 cm in length. It has a long tail with a pointed tip. Its coloration is whitish with orange reflections, and on the dorsal part of its body it has between 8 and 10 groups of narrow and elongated appendages called cerata on each side. The color of these cerata is variable, ranging from deep red, purple, brown or blue, with the tips colored with an iridescent blue. In fact, the cerata or dorsal papillae are extensions of their digestive system, and part of the stomach gland will be visible inside them with an intense red color. In the cephalic region we find two pairs of elongated rhinophores with flattened shape and orange color. These rhinophores are sensitive organs used to detect changes in water pressure, and therefore if potential prey is approaching. Below these, we also find two labial tentacles longer than the rhinophores, which are whitish-white and transparent. Between the rhinophores and the labial tentacles, we find two orange spots.

The diet of Cratena peregrina consists of hydras of the genus Eudendrium. The stinging cells of these hydras are not digested by the nudibranch, but transported intact to the tips of the cerata, and used as a defensive weapon against potential predators.

Cratena peregrina is a simultaneous hermaphrodite species, therefore the same individual has both male and female gonads at the same time. Despite this, fertilization requires cross fertilization so two individuals will oppose each other to exchange sperm. The laying will take place on specimens of hydras of the genus Eudendrium, creating a whitish gelatinous spiral ribbons in which a row of salmon-colored eggs is introduced inside.