Caloria quatrefagesi is a species within the class Gasteropoda, belonging to the order Nudibranchia and the family Facelinidae, which includes a total of 30 different genera. One of these genera is the genus Caloria, formed by Caloria quatrefagesi together with 4 other species (C. elegans, C. guenanti, C. indica and C. rosea).
Until 2016 this species was considered to be a juvenile form of the species Caloria elegans, although it has subsequently been shown through genetic studies that it is a phylogenetically distinct species, although similar to each other. This species is found in the waters of the Atlantic Ocean, as well as in the western Mediterranean Sea. It is usually associated with rocky bottoms with abundant algae and hydrozoa on which it feeds.
The body of Caloria quatrefagesi is elongated (1-2 cm), whitish in color and has numerous protuberances called cerata on its dorsal side. These cerata are arranged in groups of about 7, with the innermost cerata being longer than the outer ones. A characteristic feature of the species belonging to the genus Caloria is that at the end of the cerata there is a black gland. In the case of Caloria quatrefagesi, these black glands correspond to the end of the digestive glands, being preceded by a more extensive region of the gland of reddish color. In the anterior part of the body we find two relatively long oral tentacles of the same whitish color as the body. Behind them are the two vertically located rhinophores, which are somewhat shorter in length than the oral tentacles. The upper two thirds of the rhinophores have olfactory structures in the form of perpendicular lamellae (this feature diferentiates Caloria quatrefagesi from the straight rinophores found in Caloria elegans). If we pay close attention, we will find at the base of the rhinophores the eyes of the specimen, which are two small dots of a slightly dark color. The rear part of the body of Caloria quatrefagesi is slender and elongated, with the dorsal part of a more intense white color than the rest.
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