Calmella cavolini is a nudibranch belonging to the family Flabellinidae and the order Nudibranchia. The genus name ‘Calmella‘ derives from Latin and refers to its delicate and small body structure, while ‘cavolini‘ is in honor of the Italian naturalist Filippo Cavolini, who contributed significantly to the study of marine invertebrates. This species is found in the Mediterranean Sea, especially off the coasts of Spain, Italy and southern France. Calmella cavolini inhabits shallow subtidal zones, among algae, rocks and on hydrozoans, in waters with moderate to strong currents, which facilitate dispersal and foraging.
Calmella cavolini is a small nudibranch that usually measures between 5 and 10 millimeters in length, although it can reach up to 15 millimeters. Its body is elongated and narrow, translucent white to pinkish in color, with bluish or purple tones at the tips of the cerata (dorsal appendages). The cerata are arranged in clusters along the upper body and contain branches of the digestive system, being visible through their translucent skin. These cerata can also present darker shades towards their ends, with a characteristic coloration that helps to differentiate Calmella cavolini from other similar species such as Flabellina affinis, which has a larger size, longer cerata and a more intense color. The rhinophores (sensory tentacles on the head) are smooth and paler in color, which also distinguishes them from other species that have rhinophores with papillae or rings.
Calmella cavolini is a specialized carnivore that feeds mainly on hydrozoans, small colonial invertebrates that are attached to hard substrates such as rocks or algae. It uses its radula, a tongue-like structure with chitinous teeth, to scrape and consume the polyps of the hydrozoa, from which it obtains both nutrients and the stinging cells (cnidocytes) stored in its cerata. These stinging cells are used by Calmella cavolini as a defense against predators, a mechanism known as kleptocnidia. It is therefore common to find specimens of this species on hydrozoans such as Halecium pusillum and Eudendrium racemosum, on which it feeds. Specimens of Calmella cavolini have also been found on Paramuricea clavata gorgonia.
Reproduction of Calmella cavolini is sexual, and like all nudibranchs, it is simultaneously hermaphroditic, meaning that each individual has both male and female reproductive organs. Mating involves the mutual transfer of sperm between two individuals, followed by the laying of eggs on gelatinous ribbons that adhere to substrates near their food sources. These whitish-colored ribbons may contain hundreds of tiny pinkish eggs, which develop into planktonic veliger larvae before settling to the seafloor and metamorphosing into juveniles.
A curious fact about Calmella cavolini is its ability to change color slightly depending on its diet, as pigments from the hydrozoans it consumes can influence the coloration of its cerata. This species is also notable for its gregarious behavior, often forming small groups in areas where there is an abundance of hydrozoans, which facilitates its reproduction and defense against predators. In addition, its small size and beautiful pink color make it an object of interest for divers and underwater photographers exploring the Mediterranean seabed.
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