Prostheceraeus giesbrechtii is a species of flatworm belonging to the order Polycladida which together with 19 other species make up the genus Prostheceraeus. This very showy flatworm inhabits the waters of the Mediterranean Sea and the eastern Atlantic Ocean, where it can be found on rocky bottoms covered by calcareous algae, in cavities, under rocks or on certain sessile organisms. The bathymetric range in which we can find specimens of Prostheceraeus giesbrechtii is no more than 50 meters.
As with all planarians, Prostheceraeus giesbrechtii is characterized by a very flattened body, slightly oval in shape and a striking coloration. The back is blue, with white longitudinal lines, as well as a thicker central line of intense yellow color. On the anterior part of the body there is a pair of blue tentacles. Individuals of this species can reach 4 cm in length and up to 1 cm in width. It is common that when we observe specimens of this species, we notice that the edges of its body are very wavy. These undulations are used by some of the specimens to swim through the water column, although we will usually observe them moving along the seabed, which they do with extraordinary speed and fluidity given their size.
The diet of Prostheceraeus giesbrechtii is very varied, finding among its food different unicellular organisms, small invertebrates such as crabs or annelids, acidians. Once they have captured their food, they wrap their bodies around it and secrete a mucus that allows them to ingest it more easily.
Prostheceraeus giesbrechtii is a hermaphrodite species with crossed reproduction, and once the sexual act is concluded, they deposit the eggs wrapped with a gelatinous layer on the stones or algae of the place.
There is frequent confusion not only with Prostheceraeus giesbrechtii, but with all flatworms in general. It is often mistaken for Prostheceraeus roseus, which may have a similar livery, only its background color is pink/blue, and it lacks the central yellow line.
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