Calliactis parasitica

Calliactis parasitica is a species of anemone in the family Hormathiidae, which includes 26 genera with a total of 113 species. It is a parasitic anemone that lives between the surface and 60 meters deep in both the Atlantic Ocean and the Mediterranean Sea.

With a wider base than the rest of the trunk, Calliactis parasitica can reach up to 10 cm in height and 8 cm in width. The color of the trunk is generally cream-colored with various reddish and/or grayish vertical spots and streaks. The basal disc allows them to firmly attach to the substrate. In the most apical part of the specimen we find the mouth located in the center, surrounded by about 700 thin and moderately long tentacles that are yellow or translucent orange with some reddish and brown longitudinal lines. These tentacles are arranged in several rows.

Although occasionally we can find isolated specimens of Calliactis parasitica, the most common is to find it attached to the shells of hermit crabs, especially with Dardanus calidus or Dardanus arrosor, although it can also be associated with other hermit crabs of the genus Pagurus. It is common to find more than one specimen of this anemone in the shells of hermit crabs. This relationship it establishes with the hermit crabs is a mutualistic relationship, in which both species benefit. On the one hand, the hermit crab obtains protection from predators due to the presence of the anemone's stinging cells. On the other hand, the anemone receives in return an advantage in accessing a wider distribution of food sources, since the crab moves along the ocean floor.

A compound called Calitoxin was isolated from this anemone species. It is a neurotoxin that causes neuromuscular paralysis. This toxin, present in the stinging cells located in the tentacles of the anemone, is used to capture and immobilize its prey, mainly microorganisms such as zooplankton, although occasionally it can also capture organic particles.