Cerata

Aeolidia papillosa showing cerata

Cerata are dorsal and lateral outgrowths on the body of nudibranches. The singular is ceras, and the name is from the classical Greek word "keratos" = "horn" and is a reference to its shape.

They are a blood-filled tube which contains a duct of the digestive gland. At the tip of the ceras in most aeolid nudibranches is a sac, called the cnidosac which stores stinging nematocysts from the cnidarians (sea anemones, hydroids etc) on which they feed. Aeolids can discharge these nematocysts in their own defence. Some aeolids, such as species of Phyllodesmium which feed on soft-corals, do not have a cnidosac because the nematocysts of soft-corals are of little use in defence. Instead their cerata produce a sticky secretion at the tip of the ceras. Species of Phyllodesmium are able to break off their cerata when attacked.

Cerata come in many different shapes. As a thin-walled sac of blood it has a major function as a gill, and with a branch of the digestive gland it provides an ideal organ for the exchange of gases and molecules into, and out of, the blood system. The cnidosac also makes it an important defensive organ. Its ability to change colour also assists it in camouflage.

References

• Sea Slug Forum