Barnacle
Barnacles are a subclass of arthropods, belonging to the subphylum Crustacea. They are closely related to crabs and lobsters. Barnacles are exclusively found in marine environments, particularly in shallow and tidal waters with erosive conditions. Currently, there are approximately 1,000 known species of barnacles.
These organisms are sessile, meaning they are non-mobile, and most of them are suspension feeders. However, the infraclass Rhizocephala consists of highly specialized barnacles that are parasitic on other crustaceans. Barnacles go through four larval stages, during which they are capable of active swimming.
Barnacles can be encrusters, attaching themselves temporarily to hard substrates or forming symbiotic relationships with other organisms such as whales, sea snakes, crabs, or lobsters. One common type of barnacle, known as "acorn barnacles," grows their shells directly onto the substrate. On the other hand, pedunculate barnacles, including goose barnacles, use a stalk to attach themselves.
The attachment of free-living barnacles to the substratum is facilitated by cement glands located at the base of their first pair of antennae. This effectively fixes the animal upside down. In some barnacles, the cement glands are connected to a long, muscular stalk, while in others, they are part of a flat membrane or calcified plate. The natural cement secreted by these glands is composed of complex protein bonds and other trace components like calcium. This cement has impressive adhesive properties, withstanding high pulling and sticking forces.
The body of a barnacle is surrounded by a ring of plates that resemble the carapace found in other crustaceans. This ring includes the rostrum, two lateral plates, two carinolaterals, and a carina. In sessile barnacles, the apex of the plate ring is covered by an operculum, which can be recessed into the carapace. The plates can be fused together in some species.
Within the carapace, the barnacle's body lies on its stomach, with the limbs extending downwards. The segmentation is usually indistinct, and the body is evenly divided between the head and thorax, with little or no abdomen. Adult barnacles have few appendages on their heads, including only a single, vestigial pair of antennae attached to the cement gland. The eight pairs of feathery and long limbs on the thorax are referred to as "cirri." These cirri are used to filter and move food, such as plankton, towards the mouth.
Barnacles do not possess a true heart, but a sinus located near the esophagus performs a similar function, pumping blood through a series of muscles. The blood vascular system in barnacles is minimal, and they lack gills. Instead, they absorb oxygen through their limbs and the inner membrane of their carapaces. The excretory organs in barnacles are known as maxillary glands.
The primary sense of barnacles appears to be touch, with their limb hairs being highly sensitive. They also have three photoreceptors, one median and two lateral, which can detect changes in light intensity. These photoreceptors play a role in the barnacle's shadow reflex, where a sudden decrease in light triggers the cessation of the fishing rhythm and closing of the opercular plates. However, these photoreceptors are likely only capable of sensing the difference between light and dark. The primary eye of barnacles is derived from their naupliar eye.