### Identification Look for a tiny, rose-tinted "volcano" perched precariously on the side of a stony coral. *Adna anglica*, often called the English Cup-coral Barnacle, is a small, sessile cirripede reaching only about 6mm in diameter. Its shell is a beautiful pale pink to deep mauve, forming a steep-sided cone. Unlike the common barnacles of the intertidal zone, *Adna* is almost exclusively found growing on the Devonshire cup coral (*Caryophyllia smithii*). To distinguish it from similar species, look for its distinctive opercular valves—the "trapdoor" at the top—which are recessed and often slightly overgrown by the host coral’s own skeletal material, making the barnacle look like a natural extension of the coral itself.
### Habitat & Range This specialist is found in the cool, temperate waters of the Northeast Atlantic, ranging from the British Isles down to the Mediterranean. You won’t find them on the shore; they are strictly sublittoral, dwelling at depths of 10 to 100 meters. They favor rocky reefs and vertical faces where their host corals thrive, particularly in areas with moderate to strong currents that bring a steady supply of nutrients.
### Behaviour *Adna anglica* is a master of the "long-term hitchhike." Once the microscopic larva settles on a host coral, it becomes a permanent resident. An observer with a macro lens might see its delicate, feathery appendages—called cirri—rhythmically sweeping the water column. Unlike many parasites, it doesn't appear to harm the coral; instead, it utilizes the coral’s elevated position to reach better food currents, while the coral’s stinging tentacles may provide the barnacle with a degree of protection from predators.
### Diet Like its cousins on the rocks, *Adna* is a specialized filter feeder. It extends its cirri to "comb" the passing current for tiny zooplankton and organic detritus. It is a passive hunter, relying entirely on the movement of the tide to deliver its meals.
### Fascinating Fact The relationship between this barnacle and its host is so intimate that *Adna anglica* actually manipulates the coral's biology! As the barnacle grows, it stimulates the coral to deposit calcium carbonate around its base, effectively "anchoring" the barnacle into the coral’s own skeleton. It is a rare example of a crustacean forcing a cnidarian to build its home for it.