''Adna'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of acorn barnacles that grows in association with or semi-parasitically on

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...

s and

octocorals Octocorallia (also known as Alcyonaria) is a class of Anthozoa comprising around 3,000 species of water-based organisms formed of colonial polyps with 8-fold symmetry. It includes the blue coral, soft corals, sea pens, and gorgonians (sea fans ...

. Its only species is ''Adna anglica''. It is found in the

intertidal zone The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species o ...

on the coasts of

northwestern Europe Northwestern Europe, or Northwest Europe, is a loosely defined subregion of Europe, overlapping Northern and Western Europe. The region can be defined both geographically and ethnographically. Geographic definitions Geographically, Northw ...

and in the

Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ea ...

Description

''Adna anglicum'' has a pink conical shell on a cup-shaped base and can grow to a diameter of up to . There are longitudinal striations on the shell and the operculum, the lid to the opening through which the barnacle's feeding parts protrude, is depressed, so that the structure is vase-shaped. Another barnacle, '' Verruca stroemia'', is sometimes found at the foot of the coral's calcified cup, but ''M. anglicum'' seems to be the only barnacle species adapted to live in close proximity to the coral's tentacles.

Ecology

Caryophyllia smithii Stokes & Broderip, 1828

In British waters, where it is at the northern limit of its range, ''Adna anglicum'' is found in association with the solitary cup coral ''

Caryophyllia smithii ''Caryophyllia smithii'', the Devonshire cup coral, is a species of solitary coral in the family Caryophylliidae. It is native to the northeastern Atlantic Ocean, the North Sea and the Mediterranean Sea. There are shallow and deep-water forms wh ...

'', and in the Mediterranean it is often associated with the sunset cup coral (''Leptopsammia pruvoti''). However, it has been found growing on corals of at least eight different

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

. The cyprid larvae of the barnacle settle around the rims of the stony cups of the coral where they seem unaffected by the

nematocysts A cnidocyte (also known as a cnidoblast or nematocyte) is an explosive cell containing one large secretory organelle called a cnidocyst (also known as a cnida () or nematocyst) that can deliver a sting to other organisms. The presence of this c ...

(stinging cells). They become firmly attached and, except for their operculum, are overgrown by the

epithelium Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...

of the coral. The barnacles benefit from this arrangement by being in an elevated position for better harvesting of food particles and by being free of competition from other organisms that are unable to withstand the coral's stinging cells. The coral probably derives no benefit from the arrangement so the barnacle can be considered an obligate semi-parasite. It is also likely to benefit from some diffusion of nutrients from the coenosarc (living tissue) of the coral which overgrows it, another reason for considering it parasitic.

References

{{Taxonbar, from1=Q25996824, from2=Q21233261 Barnacles Crustaceans described in 1823