''Adna'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

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

coral Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...

s and

octocorals Octocorallia, along with Hexacorallia, is one of the two extant classes of Anthozoa. It comprises over 3,000 species of marine and brackish animals consisting of colonial polyps with 8-fold symmetry, commonly referred informally as "soft cora ...

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

intertidal zone The intertidal zone or foreshore is the area above water level at low tide and underwater at high tide; in other words, it is the part of the littoral zone within the tidal range. This area can include several types of habitats with various ...

on the coasts of

northwestern Europe Northwestern Europe, or Northwest Europe, is a loosely defined subregion of Europe, overlapping Northern and Western Europe. The term is used in geographic, history, and military contexts. Geographic definitions Geographically, Northwestern ...

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 east by the Levant in West Asia, on the north by Anatolia in West Asia and Southern Eur ...

Description

''Adna anglica'' 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 ''A. anglica'' seems to be the only barnacle species adapted to live in close proximity to the coral's tentacles.

Ecology

In British waters, where it is at the northern limit of its range, ''Adna anglica'' is found in association with the solitary cup coral '' Caryophyllia smithii'', 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 (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

. 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) is a type of cell containing a large secretory organelle called a ''cnidocyst'', that can deliver a sting to other organisms as a way to capture prey and defend against predators. A cnidocyte explosively ...

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

epithelium Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...

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