Species synonymy Adna is a genus of acorn barnacles that grows in association with or semi-parasitically on corals and octocorals.

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.

[3] The cyprid larvae of the barnacle settle around the rims of the stony cups of the coral where they seem unaffected by the nematocysts (stinging cells).

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.

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.