barnacle
Barnacles are arthropods of the subclass (taxonomy), subclass Cirripedia in the subphylum Crustacean, Crustacea. They are related to crabs and lobsters, with similar Nauplius (larva), nauplius larvae. Barnacles are exclusively marine invertebra ...
mollusc
Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...
shells,
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 ...
, crinoids or hardgrounds, producing a slit-like hole in the surface known by the
trace fossil
A trace fossil, also called an ichnofossil (; ), is a fossil record of biological activity by lifeforms, but not the preserved remains of the organism itself. Trace fossils contrast with body fossils, which are the fossilized remains of part ...
name '' Rogerella''. Acrothoracicans are typically smaller than other types of barnacle, being only a few millimetres in length. Being protected by the hard surfaces into which they have bored, they have no solid carapace of plates like other barnacles but have a soft, sac-like body fixed to the surface by a
chitin
Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...
ous disc at the front of the head. They have from four to six pairs of feathery limbs, or "cirri", which they project out of their borings to catch drifting detritus for food. The mouthparts consist of mandibles, maxillules and maxillae. One pair of cirri is close to these while the others are at the other end of the body.
The cyprid larva in Lithoglyptidae and Trypetesidae have a well-developed carapace which can completely enclose the body, a developed thorax with thoracic limbs and a distinct abdomen and telson. In Cryptophialidae the cyprid larvae are reduced; the carapace is much smaller, and the thorax and the thoracic limbs are rudimentary. Unable to swim, the Cryptophialidae cyprid disperse by antennular walking.
Each individual acrothoracican is either male or female. A dwarf male is sometimes found attached to the mantle or wall of a female's burrow. The developing
larva
A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...
may omit the nauplius stage, which is always non-feeding (lecithotrophic), but always has a cyprid stage. This has
chitin
Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...
ous teeth which are used after the larva has settled to abrade the surface and commence a burrow.
Two orders, three families, 11 genera and 63 species are recognized, and many more species probably remain to be identified.