Invertebrates
Barnacle
Cirripedia · also called barnacles, acorn barnacle, goose barnacle
A barnacle spends about a fortnight as a swimming larva, tasting surfaces for the right one, then glues its own head down for good and builds a stone box around itself. From inside, lying on its back, it reaches its feathery legs up through the top and rakes the water for food. It cannot chase a mate, so it grew the longest penis, relative to its size, of any animal on Earth.
Overview
Barnacles are the group Cirripedia, and despite the shell they are crustaceans — relatives of crabs, shrimp, and krill — a fact settled in the 1830s from the unmistakable crustacean larva. They are filter feeders that comb plankton from the water with six pairs of curled, feathery legs (cirri), and they live cemented to rock, pilings, ships, turtles, and whales in every ocean.
Cemented for life
The cyprid larva settles head-down and secretes one of the strongest natural glues, fixing itself permanently before growing its calcareous plates. It never moves again. Because it cannot go to a mate, most barnacles are hermaphrodites that reproduce by reaching an extensible penis up to eight times their body length across to fertilise neighbours — which is why they settle in crowds.
Geese from driftwood
Medieval Europeans, not knowing where barnacle geese bred, believed the birds grew from the stalked goose barnacles found on driftwood, and because the goose was reckoned born of wood and water rather than flesh, some church authorities allowed eating it in Lent.
Status
Not Evaluated and in no danger — barnacles are among the most abundant shore animals, and several are significant invasive foulers. A heavy hull encrustation raises a ship’s fuel use sharply, and percebes (gooseneck barnacles) are an expensive, locally overharvested delicacy in Spain and Portugal.
Barnacle: common questions
Is a barnacle a mollusc like a mussel or a clam?
No, and this is the most common misconception about them. A barnacle looks like a small shelled mollusc, but it is a crustacean, in the same broad group as crabs, shrimp, lobsters, and krill. The giveaway is the larva. A barnacle hatches as a nauplius, the distinctive one-eyed, triangular, swimming larva that only crustaceans produce, and later becomes a cyprid larva before settling. The adult barnacle inside the plates is a legless-looking animal that is actually lying on its back, with its six pairs of jointed, feathery legs, homologous to the legs of a shrimp, sticking up and out through the opening to sweep for food. The shell is not a mollusc shell but a set of calcareous plates the crustacean builds around itself.
How do barnacles reproduce if they cannot move?
Most barnacles are hermaphrodites, each individual having both male and female organs, and they cross-fertilise with their neighbours using an extraordinarily long, thin, extensible penis. A barnacle unrolls this organ, which in some species is several times and up to about eight times the length of its own body, and probes into the shell openings of nearby barnacles to deposit sperm, so a barnacle needs at least one other barnacle within reach to breed, which is why they settle in crowds. The fertilised eggs are brooded inside the shell and released as nauplius larvae. A few deep-sea and isolated barnacles have separate sexes, with tiny dwarf males living attached to a large female.
What is the difference between an acorn barnacle and a goose barnacle?
They are the two main body plans within the barnacle group. An acorn barnacle sits directly on the surface, with a low, wide, volcano-shaped ring of fused plates and a lid of movable plates on top, and it is the kind that crusts intertidal rocks, boat hulls, and whales. A goose or gooseneck barnacle attaches by a long, tough, flexible, muscular stalk, with the feeding body and its plates held at the end of the stalk, and it typically hangs from driftwood, ship hulls, buoys, and floating debris in clusters. Gooseneck barnacles are the ones eaten as the delicacy percebes, and the ones tied to the medieval barnacle goose legend.
Are barnacles harmful, and how do you remove them?
Barnacles do not harm the rock, hull, or animal they attach to directly, but a heavy encrustation adds weight and drag to a boat, can foul intake pipes and equipment, and on a slow-moving or sick animal such as a manatee or an old sea turtle a large barnacle load is a burden. On boats, the standard approach is prevention with antifouling paint, plus regular haul-out and scraping or pressure-washing, since established barnacles cement so firmly that removing them takes a scraper and often leaves a mark. On wildlife, barnacles are usually left alone, as they generally drop off naturally and removal can injure the animal, and they are only cleared by rehabilitators when the load is clearly causing harm.