Barnacles are an entirely marine group of crustaceans with bodies that have been hugely modified for their sessile way of life. Although they don’t look much like other crustaceans, the link can be seen when they feed. From inside a small fortress made of interlocking, limy plates they extend jointed limbs covered in fine hairs and actively sieve the water for food. It is these jointed legs, and their method of reproduction, involving several larval stages that give them away as crustaceans. Although barnacles are thought to be sedentary and immobile, some make epic ocean voyages as a passenger on larger, migratory animals such as whales, or oceanic flotsam such as litter, wood, pumice stone from volcanic eruptions and even invertebrates that float on the surface of the open ocean. There are forms with long, fleshy stalks (eg goose barnacles) and nearly unrecognisable parasitic forms. While they are common and abundant in easily accessible temperate habitats such as intertidal reefs, they are understood by few people. This is because we mainly observe them when they are inactive, during low tide. They are fascinating, tenacious invertebrates with the following characteristics

• 1. A hugely modified crustacean body housed in a fleshy mantle and usually strengthened with calcareous plates.
• 2. Six pairs of jointed, fringed limbs which protrude from the mantle and comb water currents for suspended food particles.
• To be permanently attachment to a solid object.
• 4. To have a free swimming,feeding nauplius larva, followed by a two-shell, non-feeding cyprid stage.This stage settles and attaches to a solid object

Acorn barnacle, goose barnacle, stalked barnacle.

Acorn barnacles have calcareous plates that completely cover the animal inside and are cemented to a firm surface. The calcareous plates are mostly white but can be pink or deep purple. The number, size and shape of the plates varies with the species. During low tide the plates fit firmly together, but when submerged the top plates can open slightly, allowing the feeding appendages to emerge. Stalked barnacles live suspended in the sea from floating objects. They have a fleshy stalk and white plates which cover the body loosely.

Barnacles live in all seas in all temperature zones between mid and low tides and on floating objects such as driftwood, buoys and ships. Some very large species of both main types live attached to whales. Goose barnacles, sometimes of great size, have been found dredged up from deep benthic regions. Some species of goose barnacles occur all around the world (cosmopolitan). Sometimes barnacles attach to other invertebrates such as limpets or other crustaceans. They can occur in huge numbers. One species of goose barnacle Lepas fascicularis secretes its own spongy bubble raft to aid in flotation. On coral reefs barnacles are found on boulders, beach rock and on floating objects.

Barnacles use their limbs fringed with cirri (hence “cirripede”) to comb the surrounding ocean currents for small particles. These limbs are swept down through the water toward the mouth where specialised appendages wipe off and sort the particles. Organisms up to 1mm. long are caught in the net of the cirri and eaten. Also eaten are very small particles (1/500mm long) such as single-celled plants and bacteria that are swept into the barnacle by the feeding currents. Barnacles capture food that is proportional to their size, that is, larger barnacles probably eat larger particles, and smaller one filter out smaller particles.

Adult barnacles would appear to be “sitting ducks” for predators, but only if the predator has the right equipment. On temperate reefs, a few species of whelks (see “Gastropods”) have become specialist predators on barnacles. They detect barnacles by “smell”, and use specialised parts of their foot to slowly drill a hole through the barnacle’s plates, and eat the flesh inside. It may take a whelk more than two days to drill through and eat a barnacle! Shore birds such as oyster catchers and gulls may attempt to break open barnacles with their beaks, but no one knows how important they are as barnacle predators. While in the plankton barnacle larvae (nauplii and cyprid stages) are eaten by other members of the plankton, and can even be filtered out and eaten by adult barnacles.

Each barnacles has both male and female organs (hermaphroditic), and can act either as a male or female, but not usually at the same time. No one knows what factors influence barnacles to act as males or females during mating. “Male” barnacles have a very long penis which is unrolled to transfer sperm to a nearby barnacle. Barnacle eggs are fertilised inside the body cavity and develop there through several stages. Most barnacles release young in the form of many nauplii larvae after about 4 months, often coinciding with periods when food in the plankton is plentiful. The nauplius larva looks nothing like an adult barnacle. It has a shield-shaped body, one eye, a spiny tail and 3 pairs of limbs with which it swims jerkily. It feeds, moults several times and then changes into a cyprid larva which reassembles a bivalve mollusc. In this stage it does not feed but eventually drifts and settles down head first on a suitable surface, and glues itself down using cement from a special gland near its head. It rapidly changes (metamorphoses) into a miniature version of the adult form with no antennae, eyes and reduced body.

Barnacles are usually found groups, which can sometimes consist of thousands of animals. Settling near other barnacles of the same species ensures that a barnacle can reproduce, as the barnacle penis can extend only about ten times its body height to fertilise another barnacle’s eggs. Barnacles are often found attached to mobile animals such as whales and turtles, or other invertebrates that can provide a suitable surface. They may benefit from this arrangement by filtering out some of the smaller particles produced by the feeding activities of their host. A few barnacles are parasitic, with weird, highly modified bodies. Hosts of parasitic barnacles include swimmer crabs

The habit of settling on timbers, concrete and even metal surfaces makes barnacles a considerable nuisance to shipping, as heavy infestations reduces a ship’s speed considerably. Vessels used to be regularly dry docked to have their bottoms scraped below the water line. These days anti-fouling paints are employed to deter larvae of barnacles and many other organisms from settling. TBT (tri- butyl tin) was one ingredient used in these paints, but its use is now restricted because it was found to be toxic to many marine organisms, and, in particular, causes oysters to become deformed.

Coronula diadema The Diadem Whale barnacle, grows to a massive 65mm. and is only found on the throat and belly of whales such as humpbacks. They appear as white encrusting blobs. The barnacles metabolism must have to adapt to the large changes in temperatures it experiences as the whale migrates. A second species of barnacle, a goose barnacle Conchoderma is often found attached to the whale barnacle. Goose barnacles acquired their common name from a weird medieval myth. According to this myth, stalked barnacles attached to floating tree trunks were the fruit of the trees themselves, and had the tree kept growing, each fruit would have developed into a sea bird called the Barnacle Goose!!