Some barnacles burrow into the shells of molluscs and dead coral, others are fully parasitic - but most live on the sea shore.
Burrowing Barnacles
There are about forty species of barnacle that burrow into a variety of substrates. They have separate sexes, but the males are small ‘parasites’ found only attached to females. This phenomenon fascinated Charles Darwin (who wrote the definitive monographs on living barnacles in 1851and 1854), and it made him start thinking about how such a way of life could come to be.
Parasitic Barnacles
Some barnacle species have become fully parasitic, usually on other crustaceans. The adults have no real body-form and only exist as root-like threads that fan out through the host’s body to feed. It is only when they reproduce that their larvae show that they are barnacles (typical barnacle nauplius or cypris larvae).
Commensal Barnacles
There is a way of life between ‘parasitic’ and ‘free-living’. Some barnacles live only attached to other animals but do them no harm – they simply ‘hitch a ride’. These commensal species are found on a variety of marine animals, but the most famous are the twenty or so species that can be found on the marine mammals. One species (the Whale Barnacle (Coronula diadema)) is found only on the Humpback Whale, and it seems likely that it uses chemical clues to find them.
Free-living Barnacles
Most of the thousand or more species of barnacle live permanently attached to rocks on the sea shore where they filter plankton from the water. There are two types:
- Stalked or Goose Barnacles - Goose Barnacles are eaten in parts of the Mediterranean (see: Spanish Barnacles – where you can also watch a short video of the dangerous business of collecting them from the Atlantic coast!). They stick their long rubbery ‘neck’ to the rocks and protect their delicate bodies with a number of tough shell-like plates.
- Acorn Barnacles - These are the familiar barnacles of the rocky shore. The adults are permanently stuck on the rock, but their larvae (like those of all barnacles) are free-swimming (see: Anatomy and animations of all stages in the life-cycle). Acorn Barnacles are hermaphrodites, but being permanently attached makes mating problematic! They have solved this by developing one of the largest penises in the animal world (rivaled only by certain banana slugs). This amazing organ seems to creep over the rocks with a mind of its own until it locates a nearby barnacle of the same species. Once mating is over the penis is shed to become an important, and nourishing, part of the plankton.
(See also my blog on ‘The Barnacle Heart’)
Other articles by John Blatchford
