Darwin’s Opera House Barnacle - species of the month

Andrew Hosie's blog | Created 1 decade ago

Calantica darwini Jones & Hosie, 2009

Described and named only last year, these tiny stalked barnacles are only known from the deep water off the coast of north Western Australia and are easily overlooked due to their small size (<1 cm in height!) and their habit of attaching to the branches of deep sea corals. With a bit of imagination, a cluster of these tiny barnacles resembles the Sydney Opera House, hence their common name.

Each barnacle’s body is enclosed by a protective arrangement of hard shell plates and its muscular stalk allows it to twist into the water current so that it can feed more efficiently on the minute planktonic organisms by extending its long, feathery legs, or cirri, out of its shell.

Unlike the more typical crustaceans such as crabs and shrimp, barnacles are sedentary, stuck to rocks and other hard substrates as adults and unable to move about. However, like other active crustaceans, they are still able to reproduce through internal fertilisation.

Most other sedentary animals, like corals, sponges and mussels, typically release eggs and sperm into the water column, where the meeting and union of the right type of sperm and egg is very chancy. Barnacles have solved this wastage problem through their most famous appendage - an extremely long muscular penis. This may be many times their own body length and with it the barnacles can reach out to contact and fertilise their neighbours.

Most species of barnacles are hermaphrodites, each individual having both male and female reproductive organs. However, Darwin’s Opera House barnacles are much less well endowed than are some of their relatives, each having a penis of only about half of its short body length. The species gets around this seemingly unfortunate limitation by having some individuals that are strictly male. These males are very small, being only about 1/5 the size of the hermaphrodites. They live attached, not to the coral, but to the bases of the shells of “normal” Opera House barnacles. From those vantage points these tiny males can reach with their very short penises past their hosts’ shelly plates to the female parts of their hosts’ reproductive systems.

This unusual species, with the problems of its reproduction solved through the evolution of a completely novel social organisation, was named after Charles Darwin in 2009 on the 150th anniversary of the publication of The Origin of Species by Natural Selection, and has been nominated in the top ten new species of 2009 (http://www.bushblitz.org.au/topten.php).