| Gooseneck barnacle from the Ross Sea, Antarctica. Image by Karen Osborn of specimen collected by James Barry. |  |
There are approximately 1000 species belonging to four groups within the Cirripedia. They are the acorn and gooseneck barnacles (Thoracica), the tiny Acrothoracica, and two parasitic groups, the Rhizocephala and Ascothoracica. Charles Darwin spent much time studying barnacles and wrote four important monographs in the 1850's on living and fossil cirriped groups. These works are still referenced today because of his careful observations and the meticulus illustrations.
Tiny Acrothoracica burrow into calcareous substrates such as mollusk shells or coral skeleton. Because they burrow into hard substrates, they only have to build their own calcareous plates to cover the opening of their burrow. Acorn barnacles on the other hand, secrete calcareous plates to completely cover their bodies. These plates protect the animal from many predators and from drying out when they are exposed to the air during low tides. Acorn barnacles (Thoracica) are the rough white barnacles seen on rocks in the intertidal all over the world. Members of the Thoracica are found many other places besides intertidal rocks. A giant acorn barnacle the size of a softball is found in the depths of the Southern Ocean off of Antarctica. Some species are specialize for life attached to floating material such as buoys, logs, or seaweed. Other species attach only to whales, and still other species attach only to the species that attaches to the whales. Both the Thoracica and Acrothoracica are suspension feeders that filter particles and small organisms out of the water with their cirri (modified thoracic appendages).
Ascothoracians are sucking ectoparasites on anthozoans (anemones and kin) and echinoderms whereas rhizocephalans are endoparasites on decapods (crabs and shrimp). Members of these parasitic groups are seldom seen by anyone other than specialists. However, if you were to examine a fresh catch of shrimp you would likely find a very noticable rhizocephalan externa on the abdomen of several of the shrimp. Click here to learn more about the bizarre life of rhizocephalans.
Because of the hard calcareous plates that protect the body of most cirripeds, there was a good likelihood they would leave fossils. Likewise there is a good fossil record for the cirripeds stretching back as far as the middle Cambrian.
Sources:
Darwin, C. 1851. A monograph on the subclass Cirripedia with figures of all the species. The Lepadidae; or , pedunculated cirripedes. Ray. Soc. Publ. London. 400 pp.
Darwin, C. 1851. A monograph on the fossil Lepadidae, or pedunculated cirripedia of Great Britain. Palaeontograph. Soc., Mon. 88 pp.
Darwin, C. 1854. A monograph on the subclass Cirripedia, with figures of all the species. Ray. Soc. Publ. London. 684 pp.
Darwin, C. 1854. A monograph on the fossil Palanidae and Verrucidae of Great Britain. Palaeontograph. Soc., Mon., 44 pp.
