Cirripedia
Gooseneck
Acorn
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.
More gooseneck
Balanus