Rhizocephala
| Rhizocephala | |
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| Externa (highlighted) of mature female Sacculina on a female Liocarcinus holsatus | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Subphylum: | Crustacea |
| Class: | Maxillopoda |
| Subclass: | Thecostraca |
| Infraclass: | Cirripedia |
| Superorder: |
Rhizocephala Müller, 1862 |
| Orders | |
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Rhizocephala are derived barnacles that parasitise decapod crustaceans. Their body plan is uniquely reduced in an extreme adaptation to their parasitic lifestyle, and makes their relationship to other barnacles unrecognisable in the adult form. The name Rhizocephala derives from the Greek roots (rhiza, "root") and (cephale, "head"), describing the adult female, which mostly consists of a network of thread-like extensions penetrating the body of the host.
As adults they lack appendages, segmentation, and all internal organs except gonads, a few muscles, and the remains of the nervous system. Other than the minute nauplius stages, the only distinguishable portion of a rhizocephalan body is the externa or reproductive portion of adult females.
Nauplii released from adult females swim in water for several days without taking any food (the larva has no mouth and no intestine) and transform into cypris larvae (cyprids) after several moults. In some species, for example, Thompsonia, embryos develop directly into cypris larvae before they are released from adult females. A female cypris settles on a host and metamorphoses and injects its internal cell mass into host animal. The cell mass then grows into root-like threads through the host, centering on the digestive system. This network of threads is called the interna. The female then grows a sac-like externa extruding from the abdomen of the host.
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Wikipedia