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Rhizocephala are degenerate crustaceans, parasitic on decapod crustaceans.
Morphology
Rhizocephalans are derived from barnacles, a relationship which is unrecognisable from the adult forms, but can be seen by comparison of the larvae. As adults they lack appendages, segmentation, and all internal organs except gonads and the remains of the nervous system. Other than the minute naupliar stages, the only distinguishable portion of a rhizocephalan body is the externa or reproductive portion.
The name "Rhizocephala" means "root-heads" and describes the adult form, which consists of a network of threads penetrating the body of the host that resemble the root of a plant penetrating the soil.
Life cycle
A female nauplius settles on a host and metamorphoses as it penetrates the internal portion of the animal. It then ramifies, or grows in a similar manner to a root system, through the host, centering on the digestive system. Once mature, the female produces a sac-like externa on the abdomen of the host. The externa is immature until a male nauplius settles on it and fuses with it. The externa then produces two types of eggs: small ones that will become females and large ones that become males. Because the externa is located in the same location as the host's egg sac would be, the host treats it as if it were its own egg sac, and never molts again (crustaceans do not molt until they release their eggs or young from the brood pouch). This behaviour even extends to male hosts, which would never have carried eggs or young in a brood pouch, but care for the externa in the same way as females.
Classification
This article follows Martin and Davis in placing Rhizocephala as a superorder of Cirripedia and in the following classification of rhizocephalans down to the level of families:
Superorder Rhizocephala Müller, 1862
- Order Kentrogonida Delage, 1884
- Lernaeodiscidae Boschma, 1928
- Peltogastridae Lilljeborg, 1860
- Sacculinidae Lilljeborg, 1860, such as Sacculina
- Order Akentrogonida Häfele, 1911
- Chthamalophilidae Bocquet-Védrine, 1961
- Clistosaccidae Boschma, 1928
- Duplorbidae Høeg & Rybakov, 1992
- Mycetomorphidae Høeg & Rybakov, 1992
- Polysaccidae Lützen & Takahashi, 1996
- Thompsoniidae Høeg & Rybakov, 1992, such as Thompsonia
References
"Rhizocephala"