Rhizocephala
Encyclopedia
Rhizocephala are derived barnacle
s that parasitise decapod
crustacean
s. Their bauplan 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 (, "root") and (, "head"), describing the adult female, which mostly consists of a network of thread-like extensions penetrating the body of the host.
s, segmentation
, and all internal organs except gonad
s, a few muscles 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 of adult females.
Nauplius larvae released from adult females swim in water for several days without taking any foods (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. It then grows into root-like threads through the host, centring 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.
The externa remains immature until a male cypris injects its internal cells into a female's externa, where cypris cells transform into sperm
-producing cells within a pair of cypris cell receptacles which were once called the testes. The mature female's externa produces two types of eggs
: small ones, when fertilised
, become female cypris and the large ones become male cypris. A female externa produces several batches of larvae before it drops off the host. The host soon moults and a new young externa, a clonal product of interna, emerges from the host's body. In some species, such as Peltogasterella gracilis, many externae develop from a single interna by a sort of "budding".
The externa is located in the same location as the host's egg sac would be, and the host's behaviour is chemically altered: it is castrated
and never moult
s again. The host treats the externa as if it were its own egg sac. This behaviour even extends to male hosts, which would never have carried eggs, but care for the externa in the same way as females.
s which together contain 9 families
and 2 genera
which cannot be unequivocally assigned to a family:
Order Akentrogonida Häfele, 1911
Order Kentrogonida Delage, 1884
Barnacle
A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have...
s that parasitise decapod
Decapoda
The decapods or Decapoda are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most decapods are scavengers. It is estimated that the order contains nearly 15,000 species in around 2,700 genera, with...
crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s. Their bauplan 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
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
roots (, "root") and (, "head"), describing the adult female, which mostly consists of a network of thread-like extensions penetrating the body of the host.
Description and life cycle
As adults they lack appendageAppendage
In invertebrate biology, an appendage is an external body part, or natural prolongation, that protrudes from an organism's body . It is a general term that covers any of the homologous body parts that may extend from a body segment...
s, segmentation
Segmentation (biology)
Segmentation in biology refers to either a type of gastrointestinal motility or the division of some animal and plant body plans into a series of repetitive segments. This article will focus on the segmentation of animal body plans, specifically using the examples of the phyla Arthropoda,...
, and all internal organs except gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s, a few muscles and the remains of the nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
. Other than the minute naupliar stages, the only distinguishable portion of a rhizocephalan body is the externa or reproductive portion of adult females.
Nauplius larvae released from adult females swim in water for several days without taking any foods (larva has no mouth and no intestine) and transform into cypris larvae (cyprids) after several moults
Ecdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...
. In some species, for example, Thompsonia
Thompsonia (barnacle)
Thompsonia is a genus of barnacle which has evolved into an endoparasite on other crustaceans, including crabs and mantis shrimp. It spreads through the host's body as a network of threads, and produces many egg capsules which emerge through joints in the host's shell...
, 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. It then grows into root-like threads through the host, centring on the digestive system. This network of threads is called the interna. The female then grows a sac-like externa extruding from the abdomen
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
of the host.
The externa remains immature until a male cypris injects its internal cells into a female's externa, where cypris cells transform into sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
-producing cells within a pair of cypris cell receptacles which were once called the testes. The mature female's externa produces two types of eggs
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
: small ones, when fertilised
Fertilisation
Fertilisation is the fusion of gametes to produce a new organism. In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo...
, become female cypris and the large ones become male cypris. A female externa produces several batches of larvae before it drops off the host. The host soon moults and a new young externa, a clonal product of interna, emerges from the host's body. In some species, such as Peltogasterella gracilis, many externae develop from a single interna by a sort of "budding".
The externa is located in the same location as the host's egg sac would be, and the host's behaviour is chemically altered: it is castrated
Castration
Castration is any action, surgical, chemical, or otherwise, by which a male loses the functions of the testicles or a female loses the functions of the ovaries.-Humans:...
and never moult
Moult
In biology, moulting or molting , also known as sloughing, shedding, or for some species, ecdysis, is the manner in which an animal routinely casts off a part of its body , either at specific times of year, or at specific points in its life cycle.Moulting can involve the epidermis , pelage...
s again. The host treats the externa as if it were its own egg sac. This behaviour even extends to male hosts, which would never have carried eggs, but care for the externa in the same way as females.
Classification
Following the 2001 review by Martin and Davis, the Rhizocephala are ranked as a superorder and divided into two orderOrder (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
s which together contain 9 families
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
and 2 genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
which cannot be unequivocally assigned to a family:
Order Akentrogonida Häfele, 1911
- Genus Pirusaccus Lützen, 1985
- Genus Polysaccus Ho & Lutzen, 1993
- Family Chthamalophilidae Bocquet-Védrine, 1961
- Family Clistosaccidae Boschma, 1928
- Family Duplorbidae Høeg & Rybakov, 1992
- Family Mycetomorphidae Høeg & Rybakov, 1992
- Family Polysaccidae Lützen & Takahashi, 1996
- Family ThompsoniidaeThompsoniidaeThompsoniidae is a family of parasitic barnacles. They belong to the bizarre and highly apomorphic superorder Rhizocephala, and therein to the more diverse of the two orders, the Akentrogonida....
Høeg & Rybakov, 1992
Order Kentrogonida Delage, 1884
- Family LernaeodiscidaeLernaeodiscidaeLernaeodiscidae is a family of parasitic barnacles. They belong to the bizarre and highly apomorphic superorder Rhizocephala, and therein to the less diverse of the two orders, the Kentrogonida. The Lernaeodiscidae are one of the smallish families of Rhizocephala, though the only one among the...
Boschma, 1928 - Family PeltogastridaePeltogastridaePeltogastridae is a family of barnacles. They belong to the bizarre parasitic and highly apomorphic superorder Rhizocephala, and therein to the less diverse of the two orders, the Kentrogonida. The Peltogastridae are by far the largest family of Rhizocephala...
Lilljeborg, 1860 - Family SacculinidaeSacculinidaeSacculinidae is a family of barnacles. They belong to the bizarre parasitic and highly apomorphic superorder Rhizocephala, and therein to the less diverse of the two orders, the Kentrogonida...
Lilljeborg, 1860