Facetotecta
Genus of crustaceans
From Wikipedia, the free encyclopedia
Facetotecta is a poorly known subclass of thecostracan crustaceans.[1] The adult forms have never been recognized, and the group is known only from its larvae, the "y-nauplius" and "y-cyprid" larvae.[2] The only known genus is Hansenocaris in the family Hansenocarididae. They are mostly found in the north Atlantic Ocean, neritic waters around Japan,[3] and the Mediterranean Basin, where they also survive in brackish water.[4] The adults are probably endoparasites of unknown hosts.[5]
| Facetotecta | |
|---|---|
| |
| A Y-psigon escaping from its y-cyprid | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Clade: | Pancrustacea |
| Class: | Thecostraca |
| Subclass: | Facetotecta Grygier, 1985 |
| Family: | Hansenocarididae Itô, 1985 |
| Genus: | Hansenocaris Itô, 1985 |
| Species | |
|
See text | |
History
The German zoologist Christian Andreas Victor Hensen first collected facetotectans from the North Sea in 1887 but assigned them to the copepod family Corycaeidae; later Hans Jacob Hansen named them "y-nauplia", assuming them to be the larvae of unidentified barnacles.[6] More recently, it has been suggested that, since there is a potential gap in the tantulocarid life cycle, y-larvae may be the larvae of tantulocarids. However, this would be "a very tight fit", and it is more likely that the adult forms have not yet been seen.[2] Genetic analysis using 18S ribosomal DNA reveal Facetotecta to be the sister group to the remaining Thecostraca (Ascothoracida and Cirripedia).[7]
Life cycle
Y-nauplius illustration
Y-cyprid illustration
The known larval stages of facetotectans closely resemble those of parasitic rhizocephalan barnacles, this is thought to be an example of convergent evolution rather than a common origin.[8]
Nauplius
Y-nauplii are 250–620 micrometres (0.010–0.024 in) long,[2] with a faceted cephalic shield, from which the group derives its name.[9] The abdomen is relatively long, and also ornamented.[2] In common with other thecostracans, Facetotecta pass through five naupliar instars before undergoing a single cyprid phase.[6]
Cyprid
The presence of a distinctive cyprid larva indicates that the Facetotecta is a member of the Thecostraca. A number of species have been described on the basis of a y-cyprid alone.[10] As in barnacles, the cyprid is adapted to seeking a place to settle as an adult. It has compound eyes, can walk using its antennae, and is capable of producing an adhesive glue.[11]
Juvenile
In 2008, a juvenile form was artificially produced by treating y-larvae with the hormone 20-hydroxyecdysone, which stimulated ecdysis and the transition to a new life phase. The resulting animal, named the y-psigon, was slug-like, apparently unsegmented, and limbless.[11][12]
Adults
While they have never been seen, the adult facetotectans are hypothesised to be endoparasites of unknown host animals,[5] some of which could be inhabitants of coral reefs.[13]
Species
Eleven species are currently recognised,[3] while one species which is assigned to Hansenocaris – H. hanseni (Steuer, 1905) – is of uncertain affinities:[6]
- Hansenocaris acutifrons Itô, 1985
- Hansenocaris corvinae Belmonte, 2005
- Hansenocaris furcifera Itô, 1989
- Hansenocaris itoi Kolbasov & Høeg, 2003
- Hansenocaris leucadea Belmonte, 2005
- Hansenocaris mediterranea Belmonte, 2005
- Hansenocaris pacifica Itô, 1985
- Hansenocaris papillata Kolbasov & Grygier, 2007
- Hansenocaris rostrata Itô, 1985
- Hansenocaris salentina Belmonte, 2005
- Hansenocaris tentaculata Itô, 1986