Facetotecta

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

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

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

• 
  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

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

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

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.¹¹¹²

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

Eleven species are currently recognised,³ while one species which is assigned to Hansenocaris – H. hanseni (Steuer, 1905) – is of uncertain affinities:⁶