Facetotecta is a poorly known infraclass of thecostracan crustaceans. The adult forms have never been recognised, and the group is known only from its larvae, the "y-nauplius" and "y-cyprid" larvae. They are mostly found in the north Atlantic Ocean, neritic waters around Japan, and the Mediterranean Basin, where they also survive in brackish water.
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. 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. Genetic analysis using 18S ribosomal DNA reveal Facetotecta to be the sister group to the remaining Thecostraca (Ascothoracida and Cirripedia).
Y-nauplii are 250–620 micrometres (0.010–0.024 in) long, with a faceted cephalic shield, from which the group derives its name. The abdomen is relatively long, and also ornamented. In common with other thecostracans, Facetotecta pass through five naupliar instars before undergoing a single cyprid phase.
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. 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.
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 ypsigon, was slug-like, apparently unsegmented, and limbless.
- 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
- Joel W. Martin; George E. Davis (2001). An Updated Classification of the Recent Crustacea (PDF). Natural History Museum of Los Angeles County. p. 132.
- Daphne Cuvelier (April 4, 2005). "Hansenocaris Itô, 1985". World Register of Marine Species.
- Genuario Belmonte (2005). "Y-nauplii (Crustacea, Thecostraca, Facetotecta) from coastal waters of the Salento Peninsula (south eastern Italy, Mediterranean Sea) with descriptions of four new species". Marine Biology Research. 1 (4): 254–266. doi:10.1080/17451000500202518.
- E. A. Ponomarenko (2006). "Facetotecta – Unsolved Riddle of Marine Biology". Russian Journal of Marine Biology. 32 (Suppl. 1): S1–S10. doi:10.1134/S1063074006070017.
- Marcos Pérez-Losada; Jens T. Høeg; Gregory A. Kolbasov; Keith A. Crandall (2002). "Reanalysis of the relationships among the Cirripedia and the Ascothoracida and the phylogenetic position of the Facetotecta (Maxillopoda: Thecostraca) using 18S rDNA sequences". Journal of Crustacean Biology. 22 (3): 661–669. doi:10.1651/0278-0372(2002)022[0661:ROTRAT]2.0.CO;2.
- Christopher Taylor (February 23, 2008). "The secret of y-larvae". Catalogue of Organisms.
- Gregory A. Kolbasov; Mark J. Grygier; Viatcheslav V. Ivanenko; Alejandro A. Vagelli (2007). "A new species of the y-larva genus Hansenocaris Itô, 1985 (Crustacea: Thecostraca: Facetotecta) from Indonesia, with a review of y-cyprids and a key to all their described species" (PDF). Raffles Bulletin of Zoology. 55 (2): 343–353.
- Gerhard Scholtz (2008). "Zoological detective stories: the case of the facetotectan crustacean life cycle". Journal of Biology. 7 (5): 16. doi:10.1186/jbiol77. PMC 2447532. PMID 18598383.
- Henrik Glenner; Jens T. Høeg; Mark J. Grygier; Yoshihisa Fujita (2008). "Induced metamorphosis in crustacean y-larvae: Towards a solution to a 100-year-old riddle". BMC Biology. 6: 21. doi:10.1186/1741-7007-6-21.
- Mark Grygier; Jens T. Høeg; Yoshihisa Fujita (July 2004). Introduction to the tremendous diversity of y-larvae (Crustacea: Maxillopoda: Thecostraca: Facetotecta) in inshore coral reef plankton at Sesoko Island, Okinawa, Japan (PDF). 10th International Coral Reef Symposium. Biodiversity and Diversification in the Indo-West Pacific. Okinawa, Japan. Archived from the original (PDF) on 2011-06-11.