Facetotecta is a poorly known subclass of

thecostraca Thecostraca is a Class (biology), class of marine invertebrates containing over 2,200 described species. Many species have planktonic larvae which become Sessility (zoology), sessile or parasite, parasitic as adults. The most important subgroup ...

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...

s. The

adult An adult is a human or other animal that has reached full growth. In human context, the term ''adult'' has meanings associated with social and legal concepts. In contrast to a " minor", a legal adult is a person who has attained the age of major ...

forms have never been recognised, and the group is known only from its

larvae A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...

, the "y- nauplius" and "y-

cyprid A barnacle is a type of arthropod constituting the subclass 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 ...

" larvae. They are mostly found in the north

Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

neritic The neritic zone (or sublittoral zone) is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately in depth. From the point of view of marine biology it forms a relatively stable and well-illuminate ...

waters around

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

, and the

Mediterranean Basin In biogeography, the Mediterranean Basin (; also known as the Mediterranean Region or sometimes Mediterranea) is the region of lands around the Mediterranean Sea that have mostly a Mediterranean climate, with mild to cool, rainy winters and w ...

, where they also survive in brackish water.

History

The German zoologist Christian Andreas Victor Hensen first collected facetotectans from the

North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian S ...

in 1887, but assigned them to the

copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...

family Corycaeidae; later

Hans Jacob Hansen Hans Jacob Hansen (10 August 1855 – 26 June 1936) was a Danish zoologist, known for his contributions to carcinology (the study of crustacea). He was born in Bellinge and died in Gentofte. He participated on the first year of the Ingolf expedit ...

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 Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring * Life-cycle hypothesis ...

, 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 Genetic analysis is the overall process of studying and researching in fields of science that involve genetics and molecular biology. There are a number of applications that are developed from this research, and these are also considered parts of ...

using 18S ribosomal DNA reveal Facetotecta to be the

sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and t ...

to the remaining

Thecostraca Thecostraca is a Class (biology), class of marine invertebrates containing over 2,200 described species. Many species have planktonic larvae which become Sessility (zoology), sessile or parasite, parasitic as adults. The most important subgroup ...

(

Ascothoracida Ascothoracida is a small group of crustaceans, comprising around 100 species. They are found throughout the world, and are parasites on cnidarians and echinoderms. Ascothoracida was previously ranked as an order within the infraclass Cirripedia ...

and

Cirripedia A barnacle is a type of arthropod constituting the subclass 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 erosiv ...

Life cycle

Nauplius

Y-nauplii are long, with a faceted cephalic shield, from which the group derives its name. The

abdomen The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the torso. ...

is relatively long, and also ornamented. In common with other thecostracans, Facetotecta pass through five naupliar

instar An instar (, from the Latin '' īnstar'', "form", "likeness") is a developmental stage of arthropods, such as insects, between each moult (''ecdysis''), until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or ass ...

s before undergoing a single cyprid phase.

Cyprid

The presence of a distinctive

larva indicates that the Facetotecta is a member of the

. A number of species have been described on the basis of a y-cyprid alone. As in barnacles, the cyprid is

adapted In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...

to seeking a place to settle as an adult. It has

compound eye A compound eye is a visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidia, which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which disti ...

s, can

walk Walking (also known as ambulation) is one of the main gaits of terrestrial locomotion among legged animals. Walking is typically slower than running and other gaits. Walking is defined by an 'inverted pendulum' gait in which the body vaults ov ...

using its antennae, and is capable of producing an adhesive glue.

Juvenile

In 2008, a juvenile form was artificially produced by treating y-larvae with the

hormone A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required ...

20-hydroxyecdysone, which stimulated

ecdysis Ecdysis is the moulting of the cuticle in many invertebrates of the clade Ecdysozoa. Since the cuticle of these animals typically forms a largely inelastic exoskeleton, it is shed during growth and a new, larger covering is formed. The remna ...

and the transition to a new life phase. The resulting animal, named the ''

ypsigon Facetotecta is a poorly known subclass 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 Ocea ...

'', was slug-like, apparently unsegmented, and limbless.

Adults

While they have never been seen, the adult facetotectans may be

endoparasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...

s of other

animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...

s, some of which could be inhabitants of

coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. Co ...

Species

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

References

{{Taxonbar, from=Q137276 Maxillopoda Parasitic crustaceans