Facetotecta is a poorly known subclass of

thecostraca Thecostraca is a class of marine invertebrates containing over 2,200 described species. Many species have planktonic larvae which become sessile or parasitic as adults. The most prevalent subgroup are the barnacles (subclass Cirripedia), const ...

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

s. The

adult An adult is an animal that has reached full growth. The biological definition of the word means an animal reaching sexual maturity and thus capable of reproduction. In the human context, the term ''adult'' has meanings associated with social an ...

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

larvae A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect developmental biology, development such as insects, some arachnids, amphibians, or cnidarians typical ...

, the "y- nauplius" and "y-

cyprid Barnacles are arthropods of the subclass Cirripedia in the subphylum Crustacea. They are related to crabs and lobsters, with similar nauplius larvae. Barnacles are exclusively marine invertebrates; many species live in shallow and tidal water ...

" larvae. The only known genus is ''Hansenocaris'' in the family Hansenocarididae. They are mostly found in the north

Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...

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-illuminated ...

waters around

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

, and the Mediterranean Basin, 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, Denmark, Norway, Germany, the Netherlands, Belgium, and France. A sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian Se ...

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 (living in the water column), some are benthos, benthic (living on the sedimen ...

family

Corycaeidae Corycaeidae is a family of cyclopoid copepods in the order Cyclopoida The Cyclopoida are an order of small crustaceans from the class Copepoda. Like many other copepods, members of Cyclopoida are small, planktonic animals living both in the se ...

; later

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

named them "y-nauplia", assuming them to be the larvae of unidentified

barnacle Barnacles are arthropods of the subclass (taxonomy), subclass Cirripedia in the subphylum Crustacean, Crustacea. They are related to crabs and lobsters, with similar Nauplius (larva), nauplius larvae. Barnacles are exclusively marine invertebra ...

s. 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 conception to reproduction *Life-cycle hypothesis, in economics *Erikson's stages of psy ...

, 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 ...

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 ...

to the remaining

Thecostraca Thecostraca is a class of marine invertebrates containing over 2,200 described species. Many species have planktonic larvae which become sessile or parasitic as adults. The most prevalent subgroup are the barnacles (subclass Cirripedia), const ...

(

Ascothoracida Ascothoracida is a small group of Parasitism, parasitic marine crustaceans, comprising around 100 species and divided into Dendrogastrida and Laurida. They are found throughout the world on cnidarians and echinoderms. Dendrogastrida are parasites ...

and

Cirripedia Barnacles are arthropods of the subclass Cirripedia in the subphylum Crustacea. They are related to crabs and lobsters, with similar nauplius larvae. Barnacles are exclusively marine invertebrates; many species live in shallow and tidal water ...

Life cycle

Y-nauplius.png, Y-nauplius illustration Y-cyprid.png, Y-cyprid illustration

Nauplius

Y-nauplii are long, with a faceted

cephalic shield A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the unders ...

, from which the group derives its name. The

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

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, which occurs between each moult (''ecdysis'') until sexual maturity is reached. Arthropods must shed the exoskeleton in order to ...

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

s, 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 p ...

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

compound eye A compound eye is a Eye, visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidium, ommatidia, which are tiny independent photoreception units that consist of a cornea, lens (anatomy), lens, and p ...

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 as an "inverted pendulum" gait in which the body vaults over ...

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 Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs or tissues by complex biological processes to regulate physio ...

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 remnant ...

and the transition to a new life phase. The resulting animal, named the ''y-psigon'', 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 (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The en ...

s of other

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

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 group ...

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 Facetotecta is a poorly known subclass of thecostracan crustaceans. The adult forms have never been recognized, and the group is known only from its larvae, the "y- nauplius" and "y-cyprid" larvae. The only known genus is ''Hansenocaris'' in the ...

'' Itô, 1989 *''

Hansenocaris itoi Facetotecta is a poorly known subclass of thecostracan crustaceans. The adult forms have never been recognized, and the group is known only from its larvae, the "y- nauplius" and "y-cyprid" larvae. The only known genus is ''Hansenocaris'' in the ...

'' Kolbasov & Høeg, 2003 *'' Hansenocaris leucadea'' Belmonte, 2005 *'' Hansenocaris mediterranea'' Belmonte, 2005 *'' Hansenocaris pacifica'' Itô, 1985 *'' Hansenocaris papillata'' Kolbasov & Grygier, 2007 *''

Hansenocaris rostrata Facetotecta is a poorly known subclass of thecostracan crustaceans. The adult forms have never been recognized, and the group is known only from its larvae, the "y- nauplius" and "y-cyprid" larvae. The only known genus is ''Hansenocaris'' in the ...

'' Itô, 1985 *'' Hansenocaris salentina'' Belmonte, 2005 *'' Hansenocaris tentaculata'' Itô, 1986

References

{{Taxonbar, from=Q137276 Maxillopoda Parasitic crustaceans