''Xyloplax medusiformis'' is a sea daisy, a member of an unusual group of marine

taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

belonging to the phylum

Echinodermata An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...

. It is found at bathyal depths in waters around New Zealand. It was first described in 1986 by Baker, Rowe and Clark and is the type taxon of the genus ''

Xyloplax Sea daisies (infraclass Concentricycloidea; order Peripodida) make up an unusual group of deep-sea taxa belonging to the phylum Echinodermata, with three species described in the genus ''Xyloplax''. Intestine and anus are absent. Distribution S ...

''. Its generic name derives from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

"xylo" meaning wood and its specific name was chosen because its

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...

superficially resembles that of a

cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that ...

medusa In Greek mythology, Medusa (; Ancient Greek: Μέδουσα "guardian, protectress"), also called Gorgo, was one of the three monstrous Gorgons, generally described as winged human females with living venomous snakes in place of hair. Those ...

Discovery

Specimens of ''Xyloplax medusiformis'' were first discovered by accident when a submersible craft was being used to collect samples of wood on the deep sea bed in the South Pacific near New Zealand. Nine individuals were discovered from five locations. At the time these specimens puzzled researchers because of their lack of close affiliations to other echinoderms. Since then further members of the genus ''Xyloplax'' have been discovered on wood at bathyal depths, '' Xyloplax turnerae'' in the Atlantic Ocean off the

Bahamas The Bahamas (), officially the Commonwealth of The Bahamas, is an island country within the Lucayan Archipelago of the West Indies in the North Atlantic. It takes up 97% of the Lucayan Archipelago's land area and is home to 88% of the ar ...

and ''

Xyloplax janetae ''Xyloplax janetae'' is a ''Xyloplax'' of the family Xyloplacidae. It lives on the surface of wood sunken to abyssal depths. Morphology ''Xyloplax janetae'' is a flattened disk, from in diameter, and about thick. It has adambulacral spines wh ...

'' in the central Pacific Ocean.

Taxonomy

''Xyloplax medusiformis'' was the first sea daisy to be described. It was clearly an

echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...

but at first it was considered to be sufficiently different from starfish, class

Asteroidea Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish a ...

, as to warrant it being placed in a new

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...

of its own, the

Concentricycloidea Sea daisies (infraclass Concentricycloidea; order Peripodida) make up an unusual group of deep-sea taxa belonging to the phylum Echinodermata, with three species described in the genus ''Xyloplax''. Intestine and anus are absent. Distribution S ...

. The main difference was that the

water vascular system The water vascular system is a hydraulic system used by echinoderms, such as sea stars and sea urchins, for locomotion, food and waste transportation, and respiration. The system is composed of canals connecting numerous tube feet. Echinoderms m ...

of asteroids has a single ring canal circling the mouth connected to radial arms with short lateral side arms while that of the sea daisy had two ring canals surrounding the mouth, linked by five inter-ring canals. The concentric arrangement of plates on the surface of the sea daisy was novel and it had a single row of podia (tube feet) rather than the two rows typical of asteroids. DNA sequencing was done on ''Xyloplax medusiformis'' and on a range of about twenty other varied species of echinoderm. It was found that ''X. medusiformis'' was indubitably a member of class Asteroidea and was a sister taxon to '' Rathbunaster'' in the order

Forcipulatida The Forcipulatida are an order of sea stars, containing three families and 49 genera. Description Forcipulatids share with the brisingid sea stars distinctive pedicellariae, consisting of a short stalk with three skeletal ossicles. Unlike that ...

. It is considered that ''X. medusiformis'' may be

paedomorphic Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compare ...

, retaining larval characteristics into adulthood. Later it was confidently placed in Velatida (Molecular Phylogenetics and Evolution 115 (2017): 161–170).

Description

''Xyloplax medusiformis'' is a small, flattened disc or umbrella-shaped invertebrate growing to a diameter of about . The aboral (upper) surface is covered with flat plates arranged in concentric rings and there are short marginal spines projecting from the periphery. Internally the water vascular system consists of two super-oral rings and no radial canals. There is no stomach, gut or anus and it is thought that the velum (membrane) that covers the oral (lower) surface is equivalent to an everted stomach. There are five pairs of gonads, each associated with a fluid-filled sac.

Distribution and habitat

''Xyloplax medusiformis'' is known from deep waters around New Zealand. It is found on pieces of decaying timber that have sunk to the seabed and are at least beneath the surface. It is believed that it may feed on bacteria or the products of wood decay but very little is known of the biology of this echinoderm. Similar numbers of males and females have been discovered and it appears that there is a long phase of embryonic development during which time the females brood the embryos in their ovaries until they are nearly ready to undergo metamorphosis into juveniles.

References

{{Taxonbar, from=Q2321596 Peripodida Animals described in 1986