''Phoronis australis'' is a

species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...

of marine horseshoe worm in the

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclatu ...

Phoronid Phoronids (scientific name Phoronida, sometimes called horseshoe worms) are a small phylum of marine animals that filter-feed with a lophophore (a "crown" of tentacles), and build upright tubes of chitin to support and protect their soft bodies. ...

a. It is found in shallow warm-temperate and tropical waters in the eastern

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#Surface, Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Afr ...

and the Indo-Pacific region and was first detected in the

Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ...

in the late twentieth century. These worms live in association with

tube-dwelling anemone Tube-dwelling anemones or ceriantharians look very similar to sea anemones but belong to an entirely different class of anthozoans. They are solitary, living buried in soft sediments. Tube anemones live inside and can withdraw into tubes, which ...

s, particularly those in the genus ''

Cerianthus ''Cerianthus'' is a genus of tube-dwelling anemones in the family Cerianthidae. Members of the genus are found worldwide. They are predators, scavengers and omnivores. Characteristics Members of this genus do not have a pedal disc with which to ...

''.

Description

''Phoronis australis'' grows to a length of about when extended, with a diameter of about . The

lophophore The lophophore () is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Hyolitha, and Phoronida, which collectively constitute the protostome group Lophophorata.tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...

s on either side. The colour is variable and may be pink, dark red or black.

Distribution and habitat

''Phoronis australis'' is native to the Indo-Pacific region and the southeastern Atlantic Ocean. Since the 1990s it has appeared in the Mediterranean Sea, presumably arriving via the

Strait of Gibraltar The Strait of Gibraltar ( ar, مضيق جبل طارق, Maḍīq Jabal Ṭāriq; es, Estrecho de Gibraltar, Archaism, Archaic: Pillars of Hercules), also known as the Straits of Gibraltar, is a narrow strait that connects the Atlantic Ocean to ...

as early records were from Spain. In the Mediterranean, it is normally associated with the tube-dwelling anemone ''

Cerianthus membranaceus ''Cerianthus membranaceus'', the cylinder anemone or coloured tube anemone, is a species of large, tube-dwelling anemone in the family Cerianthidae. It is native to the Mediterranean Sea and adjoining parts of the northeastern Atlantic Ocean. De ...

'', with typically, about eight individual horseshoe worms being present on the tube of each

host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places *Host, Pennsylvania, a village in Berks County People * Jim Host (born 1937), American businessman * Michel Host ...

. However, it has also been recorded as living independently in muddy coarse sand in the Mediterranean, the tube being attached to the seagrass ''

Posidonia oceanica ''Posidonia oceanica'', commonly known as Neptune grass or Mediterranean tapeweed, is a seagrass species that is endemic to the Mediterranean Sea. It forms large underwater meadows that are an important part of the ecosystem. The fruit is free f ...

''.

Ecology

''Phoronis australis'' secretes a chitinous tube into which it can retreat when disturbed. The tube is attached to the much larger tube of a

which lives in shallow sheltered water immersed in soft sediment. Many horseshoe worms may be associated with a single ceriantharian anemone. The lophophore of the horseshoe worm is extended to catch the planktonic particles on which it feeds. It benefits from the protection provided by the canopy of anemone tentacles and is alerted to danger by the reactions of the anemone in withdrawing into its tube when disturbed. The anemone probably derives no benefit from the

inquiline In zoology, an inquiline (from Latin ''inquilinus'', "lodger" or "tenant") is an animal that lives commensally in the nest, burrow, or dwelling place of an animal of another species. For example, some organisms such as insects may live in the h ...

association. ''Phoronis australis'' is a

hermaphrodite In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have ...

. The embryos are at first brooded in two clumps on mucus threads secreted by nidamental glands. The actinotroch larvae are

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cr ...

ic and eventually settle and undergo metamorphosis. The horseshoe worm can also reproduce asexually by

transverse fission Strobilisation or transverse fission is a form of asexual reproduction consisting of the spontaneous transverse segmentation of the body. It is observed in certain cnidarians and helminths. This mode of reproduction is characterized by high offspr ...

References

{{Taxonbar, from=Q2341516 Phoronids Animals described in 1883