''Paranthus rapiformis'', the onion anemone, is a

species In biology, a species is the basic unit of 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 the appropriate s ...

of sea anemone in the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Actinostolidae. It was first described by the French naturalist

Charles Alexandre Lesueur Charles Alexandre Lesueur (1 January 1778 in Le Havre – 12 December 1846 in Le Havre) was a French naturalist, artist, and explorer. He was a prolific natural-history collector, gathering many type specimens in Australia, Southeast Asia, ...

in 1817 and is native to the northwestern Atlantic Ocean and the

Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of the United ...

Description

''P. rapiformis'' is a small white sea anemone which burrows into the sediment, usually subtidally, but it may be exposed at extreme low water. The base of the column is expanded which anchors it in place, and it is usually attached to a submerged stone or shell. When covered with water, the short tentacles spread out over the surface of the sand. When it is disturbed, the tentacles retract and the column inflates to form a globular shape; this is translucent, with longitudinal white stripes, and resembles a small onion.

Biology

As a burrowing sea anemone, the pedal disc of ''P. rapiformis'' is replaced by a rounded end known as a "physa" which is used for digging. First the physa is pushed into the soft sediment, then the anemone swells its body wall to anchor itself in place while pushing the physa further into the sand. Now it inflates the physa and deflates the body wall, using its strong longitudinal muscles to draw the upper part towards the base, during which process, the physa inverts. These two steps are repeated as it works its way deeper into the sediment. The burrow is cylindrical and the vertical sides are lined with

mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...

; it is commonly long and wide, but can be much larger, however it collapses if the sea anemone is removed. A dislodged anemone may adopt a globular shape and be rolled about on the sand by the sea until it finds a less turbulent place where it can burrow.

References

{{Taxonbar, from=Q2194087 Actinostolidae Animals described in 1817 Taxa named by Charles Alexandre Lesueur Cnidarians of the Atlantic Ocean