Paranthus rapiformis

Paranthus rapiformis, the onion anemone, is a species of sea anemone in the family Actinostolidae. It was first described by the French naturalist Charles Alexandre Lesueur in 1817 and is native to the northwestern Atlantic Ocean and the Gulf of Mexico.[2]

Paranthus rapiformis
Fig 28. Ammophiladis rapiformis.
Scientific classification
Kingdom: Animalia
Phylum: Cnidaria
Class: Anthozoa
Order: Actiniaria
Family: Actinostolidae
Genus: Paranthus
Species:
P. rapiformis
Binomial name
Paranthus rapiformis
(Le Sueur, 1817)[1]
Synonyms[2]
  • Actinia rapiformis Le Sueur, 1817
  • Ammophilactis rapiformis (Le Sueur, 1817)
  • Paractis rapiformis (Le Sueur, 1817)

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.[3] 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.[4]

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.[4]

The burrow is cylindrical and the vertical sides are lined with mucus; it is commonly 8 cm (3 in) long and 2.5 cm (1 in) 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.[5]

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References
  1. Le Sueur, C.A. (1817). "Observations on several species of the genus Actinia; illustrated by figures". Journal of the Academy of Sciences of Philadelphia. 1 (1): 149–154, 169–189, 171.
  2. Fautin, Daphne (2015). "Paranthus rapiformis (Le Sueur, 1817)". WoRMS. World Register of Marine Species. Retrieved 17 July 2017.
  3. Lippson, Alice Jane; Lippson, Robert L. (2006). Life in the Chesapeake Bay. JHU Press. p. 80. ISBN 978-0-8018-8338-5.
  4. Rothschild, Susan B. (2004). Beachcomber's Guide to Gulf Coast Marine Life: Texas, Louisiana, Mississippi, Alabama, and Florida. Taylor Trade Publications. p. 42. ISBN 978-1-58979-061-2.
  5. Martin, Anthony J. (2013). Life Traces of the Georgia Coast: Revealing the Unseen Lives of Plants and Animals. Indiana University Press. pp. 256–257. ISBN 0-253-00609-0.
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