''Allantactis'' is a

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispec ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

sea anemone Sea anemones are a group of predation, predatory marine invertebrates of the order (biology), order Actiniaria. Because of their colourful appearance, they are named after the ''Anemone'', a terrestrial flowering plant. Sea anemones are classifi ...

s, and ''Allantactis parasitica'' is the only

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 ...

in the genus. This sea anemone lives at bathyal depths in the North Atlantic Ocean and has a symbiotic relationship with a gastropod mollusc.

Description

''A. parasitica'' has a strong base and a smooth column with thick walls and no cuticle or perforations of the body wall that are termed ''cinclides''. The margin is distinct and the sphincter powerful. The

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 are fairly short and arranged in whorls of six, there being a maximum of 96. The oral disc has a central mouth with two broad siphonoglyphs. The six pairs of mesenteries are complete, in that their sheetlike membranes join the gastrodermis of the body wall with that of the pharynx, and are sterile, in that they do not bear

gonad A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sper ...

s. The

nematocyst A cnidocyte (also known as a cnidoblast or nematocyte) is an explosive cell containing one large secretory organelle called a cnidocyst (also known as a cnida () or nematocyst) that can deliver a sting to other organisms. The presence of this ce ...

s consist of a mixture of spirocysts, basitrichs and microbasic p-mastigophors.

Ecology

The species lives on the abyssal seabed of the North Atlantic at depths of between . Although sometimes living attached to a pebble or empty shell, it prefers to attach to the shell of a living gastropod mollusc when available. Sea anemone larvae settling on the seabed do so preferentially on any gastropods that are present, and where the larvae settle onto mud, the presence of gastropods nearby stimulates them to move towards and onto the molluscs. Sometimes as many as six anemones can be found crowded onto the shell of a single living gastropod. Gastropods with one or two sea anemone on their shells were found to have greater feeding success and more varied stomach contents than those with either no epibionts, or with three or more. The sea anemones also benefited from the mutualism because of the longer time spent in food-rich areas in the deep sea floor, a place where food resources are limited. The chief

predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...

of gastropods at these depths in the North Atlantic is the starfish '' Leptasterias polaris''. Researchers found that starfish avoided molluscs with anemones on their shells, and that molluscs relied on the presence of their epibionts and did not take evasive action when they encountered the starfish. If their attached anemones were removed, they lost the protection that their toxin-producing

s provided, and were attacked. The sea anemone gained advantage from being an epibiont as it is itself sometimes attacked by the starfish ''

Crossaster papposus The common sunstar is a species of sea star (aka starfish) belonging to the family Solasteridae. It is found in the northern parts of both the Atlantic and the Pacific Oceans. Distribution The common sunstar is distributed from the Arctic dow ...

'', but was able to escape from this predator when attached to a living gastropod mollusc. Despite living at bathyal depths where no light penetrates, ''A. parasitica'' is one of a number of deep sea species that show synchronisation in the release of eggs and sperm to the

phases of the moon Concerning the lunar month of ~29.53 days as viewed from Earth, the lunar phase or Moon phase is the shape of the Moon's directly sunlit portion, which can be expressed quantitatively using areas or angles, or described qualitatively using the t ...

References

{{Taxonbar, from=Q2013147, from2=Q21445761 Hormathiidae Monotypic genera Hexacorallia genera