''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 "unisp ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

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

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

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

in the genus. This sea anemone lives at bathyal depths in the North Atlantic Ocean and has a

symbiotic relationship Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biolo ...

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

siphonoglyph The siphonoglyph is a ciliated groove at one or both ends of the mouth of sea anemones and some corals. The siphonoglyph extends into a pharynx and is used to create currents of water into the pharynx. These water currents are important for respirat ...

s. The six pairs of

mesenteries In human anatomy, the mesentery is an organ that attaches the intestines to the posterior abdominal wall, consisting of a double fold of the peritoneum. It helps (among other functions) in storing fat and allowing blood vessels, lymphatics, a ...

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 Heterocrine gland, mixed gland and sex organ that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gon ...

s. The

nematocyst A cnidocyte (also known as a cnidoblast) is a type of cell containing a large secretory organelle called a ''cnidocyst'', that can deliver a sting to other organisms as a way to capture prey and defend against predators. A cnidocyte explosively ...

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 in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...

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

epibiont An epibiont (from the Ancient Greek meaning "living on top of") is an organism that lives on the surface of another living organism, called the basibiont ("living underneath"). The interaction between the two organisms is called epibiosis. An ep ...

s 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 (''Crossaster papposus'') 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 distri ...

'', 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 A lunar phase or Moon phase is the apparent shape of the Moon's directly sunlit portion as viewed from the Earth. Because the Moon is Tidal locking, tidally locked with the Earth, the same Hemisphere (geometry), hemisphere is always facing the ...

References

{{Taxonbar, from2=Q2013147, from1=Q21445761 Hormathiidae Monotypic cnidarian genera Hexacorallia genera