Apolyp inzoology is one of two forms found in thephylumCnidaria, the other being themedusa. Polyps are roughlycylindrical in shape and elongated at the axis of thevase-shaped body. In solitary polyps, theaboral (opposite to oral) end is attached to thesubstrate by means of a disc-likeholdfast called apedal disc, while incolonies of polyps it is connected to other polyps, either directly or indirectly. The oral end contains themouth, and is surrounded by a circlet oftentacles.
In the classAnthozoa, comprising thesea anemones andcorals, the individual is always a polyp; in the classHydrozoa, however, the individual may be either a polyp or amedusa,[1] with most species undergoing alife cycle with both a polyp stage and a medusa stage.
In the classScyphozoa, the medusa stage is dominant, and the polyp stage may or may not be present, depending on thefamily. In those scyphozoans that have thelarval planulametamorphose into a polyp, the polyp, also called a "scyphistoma," grows until it develops a stack of plate-like medusae that pinch off and swim away in a process known asstrobilation. Once strobilation is complete, the polyp may die, orregenerate itself to repeat the process again later. Withcubozoans, theplanula settles onto a suitable surface, and develops into a polyp. The cubozoan polyp then eventually metamorphoses directly into a medusa.[citation needed]
 | This sectionrelies largely or entirely on asingle source. Relevant discussion may be found on thetalk page. Please helpimprove this article byintroducing citations to additional sources. Find sources: "Polyp" zoology – news ·newspapers ·books ·scholar ·JSTOR(January 2025) |
The body of the polyp may be roughly compared in a structure to a sac, the wall of which is composed of two layers ofcells. The outerlayer is known technically as theectoderm, with the inner layer as theendoderm (orgastroderm). Between ectoderm and endoderm is a supporting layer of structureless gelatinous substance termedmesoglea, secreted by the cell layers of the body wall.[1] The mesoglea can be thinner than the endoderm or ectoderm or comprise the bulk of the body as in largerjellyfish. The mesoglea can containskeletal elements derived from cellsmigrated from ectoderm.[citation needed]
The sac-like body built up in this way is attached usually to some firm object by its blind end, and bears at the upper end the mouth which is surrounded by a circle oftentacles which resemble glove fingers. The tentacles areorgans which serve both for thetactile sense and for the capture of food.[1] Polyps extend their tentacles, particularly at night, containing coiled stinging nettle-like cells, ornematocysts, which pierce, poison, and firmly hold living prey paralysing or killing them. Polyp prey includescopepods and fishlarvae.[2] Longitudinalmuscular fibrils formed from the cells of the ectoderm allow tentacles to contract when conveying the food to the mouth. Similarly, circularly disposed muscular fibrils formed from the endoderm permit tentacles to be protract or thrust out once they are contracted. These muscle fibres belong to the same two systems, allowing the whole body to retract or protrude outwards.[1]
We can distinguish therefore in the body of a polyp the column, circular or oval in section, forming the trunk, resting on a base or foot and surmounted by the crown of tentacles, which enclose an area termed theperistome, in the centre of which again is the mouth. Generally, there is no other opening to the body except the mouth, but in some casesexcretory pores are known to occur in the foot, and pores may occur at the tips of the tentacles. A polyp is an animal of very simple structure,[1] aliving fossil that has not changed significantly for about half a billion years (per generally accepted dating ofCambriansedimentary rock).[citation needed]
The external form of the polyp varies greatly in different cases. The column may be long and slender, or may be so short in the vertical direction that the body becomes disk-like. The tentacles may number many hundreds or may be very few, in rare cases only one or two. They may be long and filamentous, or short and reduced to mere knobs or warts. They may be simple and unbranched, or they may be feathery in pattern. The mouth may be level with the surface of the peristome, or may be projecting and trumpet-shaped. As regards internal structure, polyps exhibit two well-marked types of organization, each characteristic of one of the two classes, Hydrozoa and Anthozoa.[1]
In the class Hydrozoa, the polyps are indeed often very simple, like the common littlefresh water species of the genusHydra. Anthozoan polyps, including the corals and sea anemones, are much more complex due to the development of a tubularstomodaeum leading inward from the mouth and a series of radial partitions calledmesenteries. Many of the mesenteries project into the enteric cavity but some extend from the body wall to the central stomodaeum.[citation needed]
It is an almost universal attribute of polyps to reproduceasexually by the method ofbudding. This mode ofreproduction may be combined withsexual reproduction, or may be the sole method by which the polyp producesoffspring, in which case the polyp is entirely withoutsexual organs.[1]
In many cases the buds formed do not separate from the parent but remain in continuity with it, thus formingcolonies or stocks, which may reach a great size and contain a vast number of individuals. Slight differences in the method of budding produce great variations in the form of the colonies. Thereef-buildingcorals are polyp-colonies, strengthened by the formation of a firm skeleton.[1]
Among sea anemones,sexual plasticity may occur. That is, asexually produced clones derived from a single founder individual can contain both male and female individuals (ramets).[3] When eggs and sperm (gametes) are formed, they can producezygotes derived from "selfing" (within the founding clone) or out-crossing, that then develop into swimming planula larvae.[4]
The overwhelming majority of stony coral (Scleractinia) taxa arehermaphroditic in their adult colonies.[5] In these species, there is ordinarily synchronized release ofeggs andsperm into the water during brief spawning events.[6] Although some species are capable ofself-fertilization to varying extents,cross-fertilization appears to be the dominant mating pattern.[5]
The name polyp was given byRené Antoine Ferchault de Réaumur[7] to these organisms from their superficial resemblance to anoctopus (French:poulpe, ultimately from Ancient Greek adverbπολύ (poly, "much") + nounπούς (pous, "foot")), with its circle of writhing arms round the mouth. This comparison contrasts to the common name "coral-insects", applied to the polyps which formcoral.[1]
75% of the world's corals are threatened[8] due tooverfishing, destructive fishing, coastal development,pollution, thermal stress,ocean acidification,crown-of-thorns starfish, and introducedinvasive species.[9]
In recent decades the conditions that corals and polyps have found themselves in have been changing, leading to new diseases being observed in corals in many parts of the world, posing even greater risk to an already pressured animal.[10]Aquatic life has been put under a substantial amount of stress because of the pollutants caused by land-basedagriculture. Particularly, exposure to the insecticideprofenofos and the fungicide MEMC have played a major part in polyp retraction and biomass decrease.[11][12]
There have been many experiments supporting the hypothesis that heat stress inAcropora tenuis juvenile polyps provokes an up-regulation of protein in theendoplasmic reticulum. The results vary based on the polyp characteristics such as age, type, and growth stage.[citation needed]
One or more of the preceding sentences incorporates text from a publication now in thepublic domain: Minchin, Edward Alfred (1911). "Polyp". InChisholm, Hugh (ed.).Encyclopædia Britannica. Vol. 22 (11th ed.). Cambridge University Press. p. 37.
{{cite journal}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link){{cite journal}}: CS1 maint: multiple names: authors list (link)