Cnidaria

Tree following Werner (1973) and Bridge et al. (1995).

Introduction

The exclusively aquatic phylum Cnidaria is represented by polyps such as sea anemones and corals, and by medusae such as jellyfish. A polypoid or a medusoid cnidarian is a radially or biradially symmetrical, uncephalized animal with a single body opening, the mouth. The mouth is surrounded by tentacles studded with microscopic stinging capsules known as nematocysts that are the agents of offense and defense. The possession of intrinsic nematocysts is the defining characteristic of the phylum (Hessinger and Lenhoff 1988); nematocysts are the most diverse and widespread of three types of cnidae (cnidos = thread) -- hence the preferred name of the phylum.

Cnidarians are diploblastic -- that is, the body and tentacles consist of two cell layers, the endoderm (sometimes referred to as the gastrodermis) and the ectoderm (the epidermis). Between the two cell layers is the mesoglea, which ranges from little more than a glue to bind the layers (for example, in Hydra) to the vast bulk of the animal (for example, in jellyfish of Class Scyphozoa). The body encompasses a single sac-like body space, the coelenteron (koilos = cavity; enteron = intestine), which communicates with the surrounding medium through the mouth. The less preferred name of the phylum, Coelenterata, is based on this attribute. The coelenteron (also termed the gastrovascular cavity) serves for gas exchange and digestion.

All cnidarians are carnivorous, with cnidae and tentacles active in prey capture. Because polyps are typically sessile, and only some medusae possess sensory structures (the most sophisticated occur in the Cubozoa; Pearse and Pearse 1978), cnidarians are generally believed to be passive predators, feeding on prey items that blunder into their tentacles. Some cnidarians can absorb dissolved organic matter directly from seawater (e.g. Schlichter 1975), but it is not known how widespread this ability is. Living within the tissues of anthozoans of many species and hydrozoans and scyphozoans of a small number of species are unicellular algae from which the animals derive reduced carbon (Shick 1991). Dinoflagellate symbionts, termed zooxanthellae, are by far the most common algal symbionts; they are exclusively marine. Green algal symbionts, termed zoochlorellae, occur in both marine and freshwater cnidarians.

The text-book depiction of the typical cnidarian life cycle is an alternation between a medusa and a polyp (termed metagenesis), the former the sexually reproductive stage and the latter the asexual stage. In fact, an attribute of the entire class Anthozoa is the absence of a medusa. At least some individuals of all anthozoan species form gametes; those of some species may reproduce vegetatively as well. The other three classes -- Cubozoa, Hydrozoa, and Scyphozoa -- are often grouped as the "Medusozoa" because the medusa phase is present in them all. Indeed, the medusa dominates the life cycle of members of the classes Cubozoa and Scyphozoa (Cubozoa was formerly considered an order of Scyphozoa, and some specialists still consider it as such). Life cycles of the Hydrozoa are the most diverse in the phylum: although the polyp is the more conspicuous and persistent stage in most taxa, some lack the medusa phase, whereas others lack the polyp phase. Hydra, which is used in many textbooks to illustrate the phylum, is utterly atypical: a hydrozoan, it lacks a medusa, it has aggregations of gametogenic tissue that function as gonads, and it is among only a handful of freshwater cnidarian species.

The cnidarian larva is the planula, a pear-shaped, entirely ciliated animal. In the "typical" cnidarian life cycle, male and female medusae spawn freely into the sea, where fertilization occurs and a planula develops. At metamorphosis, the planula settles on and attaches to the substratum, where it metamorphoses into a polyp. The primary polyp produces additional polyps asexually, by budding, stolonic outgrowth, or some other process, to form a clone or a colony. At the appropriate time, determined perhaps by size of the colony or environmental conditions, rather than or in addition to polyps, medusae are produced asexually (in Cubozoa, each polyp metamorphoses into a medusa). They are released to take up a pelagic existence and the cycle begins anew.

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Idealized lifecycle of the Cnidaria.

Characteristics

The cnida, or nematocyst, which is the sine qua non of the phylum, is secreted by the Golgi apparatus of a cell termed a cnidoblast (Watson 1988). A cnida therefore is technically not an organelle, but, rather, the most complex secretory product known. Upon receiving the appropriate physical and/or chemical stimulus, a cnida fires, everting a tubule many times the length of the capsule. The tubule may deliver a toxin, may stick to a prey item, or may entangle an object, depending on the type of cnida. A cnida can fire but once. There are three major types of cnidae: nematocysts, spirocysts, and ptychocysts. Nematocysts occur in all classes of Cnidaria, but some of the 30-plus varieties of nematocysts are restricted to members of certain classes (Fautin and Mariscal 1991). Spirocysts are found only in Anthozoa; they are adhesive in nature. Ptychocysts are the most taxonomically restricted in distribution, occurring only in the anthozoan order Ceriantharia; their function is to entangle bits of mud among their robust tubules to form the feltwork that constitutes the tube of these burrowing animals.

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Left: Fired "basitrich" (basitrichous isorhiza) from a sea anemone. The now empty capsule is in the lower right of the photo; the spiny basal part of the fired tubule extends to the upper left; beyond the frame of the photo is the non-spiny, distal part of the tubule, which is many times longer than the capsule. Middle: "Holotrich" (holotrichous isorhiza) from a corallimorpharian. Right: Unfired "basitrichs" (basitrichous isorhizas) from a sea anemone. The longitudinal line inside each capsule is the spiny basal part of the unfired tubule.

Two body forms are characteristic of cnidarians -- the polyp and the medusa. With a few exceptions, a columnar polyp is sedentary, being attached to or burrowed into the substratum by the end opposite the mouth. Thus its tentacles are typically considered to point upward and outward. Polyps of some species propagate vegetatively, forming colonies (if the progeny remain attached to one another) or clones (if the progeny separate). Polymorphism occurs in colonies of some species of hydrozoans and anthozoans, the polyps being specialized for functions such as feeding, defense, and sexual reproduction. Polyps of some taxa form a skeleton within or external to their tissues; some skeletons are mineralic (of calcium carbonate), others are organic (of chitin or another carbohydrate), and some are both. The spheroidal or discoidal medusae are solitary, and those of most species are pelagic. Although typically depicted as living with mouth and tentacles pointing down, medusae assume all orientations in the water. Medusae of few species possess the ability to propagate vegetatively. The common name of medusae, jellyfish, alludes to the massive amount of mesoglea that contributes to their buoyancy.

All cnidarians have hydrostatic skeletons, regardless of whether they also have mineralic and/or organic exoskeletons or endoskeletons. The muscles of the body wall operate against the fluid in the coelenteron to extend individual polyps and to effect the swimming of medusae, for example. The hollow tentacles of anthozoans are extended through hydrostatic action as well.

Discussion of Phylogenetic Relationships

Cnidaria is thought to have one of the longest fossil histories of metazoan phlya with representatives in the Ediacaran fauna of the late Precambrian (Scrutton 1979). These earliest fossils are both medusoid and polypoid, and thought to represent all cnidarian classes (Scrutton 1979).

The four extant cnidarian classes are identifiable as early as the Ordovician (Robson 1985), but evolutionary relationships among them have been the subject of much debate (e.g. Brooks 1886, Hyman 1940, Jagersten 1955, Hand 1959, Pantin 1960, Werner 1973, Petersen 1979, Barnes 1987, Ax 1989). Anthozoa is alternatively considered the most basal or the most derived group. The former hypothesis posits that the polyp is the original body form, with the medusa (and metagenesis) being derived (Fig. 1A). The latter perspective is that, in the "typical" life cycle, the medusa is gametogenic, and so constitutes the definitive, or adult, stage, with the polyp being a persistent larva. Thus, it is reasoned, the polyp evolved secondarily, and loss of the original body form, the medusa, places Anthozoa as the most derived taxon (Fig. 1B). A comprehensive morphological cladistic analysis by Schuchert (1993) supports the basal position of Anthozoa with the Scyphozoa and Cubozoa being more closely related to each other than to Hydrozoa. Morphological, mtDNA, and 18S rDNA data separately and together also support the basal position of Anthozoa but do not resolve the relationships among Scyphozoa, Cubozoa and Hydrozoa (Bridge et al. 1995). The phylogenetic tree at the beginning of this page is that of Bridge et al. (1995). Neither of these treatments attempts to include the extinct class Conulata, which has been considered by most paleontologists to be related to the Scyphozoa.

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Alternative views of cnidarian life-cycle evolution and systematic relationships. (After Bridge et al. 1995.)

Their diploblastic structure and their single body opening and cavity had been thought to ally cnidarians with ctenophores. Indeed, until relatively recently the phylum Coelenterata was considered to include animals now placed in Cnidaria and Ctenophora. However, ctenophores lack a metagenetic life cycle and cnidae. Cnidae have been found in one ctenophore, but it is now known that the ctenophore acquires those cnidae from the hydromedusae upon which it preys (Mills and Miller 1984). Thus, it is generally agreed that the similarity in body form between pelagic ctenophores and pelagic cnidarians is convergent; benthic ctenophores do not resemble cnidarians at all. Cnidaria, therefore, is a well circumscribed taxon; it is considered by many to be a sister group of all metazoans other than sponges.

References

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