Cnidaria
Sea anemones, corals, jellyfish, sea pens, hydra
Daphne G. Fautin and Sandra L. Romano
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.
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.
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.
D i sc u ssi o n o f P h yl o g e n e ti c R el a ti o ns hi ps
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.
Alternative views of cnidarian life-cycle evolution and systematic
relationships. (After Bridge et al. 1995.)