WHAT IS A CORAL
Corals
are members of a large, related group (subphylum) of stinging
marine animals called the Cnidaria (cnida = nettle), which
collectively includes hydroids, jellyfish or medusae, sea fans,
sea pens, soft corals, and sea anemones. Historically, corals
included a larger variety of animals, as well as certain plants
which had the ability to secrete hard skeletal parts or incorporate
limey deposits within or around their tissues. As the affinities
of the group became more well known the term has become more restricted
in its usage and now generally refers to cnidarians which secrete
mineralized or hard skeletons.
The cnidarians
are subdivided into three smaller groups called classes, primarily
based upon the pattern of their life cycles, but only two of these
the Hydrozoa and Anthozoa possess living members with skeletons
and are thus included in the handbook as corals. The hydrozoan
corals or hydrocorals (named after Hydra - the mythical Greek
marsh serpent with nine heads) are included in two orders (next
classification category below the class level). These are
the Milleporina (mille = a thousand + pora = hole)
or fire corals which include some important reef-building species
and the Stylasterina (style = spike + aster = star)
which are small delicately branched corals that generally live
in cryptic (hidden places such as cracks and holes) habitats and
are not important as reef builders. Anthozoan corals, named
because of the living individuals resemblance to the petal arrangement
in flowers, are a much larger class of which three coral orders
are included in this handbook. These are in the Stolonifera
(stolon = branch + fera = bearer), a small group
with a single reef-building species called the red organ pipe
coral; the Coenothecalia (Coen = shared + theca
= box) also comprised of a single reef-building species called
the blue stony coral; and the Scleractinia (sclera = hard
+ actinia = ray) or stony star corals which to date include
about 275 reef-building and deep-sea species known to occur in
Guam waters.
The remaining
lower categories or levels of organization within the cnidarian
corals (family, genus, and species) are classified and identified
to a large extent on the form and architectural features of the
nonliving skeleton. Since it is hoped that many of the users
of this handbook will be able to observe corals in their natural
habitats they should be familiar with certain features of the
living animal as well as it skeletal parts.
General Features of the Animals
The most
outstanding overall characteristic of the cnidarians is their
possession of stinging capsules called nematocysts. Although
nematocysts are primarily used for food capture and aggression
and defense against other organisms, some hydrocorals can give
a swimmer a painful sting, especially on tender areas of the skin.
Cellular organization in the corals is at the tissue grade
(tissues not organized into organs) with a body wall that encloses
a cavity called the coelenteron. The coelenteron has a mouth
opening at one end (oral end) and is closed at the other end (aboral
end). The body wall consists of an outer layer of ectodermal
tissue and an inner layer of endodermal tissue separated by a
jelly-like middle layer called the mesogloea. Body parts
are arranged in a radiating pattern around the central oral-aboral
axis, like spokes from the hub of a wheel. Although polymorphism
(expression of the body in more than one form) is common in some
cnidarian groups, it is generally expressed in two forms consisting
of polypoid and medusoid individuals (see Text
Fig. A). The polypoid form or polyp is shaped like an
elongate cylinder, generally attached to the substrate (surface
upon which an organism lives) at its aboral end and free with
tentacles arranged in a circle around the mouth at the oral end.
The medusae are free-swimming forms, shaped like an umbrella
with tentacles arranged around the rim and a mouth located in
the center of the concave side. Life cycles involving medusoid
forms are not the general rule in cnidarian coral groups.
