In the previous tutorial we went over some
general information broadly pertaining to the phlyum Cnidaria, and we mentioned a
number of species that belong to this phylum. But let’s now begin a more thorough and
detailed investigation. The five best-known classes of Phylum Cnidaria are Hydrozoa,
Scyphozoa, Staurozoa, Cubozoa, and Anthozoa. Lesser known are Myxozoa and Polypodiozoa,
though their classification is a bit trickier. So let’s start with the more well-known Cnidarians
before we cover some of the more enigmatic ones. The animals that make up class hydrozoa
are generally very small predatory polyps. Some are colonial and others are solitary. They
are found in marine waters around the world, and some, like the common hydra, inhabit
freshwater environments throughout the world. Individual hydras grow to a length of about
25-30 millimeters, and are generally found on the underside of leaves in shallow pools
and streams. Their bodies are cylindrical tubes descending into a slender “stalk”,
ending in a basal disc used for attachment and also to secrete a gas bubble for floatation.
The mouth is located at the other end of the body and is encircled by hollow tentacles
that open into the gastrovascular cavity, which functions as a multipurpose cavity in
hydra and most other Cnidarians. During feeding, the hydra extends its tentacles out into the
water column and attempts to snag passing prey with its lasso-like, volvent type cnidocytes.
Hydra can morph their bodies into different forms. For example, when frightened, they
can contract into small gelatinous spheres, and when feeding they extend their bodies to
over four times their usual length. Though they are often sessile animals, they are
capable of moving not only by detaching from the substrate and floating, but also by
“looping” or “somersaulting” which involves an intricate tumbling motion that results from an
expansion and contraction of their whole bodies. Hydra reproduce asexually by budding, or splitting
in two when food is plentiful, as well as sexually often right before winter or in poor feeding
conditions. During sexual reproduction, hydra form swellings on their epidermis that
develop into male or female gonads. The testes release free swimming sperm into the water to
fertilize the eggs attached to another hydra. Once an egg is fertilized, it undergoes cleavage
to form a diploblastic blastula. A tough outer covering, called a cyst, then encapsulates the
developing embryo to protect it through a period of harsh conditions as both parents slowly die.
When conditions are favorable, young hydras hatch. Now, hydras are unique in that although
they are somewhat fragile creatures, they possess the ability to undergo
morphallaxis, or tissue regeneration, when damaged, often regrowing whole body parts
and even new individuals. For example, a hydra that is cut in half forms two new hydra. However,
this is nothing compared to the hydra’s ability to regenerate their stem cells indefinitely, making
them biologically immortal. Though hydra can, and often do, die easily in poor conditions,
they do not senesce, meaning they do not age. And they’re not the only hydrozoans
that are biologically immortal. Turritopsis dohrnii, for example, often called
the “immortal jellyfish” is not a true jellyfish at all, but rather a member of class hydrozoa that
has a medusa stage. The immortal jellyfish spends most of its time as a member of a polyp colony.
The colony consists of multi-branching zooids, some of which are specialized for reproduction.
These zooids reproduce asexually when tiny medusas in the “jellyfish” stage pop off
and swim away. The medusas can, and often do, reproduce sexually by releasing sperm and eggs
into the water. If an egg is fertilized it develops into a planula larva, which settles
into the substrate and forms a new colony. “Immortal jellyfish” are tiny creatures, with a
bell of only about 4.5 millimeters in diameter, and many of them are eaten or otherwise killed.
However, under conditions of extreme stress, such as starvation, tissue damage, or
temperature changes, medusas of any age can settle on the sea floor and through the process
of reverse metamorphosis and transdifferentiation, they become toughened cysts which grow into polyps
and polyp colonies, that reproduce asexually forming new medusas. This reverse biotic cycle
allows the “immortal jellyfish” to escape death, rendering it, like the hydra and other
hydrozoans, biologically immortal. Not all hydrozoans are capable of such feats,
though there are many others that are colonial. Let’s use the genus Obelia as an example.
A single hydroid Obelia has a base, stalk, and one of a few types of terminal zooids.
The base of the colony attaches to the substrate using a rootlike “stolon” or hydrorhiza. Stalks
called hydrocauli or hydrocaulus extend upwards and are often protected by a nonliving chitinous
sheath called a perisarc. Attached to the end of the hydrocaulus are individual polyp animals, the
zooids. Most of these polyps are gastrozooids, specialized for feeding. Food that they catch
is partially digested and then distributed to the rest of the colony. Other polyps,
like the gonozooids, contain medusa buds which asexually reproduce individual medusas, that
then swim away to sexually mature and reproduce. Most colonial hydroids are small and sessile,
but some, most notably the siphonophores, are able to drift and float in the ocean.
These creatures look like individual animals, and ecologically speaking, they do behave
as one. However, they are actually a colony composed of a myriad of different polyps.
The largest recorded siphonophore colony, Apolemia, can reach lengths of 40 to 45 meters,
which is longer than the blue whale! Unlike benthic colonial hydroids, siphonophore colonies
are made of zooids that can be either polyps or medusae. Common siphonophore zooids
include nectophores, which aid in propulsion, bracts which maintain buoyancy, gastrozooids
for feeding, gonophores for reproduction, and pneumatophores that create gas-filled
floats found in some siphonophores, most famously, the Portuguese man o' war and the
Flying Spaghetti Monster, which is the name of a real animal in addition to the satirical deity.
Other colonial hydrozoans include the hydrocorals, such as the fire corals and rose corals, which
are not true corals at all, though they do resemble them from a distance. However, on closer
inspection it is clear they are instead hydrozoan colonies with calcareous skeletons. A few other
notable hydrozoans include the chondrophores, tiny colonial hydroids with a central gas-filled
disk, deep red jellyfish, which are free living hydrozoans without a polyp stage, air ferns,
colonial hydroids which are sometimes dried out and sold as “indoor plants,” and freshwater
jellyfish, a type of hydromedusae now found as an invasive species in bodies of water around
the world. So that more or less sums up the hydrozoans. Let’s move forward and discuss
some more classes of the phylum Cnidaria.