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.