We just covered the phylum Porifera, which consists of organisms that are quite similar
to some of the first animals that ever existed. Moving forward in a roughly chronological
evolutionary context, we arrive at the phlyum Cnidaria. Cnidaria consists of over 10,000 species
whose fossil record reaches back about 580 million years. The name, Cnidaria, comes from the Greek
word “knide” meaning nettle, and “aria” meaning like or connected with. The phylum itself is named
after their unique cells called cnidocytes which contain stinging organelles called nematocysts.
These cnidocytes are characteristic of phylum Cnidaria and are found in no other animals.
Now, if you’ve ever been stung by a jellyfish, you’ve had firsthand experience with cnidocytes.
Let’s take a closer look at these now. It’s important to note that cnidarians don’t
“electrocute” anything, though a sudden sting may feel like an electric shock. Instead,
they release hundreds or thousands of tiny venom-filled harpoons that fire when “triggered.”
Before triggering, when a nematocyst is undischarged,
it is coiled within the cnidocyte and covered by the hinged operculum.
A single nematocyst can discharge in as little as 700 nanoseconds, which is less than a millionth
of a second. This discharge begins when the cnidocil, a chemical and mechanoreceptor,
is activated. Some cnidocils are extremely sensitive and easily triggered, but others are
only triggered by a specific stimulus and rely on surrounding cells for confirmation before firing.
This makes sense because cnidocytes are valuable “single-use” cells that are lost once discharged.
So a confirmation from other cells not only prevents unnecessary loss of cnidocytes,
but also prevents the animal from stinging itself and other mutualist species, such as clownfish,
which are able to move within anemones without getting attacked because they clean the anemone
and provide it with nutrients, while receiving protection in a form of symbiosis. The trigger
mechanism of anemones is a bit different in that they have ciliary mechanoreceptors which “tune”
the nematocysts so as to only trigger when touched by specific prey species or potential predators,
like humans. Such nematocysts remain dormant in the presence of the mutualistic clownfish.
In any case, nematocyst capsules usually contain a high concentration of calcium ions.
When the cell is activated by the cnidocil, these ions flood into the cytoplasm of the cnidocyte.
This sudden shift in gradient causes an astonishingly large change in osmotic pressure,
which causes water to rush into the nematocyst, forcing the operculum open, and everting the
nematocyst. At the very end of the nematocyst filament the tiny stinging barbs flick open,
like a switchblade, and venom is pumped into the victim. This particular type
of nematocyst is called a “penetrant.” Over 30 different types of cnidocytes have been
identified, though the three most widely known are the penetrant type, which we just covered,
the volvent type, a much smaller nematocyst which contains a spineless elastic thread
that lassos prey and holds it firm, and the glutinant type, a nematocyst that is always
open and secretes a sticky mucus that is often used to anchor the cnidarian to surfaces.
Now, unlike sponges, cnidarians are diploblasts, meaning they have two unique germ cell layers.
These are the ectoderm, or epidermis that contains most of the cnidocytes and sensory
cells, and the gastrodermis, or inner “digestive layer” that secretes enzymes to break down food.
Between the two layers is the water-filled mesoglea that forms the hydrostatic skeleton
which provides support for many cnidarians. It’s basically a pressurized water-skeleton,
kind of like a filled water balloon. Cnidarians have no true organs
and no circulatory, digestive, or respiratory systems. This is possible because they have
such a large surface-area-to-volume ratio, such that all of their cells are only a
short distance from the surface of the body. This means that gas and waste exchange is done
simply by diffusion where solutes move from an area of high concentration to low concentration,
instead of the more elaborate processes involved with higher animals.
However, though Cnidarians lack organs, they do have a type of nervous system that consists of
a decentralized nerve net of interconnected neurons, without any kind of cephalization,
or formation of a head. In addition, Cnidarians exhibit radial symmetry. Once again,
this means they have a top and a bottom, but their bodies are symmetrical around a central point.
So if you were to divide their bodies along any of these axes, the two halves would be mirror images.
One important characteristic of Cnidarians is polymorphism, that is, they have a few
distinctly different morphologies. Cnidarian body forms can be broadly classified as either polyps,
or hydroid forms, which are adapted to sedentary life, and medusae, or jellyfish forms, which
are adapted to floating or free-swimming. The polyp form consists of tubular bodies
with a mouth on one end surrounded by tentacles and an aboral end that is attached to the ground.
The medusa form is bell shaped with the mouth centered and surrounded by tentacles.
Some non-colonial Cnidarians exhibit both forms at different points in their lives,
like the jellyfish that reproduce sexually in the medusa form and asexually in the polyp form.
Other Cnidarians, like the anemones and corals are only polyps and do not exhibit polymorphism.
Colonial hydrozoans, which are made up of zooids, can exhibit varying degrees of polymorphism.
Hydractinia, for example, is made up of five types of polyps, and each serves a particular
function such as feeding, reproduction, or defense. Some, like the Portuguese man o’ war,
look like an individual jellyfish, but they are actually colonies of zooids.
Cnidarians are abundant in shallow marine habitats, especially in warm waters near
the equator. Some species thrive deep at the ocean floor, and others are found only in
polar regions. A few species, most notably those within the genus Hydra, are found in freshwater.
Some Cnidarians live symbiotically with other animals, like the anemones that make up the
“gloves” of boxer crabs, the Hydractinia that live on the shells of hermit crabs,
or the Magnificent sea anemones which once again, house the famous clownfish within their tentacles.
Others, like the reef-building corals and some species of freshwater hydra,
have mutualistic relationships with algae that live within their cells.
Other Cnidarians, however, do not have symbionts and must feed upon other animals. Of these
carnivorous Cnidarians, most eat small crustaceans and zooplankton, such as larval animals and tiny
invertebrates. Some, rather, are parasitic, while still others prey on larger fish species. They do
so by paralyzing their prey with their nematocysts and dragging it into their gastrovascular cavity
where it is digested alive by gland cells that secrete lubricating mucus and enzymes.
The prey is slowly reduced to a soupy broth that is absorbed into the body cells through diffusion.
Unlike most bilaterally symmetrical animals, which include everything from humans to earthworms,
cnidarians only have one opening. This means that their mouth is also their anus. The mouth opens
into the gastrovascular cavity which functions in digestion,
gas exchange, metabolic waste excretion, and reproduction. Any food items which are not
digested are expelled out through the mouth, as are sperm and eggs during mass spawning events.
Spawning, the release of sperm and eggs into the water column, is driven in Cnidarians by
the external environment. Individuals from the same species generally release their gametes
at the same time, thus increasing the chance of external fertilization. Once an egg is fertilized,
it becomes a zygote that develops into a planula that swims or crawls by means of cilia. Then this
planula settles into the substrate and develops into a polyp. In addition to sexual reproduction,
Cnidarians can reproduce asexually through budding, and in some species by splitting in two.
Lastly, all Cnidarians have the ability to regenerate. Some can only regenerate small
pieces of their bodies while others are considered to be biologically immortal, meaning they do not
deteriorate with age. Though most Cnidarians, much like humans, have a relatively “fixed” lifespan,
those that are biologically immortal do not senesce and can potentially live indefinitely.
We will revisit this concept and many others in more detail
over the next several tutorials, as with the general overview of the phylum complete,
it’s time to take a deeper dive into the various taxa within the phylum.