﻿Look out for that medusa! Coming up! Jonathan investigates the world of stingy,
squishy animals—the cnidarians! Welcome to Jonathan Bird’s Blue World!
Jellyfish—technically not fish at all. Biologists prefer to call them jellies.
Everyone has heard of them, but what are they? Jellies are squishy invertebrates, members of
a phylum of animals called Cnidarians. The phylum Cnidaria which means “stinging animal”
contains around 11,000 species worldwide. They are among the simplest of the "higher" organisms—more
complex than sponges, but not as complex as something like sea stars. The creatures in this
phylum are radially symmetrical, just like sea stars. This means that the parts of
the body extend outward from the center like the spokes on a bicycle wheel.
The Cnidarians include the hydroids, jellies, anemones, and corals. All Cnidarians have
tentacles with stinging cells in their tips, which are used to capture and subdue prey.
The stinging cells are called cnidocytes and contain a structure called a nematocyst. The
nematocyst is a coiled thread-like stinger. When the nematocyst fires, the thread uncoils,
and springs straight. The harpoon-like thread punctures through the cell wall and into the
prey. Most Cnidarians also have a toxin in their stinger that helps disable the prey.
Many Cnidarians are not harmful to humans, as the nematocyst cannot penetrate sufficiently
into human skin to inflict any harm. There are some Cnidarians however, like certain jellies and
hydroids, that can deliver extremely painful, and in a few cases, even fatal, stings to humans.
The Cnidarian can have one of two basic body types. The polypoid is
the configuration of corals and anemones, with the tentacles and mouth generally facing upward,
and the other side affixed to a substrate. In the medusoid form, the organism is a free-swimmer,
with the mouth and tentacles generally pointed down.
The hydroids, anemones, and corals are mostly all benthic animals, in a polypoid shape.
This means that they live attached to the bottom with their tentacles and mouth pointing up. The
Tealia Anemone, found in the north Atlantic, is a good example of a polypoid cnidarian.
Anemones are solitary, with each individual living alone. It sits patiently and waits for prey.
And once something wanders in to the tentacles,
the prey goes into a central mouth, then down into what is called a blind
gut. That means there is only one way in or out. Once the food has been digested,
it gets spit back out through the same mouth it went in. All Cnidarians have a blind gut.
Hydroids tend to be quite small and flower-like. They often appear as a tiny anemone on a stalk.
Both don’t let their fragile appearance fool you. They can sting their prey quite well.
There is a hydroid in the tropics that looks like peach fuzz on the reef. But it has an
appropriate name: Fire coral. It’s not actually coral, but it definitely stings like fire.
Contact with this hydroid is quite unpleasant for divers. Unlike the Tealia anemone,
coral polyps are colonial, meaning that they live in groups, connected together with living tissue,
and they share food resources. Some corals create limestone skeletons that build up
to form boulder-like structures called coral heads. Many coral heads together form reefs,
creating habitat for fish. It’s hard to believe that something so small can create structures
that can be seen from space. Cnidarians don’t have much in the way of specialized organs.
For example they have no gills. They absorb oxygen from the water directly through their skin. Having
a lot of tentacles gives them plenty of surface area exposed to the water for metabolism.
The most well-known Cnidarians are of course the members of the medusoid body form—the
jellies. Jellies drift along in the water generally with the mouth and tentacles
pointing down--a net to capture small prey. Some jellies are extremely venomous to humans.
Some, like the box jellies, can even kill people with enough contact of tentacle on bare skin,
but most are not that dangerous, producing only a mild rash and some short term pain.
All Cnidarians can reproduce asexually at some form of their development by budding off tiny
clones of themselves, or dividing themselves. But some cnidarians also reproduce sexually,
releasing eggs and sperm into the water to fertilize and disperse with the currents,
seeking new habitats. Corals in particular, which can’t move,
spread to other areas using this form of broadcast spawning.
For such simple animals, the Cnidarians make a huge impact on the oceans.
They are predators, reef-builders, and drifters. It’s hard to imagine what
the Blue World would be like without these squishy, stingy Cnidarians.
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