In the marine ecosystem, everything has its place. Whether its detrivores on the sea floor
releasing nutrients from carrion, or top predators keeping fish populations in check, all creatures
have an important role to play. But many are overlooked. One such group is the cnidarians.
A phylum of organisms that contains corals, anemones, hydroids and jellyfish. Together
they help to uphold the diversity and stability of the entire marine ecosystem, through their
roles as predators, shelter for small organisms, and filter feeders. Let’s take a closer
look at this unique group of marine creatures.
The phylum Cnidaria contains around 13,000 living species - surprisingly simple, yet
beautiful organisms. They are classified based on a number of distinct features. For example,
they tend to be radially symmetrical, meaning that their body structures extend outward
from the centre. They are multicellular, and generally consist of simply an internal cavity
and a mouth. But cnidarians come in two basic body forms - polyp and medusa - which often
occur within the life cycle of a single cnidarian.
Generally speaking, Polyps are tube shaped and sessile with a ring of tentacles around
the mouth. Medusae, on the other hand, are umbrella or bell shaped, and can move freely
with the currents.
The life cycle of a moon jellyfish sees its larva swim down to the seabed and attach onto
rocks. There, it develops into a small polyp called a scyphistoma. Over a long period of
time, the polyp undergoes a process called strobilation, rearranging itself into a tube-like
structure that resembles a stack of coins. Each coin is in fact a tiny medusa, which
are released in turn to drift off and develop into mature adults. Some species are able
to revert to an earlier stage of their life cycle, allowing them to regenerate themselves
continuously.
Commonly called jellyfish, medusas possess tentacles that hang down below the umbrella.
Their diversity is staggering. The largest, the lion’s mane jellyfish, has tentacles
that grow longer than 27 metres - longer even than the blue whale. Many are slow-moving
planktonic animals - meaning they aren’t efficient at actively swimming and must rely
on ocean currents to help them move. But don’t be fooled by their apparent placid nature.
Jellyfish have long been described as being the most important predators in the seas.
Long tentacles are used for a range of functions, including catching food, and attacking using
stinging cells called nematocysts.
It is these nematocysts which unite nearly all members of the phylum Cnidaria, from corals
and anemones to jellyfish. They are a unique feature, a capsule containing a coiled threat
which can be shot outwards to catch prey, or repel predators. A modified flagellum acts
as a sensory trigger, rapidly ejecting the nematocyst thread when it’s touched.
This adaptation is an advantage to organisms that cannot actively hunt their prey. Instead,
they adopt ‘sit and trap’, or ‘float and trap’ strategies, using their nematocysts
to stun or kill prey that collide with their transparent tentacles. One of the deadliest
examples of this is the Portuguese Man of War, a colony organism consisting of many
small animals called zooids working together. It is made up of four separate polyps. The
uppermost, a gas-filled bladder called a pneumatophore, sits atop the water like a ship at full sail.
The tentacles are the man-of-war's second polyp. Long, thin tendrils that extend 50
metres (165 feet) below the surface, covered in powerful nematocysts used to kill small
creatures. Once a prey animal is captured, muscles in the tentacles draw prey up to a
polyp containing the digestive organisms called gastrozooids. The fourth polyp contains the
reproductive organisms.
Many cnidarians produce colonies like the Man of War, composed of tiny creatures. Siphonophores
are abundant in the open ocean, consisting of medusa or polyp-like zooids. On the sea
floor, sedentary colonies of polyps secrete a calcium carbonate skeleton to provide a
rigid structure. These are the corals. Perhaps the most important cnidarians in the marine
ecosystem, building expansive reefs that support a huge diversity of creatures. Their rocky
structures provide shelter for reef fish and invertebrates, and protect coastlines from
the damaging effects of waves and tropical storms.
They also play a part in the carbon cycle as a carbon sink, sequestering carbon in their
skeletons. In doing so, they keep this carbon from returning to the atmosphere. Even after
they die, the stony structures can store it for thousands of years.
Coral polyps form a number of important associations with other organisms. Notably, stony corals’
survival depends upon a symbiotic relationships with tiny algae that live in their tissues,
called zooxanthellae. The algae use sunlight to produce sugars via the process of photosynthesis,
and provide the corals with energy. In return, the corals provide protection for the algae
within their tissues. It is this mutualistic relationship that upholds reefs, and consequently
supports 25% of all known marine species.
Another relationship forms between crabs, and sea anemones that live on their shells.
This association benefits the anemone by providing it with transport, and giving it better chances
of finding food. The sea anemone protects its host from predators in return, of course
by making use of its stinging nematocysts. Similarly, anemone fish like clown fish rely
on symbiosis with cnidarians like anemones. The fish have developed immunity to the stings
by secreting a thin layer of mucus over their bodies. Without it, the clownfish would be
stung to death and consumed by the anemone.
Other fish have a similar association with larger drifting medusae. Those that are immune
can take shelter among the jellyfish’s tentacles, baiting the trap for other fish. In all these
examples, we see creatures from vastly different groups depending on cnidarians for survival.
This goes to show just how important jellyfish and their relatives are in increasing biodiversity
in the oceans.
Overall, there are so many different kinds of cnidarians that they have many points of
importance. Coral reefs are formed of many millions of cnidarians and are home to staggering
biodiversity. Jellyfish are important predators in the open ocean food web, but they serve
as food for many other species as well. Sea anemones fulfil a similar role but in the
shallows, also serving as hiding places for clownfish. Thus, this unique and abundant
group of creatures is integral to the function of many marine communities, from the open
ocean to coasts and the benthic zone of the deep sea.
To find out more about the deep sea, and to visualise just how deep some of these creatures
live, visit the ‘exploring the zones’ page over on our Deep Sea Hub, where you can
scroll down through a scale model of our oceans.