Simple pioneering creatures
Cnidarians sound like complicated marine creatures, but in actual fact, they’re quite simple. Simple in terms of their body structure anyway. They have no proper organs to speak of, but a very basic nerve network, and with no anus to get rid of waste, their mouth doubles up as both! They may not appear to be pioneering, but the development of cnidarians marked a turning point in our evolution history, making them possibly the most important creatures to populate our earth.
What are cnidarians?
There are over 11,000 species of cnidarians, mostly of which are carnivorous living in the marine environment. They can occur in the form of either a polyp (anchored to the sea bed) or a medusa (free-swimming), with some spending their lifecycles alternating between the two. A distinguishing feature is their stinging cells, called cnidae, which contain a capsule that can be discharged in defence or to capture prey. These extraordinary weapons can come in a variety of forms, but are usually are a harpoon like structure attached to a coiled thread that shoots out under water pressure when the cell is triggered by touch or chemical stimulus.
There are three classes of cnidaria we find here in Wales:
Anthozoa (anemones and soft corals)
These are unique in the sense that they don’t have a medusa phase, and live their entire life as a polyp. Their name translates to ‘flower animals’, which is very describes these creatures beautifully. They include sea fans and sea pens.
Hydrozoa (marine hydroids, Portuguese man-o-war, Obelia)
Most of these live in colonies and their life cycle includes both polyp and medusa phase.
Spend most of their life-cycle in the medusa phase, which is what we typically think of as a jellyfish; a large floating predator.
Before cnidarians, sponges were the only living animals on planet earth. However, around 630 million years ago, the first cnidarians appeared, set to be possibly the most pivotal moment during evolution. Cnidarians were the first animals to have a mouth attached to a stomach that could digest food, an incredible invention that would spread across the animal kingdom. Their radical developments don’t stop there either. They also created movement. Cnidarians have two sets of muscles meaning they can bend in any direction, and along with a network of nerves that send electrical impulses to the muscles, they were the first animal to be able to generate movement. It may not have looked like much to us now, perhaps a very subtle stirring or swaying, but this was ground-breaking in the sense that all creatures that have evolved to soar, swim or crawl today rely on this ancient inheritance from cnidarians.
It was originally thought that hydrozoa were the most primitive of cnidaria, yet new evidence suggests that anthozoa were among the first to develop. What we are certain of is that humans, along with many other animals have so much to thank those first cnidarians for, with many later species trialling different methods of reproduction, feeding and sensing .
So the next time you see a jellyfish pulsating by, or a swaying anemone in a rockpool, take a little minute to think “you’re an ancestor of mine”.