Day 3 of Life Beneath the Keel

On Sunday 13 September, at 1800 BST, The Island Trust is live streamingLife Beneath the Keel’ from Plymouth Yacht Haven as part of the Ocean Institute’s virtual Maritime Festival. This is the third in a series of blogs about the marine life we hope to discover then.

Day 3:  2 September 2020 | Cnidarians

Today we are looking at Cnidarians, pronounced, “ny-darians.” They have stinging cells used to capture prey and for protection. There are three groups, anthozoa (anemones and corals), hydrozoa or hydroids, and jellyfish.

Metridium dianthus anemones are quite common on pontoons (image 1). Their tentacles have stinging cells which paralyse any creature which comes into contact with them (if it’s small enough, they don’t paralyse us, although the sting can hurt) allowing them to ingest the prey through the opening in the oral disc.  Cool fact – Anemones have no bottom, so once they’ve digested what they can of their prey, they regurgitate the shell or bones or whatever’s left out of the same opening.

Hydroids take many forms. They, too, have tentacles armed with stinging cells. Some just spread themselves like a net in the water (image 2), and others have shuttlecock-like arrays of tentacles which they use to filter out food (plankton, dead stuff and poo) from water passing over it (image 3).  This is a more effective way of getting food than the sponge’s suck and squirt method, and similar devices have evolved in a number of types of animals as we will see in the next few days. They also have a variety of reproductive methods. Some produce free swimming sexually active medusa. Some are tiny, and can only be seen under the microscope (image 4), others, like the Aequorea species are larger and look so much like jellyfish that they are commonly known as Crystal jellyfish (image 5).

These, like jellyfish, properly so called live in the plankton.  You might think that all plankton is small, but as plankton includes all “ocean drifters” it includes jellyfish, some of which, like the barrel jellyfish, can be can be huge. (image 6). Their sting won’t hurt you, unlike the Portuguese man o’ war’s (image 7).

Included here, because it looks like a Cnidarian although it is Ctenophore, a different phylum, is the comb jelly, which feeds on other comb jellies, engulfing them whole (image 8). The double image comes from the reflection on the underside of the sea surface.

John Hepburn, Ocean Discoverability Project Manager
2 September 2020