Random Invertebrate Day #6 Cnidarians

Today's inverts are a selection of cnidarians, a group characterised by stinging cells, bodies made up of two cell layers (in contrast to three in most multicellular animals), and radial symmetry. Above is a giant green anemone, Anthopleura xanthogrammica, taken at Brady's Beach, Bamfield, on Vancouver Island. This species has a symbiotic relationship with two algae species, which live in its body, providing it with food and giving it a vibrant green colour. Close to where I took this photo was a sea cave, and the same species living within the cave was almost white, as there was insufficient light to support the algae.

Next up is a sea nettle, Chrysaora fuscescens, taken from the Bamfield dock. This one was in the vanguard of a great swarm of sea nettles. The day I took this photo I only saw 2 or 3 sea nettles in the inlet - by the next morning, and for a week or so after, there were thousands of them.

Above is a stalked jellyfish, a member of the Stauromedusae, found in the intertidal in Victoria, BC. Although it seems to bear more resemblance to an anemone, it is actually a jellyfish that has turned upside down, and attached to the substrate at the top of its bell.

The stalked jellyfish have typically been included within the class Scyphozoa, together with the "true" jellyfish like the sea nettle above. Anemones are members of the Anthozoa, together with corals; other classes include the Cubozoa, or box jellyfish, Hydrozoa, and the parasitic classes Myxozoa and Polypodiozoa, though the evolutionary affinities of the last two have been the subject of some debate, and the matter is not yet settled.

And finally, here is a series of photos of anemones living on the underside of the Bamfield dock.

In the photo below you can also see some members of another class, the Hydrozoa. The feathery, plant-like structures in the upper left part of the photo are hydroids, a colonial form of cnidarians. Small anemone-like polyps build a home of chitin and proteins, and are joined by tubelike structures called hydrocauli.