The animal featured in a recent Guess the Creature feature is a species of Cnidarian - Cnidaria is the animal phylum that includes corals, jellyfish (also called jellies or sea jellies, since they're not really fish!), sea anemones, sea pens and hydrozoans.

With thousands of species, Cnidaria contains a very diverse set of creatures. But there are several things that all cnidarians have in common:

• They are radially symmetrical, meaning their body parts are arranged around a central axis, like the spokes of a wheel.
• Their bodies are made up of two layers of cells - the epidermis, or outer layer, and the gastrodermis (also called the endodermis), which lines the gut. In between is the mesoglea, a jelly-like substance that's most obvious in jellyfish. They also have a digestive cavity with one opening, which the cnidarian uses both to eat and expel wastes.
• They have cnidocytes, which are cells that may contain stinging nematocysts. Depending on the species, the sting of a cnidarian can be hazardous or even fatal to humans. Stay tuned to future posts to learn about the cnidarians with the most dangerous stingers!

Learn More:

• Cnidarian Profile
• Cnidaria Definition

Image: Giant Green Anemone, Courtesy Jerry Kirkhart, Flickr

Do you know this species?

Recently, this blog has focused on bony fish, one of two types of fish. The other is cartilaginous fish - that term may not be as familiar as the creatures it describes - sharks, skates, and rays - all fish whose skeleton is made out of cartilage, rather than bone.

Some other characteristics about cartilaginous fish:

• They have gill slits, rather than a bony plate covering their gills. They may also breathe through spiracles.
• They have skin covered with dermal denticles, modified teeth that are different from the flat scales found in bony fish.

Click here to learn all about cartilaginous fish.

Image: Bigeye Thresher Shark, courtesy NOAA

As several readers correctly guessed, the image in the last creature feature was of an ocean sunfish, or Mola mola.

These fish, which are often described as looking like half their body is missing, are the world's largest bony fish species - they can reach lengths over 10 feet and weights of about 5,000 pounds.

The ocean sunfish's scientific name is Mola mola. The word mola is Latin for millstone - a heavy, round stone used to grind grain, and a reference to the sunfish's circular shape.

One seemingly peculiar behavior of this fish is 'basking in the sun' up at the ocean surface. We don't know exactly why sunfish do that, but there are a couple major theories - the sunfish lies in the sun to warm up and recharge its oxygen stores in the warm, oxygen-rich water after deep dives to find food, or perhaps it lies at the surface to encourage seabirds and fish to help clean the parasites off its body. Sunfish can host up to 40 different kinds of parasites, and they can be fatal to the fish.

Have you seen an ocean sunfish in the wild, or in an aquarium? Tell me about your experience in the comments below!

Learn More About Ocean Sunfish:

• Ocean Sunfish Profile
• 10 Facts About Ocean Sunfish

Image: Ocean Sunfish in the Gulf of Maine, © Dianna Schulte, Blue Ocean Society for Marine Conservation