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Describes a close and long-term relationship between different species.

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Are Backbones All They're Cracked Up To Be
Teacher Contributed

Are Backbones All They're Cracked Up To Be??

A comb jelly from the phylum Ctenophora

Why It Matters

That's Not A Jellyfish!

People love their cute furry mammals, and we certainly appreciate other types of animals with backbones like birds and fish. But did you know of all the 34 animal phyla, animals with backbones occur in only one phyla (Chordata). Of the 34 animal phyla, most of the organisms we think of as animals occur in only one. Invertebrates are breathtaking in their diversity. The variety of their body plans tell us much about the many different ways animals can make a living. They tell us much about ecosystems and how animals interface with their environment. Looking at the fossil record, they present intriguing information about how animals change and develop through time. Yet many of them are unknown to most people. Take ctenophores for example or "jellyfish" as they are often called; the problem is ctenophores aren't jellyfish. They are ctenophores, or if you prefer "comb jellies". They may bear some superficial resemblance to jellyfish, but when you look closely, they are very different animals. Thanks to ctenophores appearing in aquariums around the world, people are starting to learn more about these intriguing animals. But not all ctenophores have made it into aquariums yet. Watch the below videos to see some truly unique ctenophores which people do not mistake for jellyfish. Nope, people don't mistake the benthic ctenophores for jellyfish, but some people do mistake them for flatworms (Phylum Platyhelminthes), which aren't really worms! Some phyla just can't catch a break!

Here you can see ctenophores living symbiotically on the sea star Echinaster callosus. Trace the tentacles back to their source to identify the ctenophores. It is not certain if this relationship is harmful to the sea star (that would be a parasitic relationship).

Take special note of the branching tentacles seen in this video.

Explore More

Use the resources below to answer the following questions:

  1. How big can ctenophores grow? Where do these types of ctenophores occur? What problems exist in studying these ctenophores?
  2. How do ctenophores feed? Do they all feed in the same manner? Explain your answer as fully as you can.
  3. How did the invasion of a ctenophore lead to the collapse of Black Sea fisheries? How was this species of ctenophore introduced to the Black Sea?
  4. Why does being hermaphroditic help ctenophores invade new ecosystems? What other characteristics do they have that can be useful for invasion?
  5. At one time, ctenophores were grouped together with cnidarians but they are now considered separate phyla. Compare and contrast the characteristics of the phylum Ctenophora and the phylum Cnidaria.
  6. Do ctenophores have a thru-gut (meaning both a mouth and an anus)? Explain your answer as fully as you can.

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