A Symbiotic Relationship

Years ago I studied Sea Stars for an article I was preparing to write. Unfortunately, I never got around to writing it due to uncontrollable circumstances, but what I learned was enlightening and a well of information I have been able to refer to since.

As I noted the numerous amount of ophuiroids (brittle stars), a relative of asteroidea (sea stars), during the recent NOAA Northeast U.S. Canyons Expedition 2013 I realized some of them are not nearly as brightly colored and appealing to look at as I once thought.

Picture courtesy of NOAA Ocean Explorer

This is a picture of a mussel bed. There is an adorable octopus but there are also many ophiuroids lying about. If you look closely you can see them lying across the mussels; they are long legged and spindly. Now not all ophiuroid look so plain but these found at over 1,000 meters in the North Atlantic are.

Brittle stars are very similar to sea stars in that they are both pentameric, a form of radial symmetry where the appendages are arranged around a central axis, among other things. The difference in their structure is that their arms are very distinct from their central disc, whereas in some species of sea star the disc is almost indistinguishable from the arms. Take a look at the picture below to see the underside of a brittle star.

Picture courtesy of NOAA Ocean Explorer

If you’ve read anything about sea stars you will have found that their main method of locomotion is through their tube feet. Water is pumped into the water vascular system that runs throughout the body via the madreporite, a porous pinprick usually on the top of the body near the perimeter of the central disk. Except in brittle stars the madreporite is underneath, along the oral surface, near the mouth.

Picture courtesy of NOAA Ocean Explorer

There are a few other differences between the two but what I noticed most was the apparent distinction in their choice of living space. It is common to see a sea star splayed out upon the ocean bottom or a rock surface but not so with brittle stars. Zoom in on a beautiful coral or sponge and what do you see but a dark serpentine creature wrapped in and out of its branches. At first I wasn’t sure what it was. Some kind of sea snake? Marine worm? No, it’s a brittle star using the coral or sponge as shelter.

My second question was the nature of this form of symbiosis. There are some species of sea star that feed on coral, like the crown-of-thorns sea star, but from what I can gather this relationship is commensal. The star depends upon the coral for shelter as well as oxygen while it collects its particles in the water that serve as its food. The coral or sponge doesn’t mind the addition to its body and everyone gets along well.

Picture courtesy of NOAA Ocean Explorer

Except in this case, I wonder if the coral finds it so agreeable. (See above picture).

Do you see the lighter color on the arms? This is the area from which the tube feet protrude. Similar to suction cups, the tube feet are essential to nearly all sea stars. Not only do they assist in movement, as I mentioned above, but they are able to “grab” food with them.

I know I have had the tendency to view sea stars as almost statue-like creatures but this study has changed my view entirely. Click here to see a video of a fast moving brittle star.

Has this post clarified a few misconceptions for you? I hope you have enjoyed reading, and be sure to leave me a comment below. I’d love to hear what my readers think!

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