Crinoids, a class of marine animals in the phylum Echinodermata, are pretty creatures. The photo at right shows a crinoid perched on a Malaysian reef with its fluffy arms extended for feeding. Looking at the photo, it’s easy to see how they acquired their common name, Feather Stars.
This is how divers usually see crinoids, and this is how they are most often photographed. As pretty as crinoids are — and they come in a vast array of colors — photographs of the whole animal don’t reveal much about the animal’s structure or behavior. Whole-animal photos of creatures like this should be just a starting point. A photo study of an animal, or class of animals, should include close-up and macro photos as well.
Today I present three close-up photos of crinoids that show something of their structure. Tomorrow I will continue with some macro photos that reveal even more about crinoids.
In the photo above, you can see the arrangement of the animal’s feathery, many-jointed arms around a central disc. The animal’s mouth is in the disc, which is covered by many cirri. Crinoids are filter feeders. They capture tiny bits of organic matter suspended in the water that passes over the pinnules on their arms. The food is then transported inward, down the length of the arms to the mouth.
This second close-up photo is an aboral view of the same individual. (It can be difficult to photograph this view, but this very accommodating individual happened to choose a protruding strand of wire coral as its perch, so I was able to swim around behind it to take the photo!) Here we have a good view of the form and function of the cirri on the underside of this crinoid. These cirri are used for mobility: the crinoid can skitter along using the cirri like little feet. Then once the animal finds a suitable perch, it uses its cirri to hold itself in place. In this case, the cirri are wrapped around the wire coral in a tight grasp.
As filter feeders, crinoids optimally position themselves in places where currents — tidal and otherwise — pass over them. The cirri of crinoids have tiny hooks at the ends, which help them to hold on and stay in place during feeding. While this may be functional for the crinoids, should they decide to perch on a soft surface, those hooks on the cirri can do damage. In this final photo (a different individual and probably a different species than the previous two photos), a crinoid has decided to perch on a sponge and has dug into the sponge’s flesh with its cirri. As you can see, the sponge has been damaged (although it will likely heal over in time).
About the photos: The crinoid in the image at the top of the page was photographed at Pulau Sipadan, Malaysia. The three close-up images were taken at Manado Tua, off the northeastern tip of Sulawesi Island, Indonesia.