MadSci Network: Zoology |
Greetings Amy! I hope that your field research with anemones is going well. You probably already know that anemones are cnidarians and members of the class Anthozoa. Cnidarians include jellyfishes, corals, and sea anemones; they are carnivores that stun their prey with stinging cells loaded with nematocysts. These stinging cells are activated through a simple neural net, and biologists are studying cnidarians for insight into how the nervous system evolved. While there are many species of anemones, each with its own unique habits and characteristics, here is a general description. Anemones are sessile, or stationary organisms; generally, if an anemone moves it is very slowly by creeping along its substrate. They attach their single sticky foot to a rock in the intertidal zone or shallow water and remain there, catching unsuspecting fish with their tentacles. While the anemone waits for prey it looks like a beautiful open flower, tentacles swaying in the current. However, as soon as the tentacles sense a fish, the nematocysts fire, and the tentacles contract bringing the prey toward the anemone's mouth. So as part of the answer to your question, the tentacles will close when prey comes in contact with them. Two biologists (Watson and Hessinger) presented information in 1990 that showed sea anemone tentacles responded to probes vibrating at the same frequencies that would be generated by a swimming fish. What are some other factors that determine the state of the tentacles? Listed below are some factors that may have an effect upon whether an anemone is open or closed. As you read, think of ways that you could test these variables on your species of anemone. If the anemone lives in an intertidal zone where it is exposed to air during low tide, the anemone keeps its tentacles shut, perhaps as a protective measure. For instance, it may be harder for birds or other predators to get to an anemone's interal parts if it's closed, and staying closed may prevent excess water loss. You could easily show that your species opens and closes in response to the presence or absence of water by observation or creating an artificial habitat. Some sea anemones can expose two different kinds of tentacles. During the daylight, these anemones expose organs on which the symbiotic dinoflagellates (zooxanthellae) live, and at night the nematocyst-bearing tentacles are present. Perhaps this suggests that sunlight acts as cue for opening and closing. Observation or manipulation of anemones in a tank may answer the question for your species. Also, perhaps anemones are like snakes in that they eat a big meal but they don't eat too often. Do anemones stay closed after ingesting a large meal and not open until they are hungry again? This might take patient observation to discover, but a video camera with timer would a be a helpful tool to observe the timing of feeding and opening again. You mentioned environmental factors that might affect the state of the tentacles. These factors could be pollutants or even natural substances secreted by prey that the anemone senses. Anemones tentacles do not respond only to the presence or absence of prey, but the anemone appears to exhibit spontaneous movement also. It can expose some or all of its tentacles and shorten or lengthen its base. This blebbing type of movement is made possible by a hydrostatic skeleton that uses water pressure to move ciruclar and longitudinal muscle fibers. Good luck with your project! Use this letter as a springboard to think of other factors and practical ways to observe or test them. Best wishes, Tinsley Cnidarian nervous systems: Grimmelikhuijzen, C.J.P. and J.A. Westfall. 1995. The nervous system of cnidarians. In: The Nervous Systems of Invertebrates: An Evoultionary and Comparative Approach (O. Breidbach and W. Kutsch, eds.), Birkhauser Verlag, Boston, pp.7-24. Hydrostatic Skeletons: Barrington, E.J.W. 1967. Invertebrate Structure and Function. Houghton Mifflin, NY, pp. 61-68. Frequency-specificity (just an abstract): Watson, G.M. and D.A. Hessinger. 1990. Frequency-specificity of vibration- sensitive mechanoreceptors in sea anemone tentacles. American Zoologist. 30:82A Anemone info written by aquarium enthusiasts, a detailed page: http://www.actwin.com/fish/species/anemone.html
Try the links in the MadSci Library for more information on Zoology.