MadSci Network: Zoology

Re: What determines when the anemone is open or closed?

Date: Sun Mar 28 18:32:34 1999
Posted By: Tinsley Davis, Grad student, Microbiology, University of Wisconsin Madison
Area of science: Zoology
ID: 921283319.Zo

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,

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. 

Anemone info written by aquarium enthusiasts, a detailed page:

Current Queue | Current Queue for Zoology | Zoology archives

Try the links in the MadSci Library for more information on Zoology.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-1999. All rights reserved.