What tells anemones to kill their enemies?

What tells anemones to kill their enemies?

Beneath the ocean surface, a tiny marine vessel receives orders to fire. A hatch blows open and a lethal payload blasts free. Within three-thousandths of a second, the missile reaches a velocity of 2 meters per second -- more than 10,000 times the acceleration experienced by shuttle astronauts at liftoff -- before crashing through the armor of its target: a tiny brine shrimp barely visible to the naked eye.

Welcome to the world of cnidarians--a family of sea anemones, jellyfish and other marine invertebrates that kill their enemies and prey by firing poisonous, microscopic projectiles called nematocysts. Scientists know little about cnidarian launch mechanisms. But new research tells a lot about the chemical and mechanical receptors that trigger an anemone's decision to fire.

The work provides the first evidence that some cnidarians, which are virtually blind, can "hear." They have a good ear, too, apparently differentiating among various planktonic pedestrians by the audio frequency of their "footsteps." Moreover, cnidarians appear unique among animals for their use of a primitive sense of smell to fine-tune their hearing organs to a few key frequencies.

Scientists have known for decades that cnidarians fire nematocysts in response to a combination of chemical and tactile stimuli. Glen M. Watson and David A. Hessinger, both of Loma Linda (Calif.) University School of Medicine, looked at the relationship between the chemical and mechanical receptors on the small sea anemone Haliplanella luciae. First they whetted the anemones' appetites by adding doses of N-acetylated sugars to their seawater. The sugars are the building blocks of chitin--the tough outer shell of shrimp and other anemone foods. Like odor molecules binding to smell receptors in the nose, these sugars bind to chemical receptors on cells adjacent to nematocyst-firing cells.

Then the researchers touched the anemones with a tiny, vibrating, gelatin-coated probe and counted the number of nematocysts shot into the probe under varying conditions.

They found that in the absence of the sugars, a 55-hertz vibration triggered the biggest volley. But with the sugars, firing was best triggered by a 5-hertz vibration -- the same frequency generated by the swimming movements of the anemone's favorite shrimp. "Nobody knew that these things were sensitive to vibration," Hessinger says. "We knew they were sensitive to contact, but this is the first time that, in a sense, we can say they hear."

More intriguing, he says, is the link between the anemone's chemical and mechanical receptors. The creature senses vibrations through tiny, hair-like cilia. When the anemone's chemoreceptors "smell" chitin-like sugars, these cilia instantly lengthen by as much as 70 percent, making them sensitive to 5-hertz frequencies instead of 50-hertz.

"I don't know of any case where any receptor can modulate the response frequency of a mechanoreceptor," Hessinger says, noting that similar sensory hairs, such as those in the human ear, have fixed lengths and respond to fixed frequencies. Apparently, he says, the anemone is "tuned to an unrealistically high frequency unless it smells the prey first. Once it gets a whiff, the tuning occurs rapidly, and it's ready to respond to the proper vibration should the prey be close enough to bring about a triggering."

Hessinger speculates that the receptor interactions evolved as a sort of "on-off" switch that helps prevent unnecessary firing of "expensive" nematocysts. "Nematocysts are very complicated structures, and it costs the anemone something to make them," he says.

The findings, described in the March 24 SCIENCE, are "pretty exciting," says Richard Mariscal, a marine researcher at Florida State University in Tallahassee. "This could be a real breakthrough to understanding the combined mechanical-chemical stimulus for a very important prey-capture structure in these animals."