Function of Acoustic Alarm Signals

Objectives of this study:
1.) Determine if stridulation acts to startle triggerfish during an attack or attract other triggerfish to interfere or both.
2.) Determine if nearby lobsters respond to stridulation by increasing responsiveness to potential predators.

Methods:
Objective 1: Lobsters will be tested against triggerfish under three conditions: lobsters which can stridulate, lobsters with the sound producing apparatus removed, and silent lobsters with playback of recorded stridulation. Experiments will be run in a large seawater enclosure with lobsters tethered near an underwater speaker. Attraction by triggerfish to stridulating lobsters will be tested by playback of stridulation and tracking the fish's position relative to the speaker.

Objective 2: At night, foraging lobsters will be exposed to alarm calls (stridulation) reproduced from an underwater speaker. Immediately following the acoustic signal, lobsters will be given a stimulus like that of a nearby predator; i.e., a pulse of water. I anticipate that lobsters exposed first to alarm calls respond with a heightened incidence and longer distance of escape behavior (tailflips).

Background:
Predation is a major selective force shaping prey behavior and morphology. Antipredator mechanisms include armor, weaponry, flight, startle behavior, retaliatory defense or other behaviors that disrupt attack. By making capture difficult, a potential prey may flee, find shelter or cause the predator to lose interest. Vertebrates commonly signal stress acoustically by alarm calls. Such calls hypothetically warn others (usually kin), elicit help from altruistic conspecifics, startle a predator into releasing its grip or attract competing predators. Most research on vertebrate alarm calls focuses on the cooperative context and less on selfish startling and predator-attraction hypotheses.

I am studing the function of putative alarm calls in the spiny lobster Panulirus argus. Spiny lobsters represent an ideal system in which to determine the function of acoustic signals given under predatory stress. When attacked, spiny lobsters loudly stridulate with a specialized apparatus at the base of the antennae. Although gregarious, lobsters form no long-term social groups and their extended planktonic dispersal phase obviates associating with close kin. Therefore, alarm stridulation probably evolved under the other selective circumstances.

My observations indicate that stridulation may startle a common predator, the grey triggerfish Balistes capriscus. Other triggerfish may be attracted by stridulation which then competitively interfere with each other's predatory attacks. Antipredator behavior in response to stridulation has not been previously reported in spiny lobsters.