Since the responses for vibration and sound were similar,
it might indicate that the ghost crab cannot really distinguish between them. However, several later studies, coupled with the fact that marine invertebrates do not have swim bladders, clearly suggest that the crab is more sensitive to vibration or particle motion. In his experiment, Horch found that the walking legs were essential for hearing sensitivity, in contrast to the claws. No change in hearing ability was observed until more than half of the legs were removed. As more legs were removed from this point, he found a gradual weakening of the response until zero when all legs were removed. Horch also reports in his paper from 1971 that the painted ghost crab (Ocypode gaudichaudii) reacts to sudden sounds such as the calls of shore birds.
3.13.2 Mating Dances Use Sound
In several species of American Fiddler Crabs, males use both visual and acoustic signals to find a mate. First, they perform their mating dance by waving their large claw in specific patterns. Ten, at night, the male produces sounds from just inside his burrow to attract females. Tese sounds begin at a low rate, and then steadily increase in frequency. In European species of Fiddler Crabs, the mating dance is similar, although the males produce two different sounds to attract a female. Te first is called a ‘short drumroll’ and is made when the male is unable to be seen by the female for a short period during his claw-waving dance. Te second sound is a ‘long drumroll’ and is used under different circumstances. Tis acoustic communication not only reaches female crabs,
but is also heard by other male crabs. When nearby males hear the mating calls of other males, they then increase their level of dancing and mating calls. It is assumed that crabs orient themselves according to
smells in the water (A. K. Woll, 2006), with visual orientation being probably of less importance. However, can underwater sound give crabs an orientation cue to find their way from the open ocean to the coast? Jeffs et al. (2003) used artificial under-
water sound sources to study if the larval and post-larval stages of coastal crabs were attracted to coastal reef sound. Te results demonstrated that in their pelagic stages, crabs respond to underwater sounds and that they may use these sounds to orient themselves towards the coast. Te orientation behaviour was modulated by lunar phase, being evident only during first- and last-quarter moon phases, at the time of neap tides. Active orientation during neap tides may take advantage of these incoming night-time tides for predator avoidance or may permit more effective directed swimming activity than is possible during new and full moon spring tides.
3.13.3 Effect of Sound on Crabs
Tere are very few studies where the focus has been to look at the effect of sound on
150 100 90 80 S1 70 E1 60
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Figure 3.97: Hearing curves for two ocy podes. Thresholds determined electrophysiologically with re cord ings made from electrodes implanted in the brains of animals suspended in air. Tones presented by loudspeaker (S) or to individual walking legs with a small earphone (E). DB SPL: sound pressure relative to 0.0002 microbar.
crabs. Some comprehensive research was undertaken in Canada by Christian et al. in 2002, in which they studied snow crab behaviour using airgun sources of 40 and 200 cubic inches. Te purpose of the test was to examine a number of health, behavioural, and reproductive variables before, during and after seismic shooting. Snow crabs reacted slightly to sound in the laboratory when sharp noises were made near them.
Figure 3.98: Photo of sensory hairs in the snow crab statocyst. The crab is equipped with at least three various hair types. One detects vibration or direct contact, another is sensitive to chemicals and the third is designed to detect pressure changes in the water. These hairs are similar to hydrophones used to record seismic signals. Experiments show that crabs do not respond to sound signals like music; however, they react instantly if you jump close to them.
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Christian et al.
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