“You cannot teach a crab to walk straight” Aristophanes, 446-386 BC, the ‘father of old comedy’
Do Crabs Hear?
The crab is one of approximately 50,000 species described as crustaceans, ranging from small animals 0.1 mm long, up to the Japanese spider crab with a length of 3.8m. There are fossils which have been dated back to the Cambrian age, so this group of animals has an impressive track record. They are also invertebrates (animals without a backbone), which means they have no bones or cartilage – which is what forms ears as we know them. Therefore, all crustaceans lack ears similar to human beings. Instead, crabs are equipped with tiny microscopic hairs covering their shells. These hairs detect changes in water pressure and transform these changes into a signal that is sent to the crab’s nervous system.
The knowledge related to hearing and sound communication for invertebrates is limited. Some invertebrates produce sound, which could mean that they also have hearing capabilities. In 1971 Kenneth Horch made a study on the hearing of the ghost crab (ocypode), and measured hearing curves between 0.4 and 3 kHz. He found that ghost crabs in air proved to be sensitive not only to tones presented by the speaker but also to talking, whistling, and playbacks of field recordings of crab sounds. Furthermore, he found that the legs were the most sensitive part of the animal to sound, with the fourth pair of walking legs generally the most sensitive. The experiment showed similar responses for vibration and sound, with the best sensitivity between 1 and 3 kHz.
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.
Mating Dances Use Sound
In several species of American Fiddler Crabs, males use both visual and acoustical signals to find a mate. First, they perform their mating dance by waving their large claw in specific patterns. Then, at night, the male produces sounds from just inside his burrow to attract females. These 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. The 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. The second sound is a ‘long drumroll’ and is used under different circumstances.
This acoustic communication not only reaches female crabs, but is also heard by other male crabs. When other nearby males hear the mating calls of other males, they then increase their amount 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 the way from the open ocean to the coast?
Jeffs et al. (2003) used artificial underwater sound sources to study if the larval and post-larval stages of coastal crabs were attracted to coastal reef sound. The results demonstrated that the pelagic stages of crabs respond to underwater sounds and that they may use these sounds to orient themselves towards the coast. The orientation behavior 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.
Effect of Sound on Crabs
There are very few studies where the focus has been to study the effect of sound on crabs. A recent comprehensive study was performed by Christian et al. in 2002, in which they studied snow crab behavior using air gun sources of 40 and 200 cubic inches. The purpose of the test was to examine a number of health, behavioral, 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. However, in the field the video camera showed that crabs on the sea bottom gave no visible reactions to a 200 cubic inch air gun array being fired 50m above them.
Effects on eggs were also conducted in this study. Here the researchers found that exposure to high levels of sound (221 dB) may retard the development of eggs. However, it is stated that this result needs further investigations.
The overall conclusion from the Canadian study is that no obvious effects were observed on crab behavior, on the health of adult crabs, and on experimental commercial catches. Despite this, it should be noted that this study was not conducted during normal seismic acquisition, and the total number of seismic signals to which these crabs were exposed was therefore less than would occur during a normal seismic acquisition. However, it should also be noted that the air gun exposures during each study trial were characterized by higher energy levels than those to which crabs would be subjected during normal seismic activities. Snow crabs tend to naturally occur in somewhat deep water so considerable signal attenuation has occurred by the time the sound reaches the crab on the seabed.
In conclusion, therefore, we can say that there is limited knowledge of the effects of seismic acquisition on crabs. Even knowledge about the hearing capabilities of various types of crabs is limited. It is commonly agreed that crabs respond to acoustic sound, and, for instance, the ghost crab has a maximum hearing sensitivity around 1–3 kHz. It is therefore likely to assume that crabs notice seismic activity. Initial experiments performed in Canada showed practically no behavioral or other impacts on adult crabs.
Acknowledgement: Many thanks to John Christian for discussions and help.
J. R. Christian, A. Mathieu, D. H. Thomson, D. White and R. A. Buchanan, Effect of Seismic Energy on Snow Crab (Chionoecetes opilio), report for National Energy Board, Calgary, Canada, 2003.
A. Jeffs, N. Tolimieri and J. C. Montgomery. Crabs on cue for the coast: the use of underwater sound for orientation by pelagic crab stages, Marine and Freshwater Research 54 (7) 841 – 845, 2003.
Horch, K., 1971, An organ for hearing and vibration sense in ghost crab ocypode, Z. vergl. Physiologie, 73, 1-21. A.K. Woll, The Edible Crab, Møreforskning, 2006.