Map of Life
Convergent Evolution Online

Cavitation conditions

The formation of bubbles in a fluid is known as cavitation. Typically this occurs at low pressures, and is perhaps best known in the xylem of plants where embolisms can be destructive to the surrounding tissues. So too the “bends” suffered by a diver who ascends too quickly and without appropriate de-pressurizing leads to gas coming out of solution in the blood. It is intensely painful and can be fatal. Interestingly, study of the bone tissue of deep-diving marine reptiles such as the mosasaurs also show evidence for the “bends”. As well as due to low pressure effects, cavitation is also familiar in high energy situations and is of major interest to marine engineers because of the damage it can cause to propellers. This is a result of the formation of tiny bubbles that rapidly implode and in doing so yield a great deal of energy. Oddly enough the same mechanism has also been identified in at least two groups of crustaceans: the alpheids and mantids.

Alpheid shrimps

The first example of cavitation in crustaceans involves snapping shrimps that are know as alpheids (in which also note eusociality has evolved three times). These are well known not only because of the versatility and multi-functionality of their snapping claws, but also because of the underwater sound this makes. So intense can this sound production be that it can interfere with man-made sonar (convergent, of course, on the echolocation of bats, dolphins and birds). In at least one species, the sound evidently arises from cavitation, the water being rapidly accelerated by the abrupt juxtaposition of an extension (or plunger) of the articulatory “finger” (the dactyl) against the rest of the claw. As the water moves very quickly its pressure drops and a small bubble forms, and then implodes so releasing energy.

Mantid shrimps

The second instance is seen in the mantid shrimps, well-known for their extremely effective predation that involves the extraordinarily rapid movement of a hammer-like claw. Whilst it is clear the assault includes a percussive blow in addition the speed of impact generates a cavitation bubble, the implosion of which contributes to the literal shock tactics of this animal. Mantid shrimps are also instructive in terms of convergence in other respects, notably colour vision and also the generation of low-frequency sounds similar to the rumbles of elephants.