EStrange cracking and crackling can be heard when pistol shrimp cavort in coral reefs, seagrass meadows or other shallow water biotopes. The little animals rightly bear their name: In the form of the larger of their two scissors, they actually have a kind of water pistol: With a special mechanism, they can close these scissors in a flash. The negative pressure in the water jet that shoots out becomes so great that an air bubble is created. This cavitation bubble then immediately implodes again with a loud bang. The crabs are less concerned with this bang effect than with the resulting pressure wave: the pistol shrimp use this to incapacitate their prey.
Smaller than a brown crab
A maximum of five centimeters long Alpheus heterochaelis, also known as the cracker shrimp – and thus smaller than the North Sea shrimp, which is known in this country as a crab. Of the more than 600 different pistol shrimps that populate coastal areas, especially in the tropics and subtropics, this species, which is distributed from North Carolina to Brazil, is the largest. It can shoot with corresponding power - and it does: off the east coast of America, the animals duel with their own kind in order to vigorously defend their territory and their shelter. Disputes of this kind are the order of the day.
This leads to unfortunate consequences. Because what the pressure wave generated by the banger shrimp does to the nervous system in particular is shown by the prey hit: worms, but also small crabs and fish are immediately stunned by the shots, if not fatally injured.
Visor in head armor protects against brain trauma
But in a duel with conspecifics, firecrackers don't seem to be harmed, even if one fires his shot just a centimeter in front of the other's head. Apparently, a visor integrated into the head armor saves the banger from an imminent brain trauma. This is reported by biologists led by Alexandra Kingston from the University of Tulsa, Oklahoma, and Sarah Woodin from the University of South Carolina in Columbia in the journal "Current Biology".
To find out what advantage the transparent shield over the crab's eyes offers during a gunfight, the researchers removed it from some test animals. If these firecrackers were then exposed to three shots from a conspecific, they then showed clear signs of neurological damage: On average, the animals needed much more time than usual to find their way back to their shelter. If they were turned onto their backs, they also took longer to get back on their feet. Firecrackers with an intact visor, on the other hand, were just as quick after being shot at three times as their peers who were spared the shots.
Sagittarius gets an equally strong pressure wave
The measurements showed that the visor, which protrudes over the eyes of the pistol shrimp, halves the strength of the blast wave. Apparently that's enough to avoid nerve damage. The head armor not only protects against slam attacks: even if the crab has fired a shot with its peculiar water pistol, nothing will happen to it. Although, as Kingston and colleagues discovered, the gunner is hit by a blast wave that is just as powerful as his target.
Surprisingly little of the violent pressure fluctuations arrives behind the transparent protective shield. However, the visor only provides optimal protection when it is open at the front. If the opening is closed artificially, the damping is much lower. Apparently, when the shock wave reaches the shield, pressure differences between inside and outside cause water to ooze out from under the visor. Much of the kinetic energy is thus deflected away from the shooter's head.
With this trick, the pistol shrimp can protect their eyes and brain from excessive pressure waves. The fact that nerve tissue reacts particularly sensitively to such mechanical stress applies to both the large human brain and the tiny one of pistol shrimp. What evolution has brought about in these crustaceans may therefore be a good inspiration for engineers who design protective helmets for humans.