How to recognise a pistol shrimp in the fossil record

Alpheid shrimps, colloquially referred to as “pistol shrimps”, exhibit a remarkable anatomical adaptation. These tiny marine crustaceans use their enlarged and highly modified claw to ‘shoot’ at their prey – hence their name. It is astonishing that the snapping claw evolved at least 30 million years ago. How do we know that? Because the fossils tell us.

Fig. 1. Habitus (body form) of alpheid snapping shrimps, exemplified by the extant species Alpheus thomasi from the Caribbean Sea. (Photo: Arthur
Anker.)

The famous snapping claw

Alpheid pistol shrimps represent a super-diverse group of benthic marine crustaceans (that is, living on the bottom of the sea, including the sediment surface and some sub-surface layers). There are more than 600 living species, nearly half of which belong to the genus Alpheus. Its representatives possess a snapping claw, a multifunctional tool used for various types of behaviour such as aggression, warning or defence, as well as for hunting prey. Although snapping claws evolved independently several times within various decapod crustaceans, only in pistol shrimps did this organ attain true perfection. The process of snapping involves a cracking sound reaching up to 210 decibels, one of the loudest produced by any animal. This noise originates from the collapse of a cavitation bubble in front of the claw, which, in addition, is accompanied by a short flash of light, a phenomenon known as shrimpoluminescence. With this remarkable organ, pistol shrimps can stun or even kill their prey. But, to track down the full story of the evolution of this organ, one must consider the geological past and find unequivocal evidence in the fossil record. Until very recently, such evidence was basically missing.

Fig. 2. Pistol shrimps ‘shoot’ with an enlarged, modified claw. (Photo: Arthur Anker.)

At the beginning of our story…

… strange, roughly triangular, tiny fossils were found across the world. Some researchers attributed these enigmatic objects to coleoid cephalopods, while others preferred assignment to portunid swimming crabs. Yet, there was also the idea that they might belong to pistol shrimps. However, these preliminary identifications were made rather intuitively, without rigorous analysis, and there were fundamental differences between these interpretations: sepias at one end of the scale and crabs on the other. Therefore, there was a question to be answered: to which group of organisms did these enigmatic fossils actually belong? At that time, we were not able to tell where the answer would lead us.


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