Pistol shrimps: How to recognise them in the fossil record

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Matúš Hyžný (Slovakia), Andreas Kroh (Austria), Alexander Ziegler (Germany) and John WM Jagt (The Netherlands)

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.

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

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.

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.

Fig. 3. An array of fossils representing pistol shrimp claw fingertips. The one in the rectangle originates from the fixed (unmovable) finger, whereas all others are tips from the movable finger. (Photo: Andreas Kroh.)

What does a fossil pistol shrimp look like?

Although modern pistol shrimps have been studied extensively, their fossil representatives have so far received only little attention. The explanation for that is relatively simple – there was simply no idea what alpheid remains would look like when preserved as fossils. As in several other crustacean groups with only a lightly mineralised exoskeleton, the only structures of the body known to fossilise are the strongly calcified claw fingertips.

Fig. 4. Claw fingertips (on the right) are the only fossil remains of pistol shrimps (on the left). The X-ray image (in the middle) shows why – because of heavy calcification of this part of the claw. (Images by Arthur Anker (left), Alexander Ziegler (centre) and Andreas Kroh (right.))

Could the strange triangular fossils truly be the claw fingertips of pistol shrimps? Using a set of invasive and non-invasive techniques, together with our colleagues, we were able to answer this question and showed that the enigmatic structures are indeed alpheid remains. Interestingly, we observed a fundamental difference in the microstructure of modern alpheid claw fingertips in comparison to the remainder of the claw, as well as in the chemical composition of the respective parts, explaining the preferential fossil preservation of the well-calcified claw fingertips.

The ancient origin of the snapping claw

The result of our analysis (published last year in the academic journal Scientific Reports) was that numerous remains previously assigned to various other groups of marine invertebrates, including cephalopods and crabs, in fact represent a compelling match with modern alpheid species. And we realised along the way that the oldest fossils attributable to alpheid pistol shrimps are actually almost 30 million years old.

Fig. 5. The shape of fossil claw fingertips matches those of modern snapping shrimps perfectly. Here, the claw of the extant species Alpheus armatus on the left is compared to a 13-million-year-old counterpart from present-day Slovakia on the right (SEM micrograph). (Images by Andreas Kroh (left) and Ján Schlögl (right).)

This was rather unexpected, because, prior to publication of our study, researchers considered the super-diverse pistol shrimps of the genus Alpheus, with some 300 living species, to be the result of a relatively recent radiation event. However, in our samples, we identified several distinct morphologies representing the remains of more than one species. Thus, it can now be stated with confidence that a substantial part of the alpheid snapping claw diversity we observe today had already evolved about 20 to 23 million years ago. The intricate snapping claw is therefore much older than previously suspected.

More questions to be answered

So far, the emphasis of our research has been the identification of pistol shrimps in the fossil record. Now that we know how to identify their fossil remains, we obviously wish to answer more questions. For example, in selected modern pistol shrimp species, a distinct sexual dimorphism in the shape of the snapping claw fingertip can be observed.

Fig. 6. Sieved palaeontological samples can be full of fossilised claw fingertips from pistol shrimps. (Photo: Andreas Kroh.)

One may ask when did this evolve and how? Yet another issue to be studied in more detail is the distribution of pistol shrimps in space and time. Today, alpheids are widespread across the tropics and subtropics. Apparently, this has not always been the case. There must have been a starting point to the extremely successful evolution of pistol shrimps and this ‘moment’ must have occurred within a limited geographical area. Where exactly did this happen? The oldest pistol shrimp remains known thus far are those from almost 30 million years old rocks in the USA. Are there any older alpheid fossils waiting to be discovered? Please take a close look at the photographs accompanying this article – you might actually have pistol shrimp remains in your own fossil collection. And maybe these specimens could help to elucidate the full story behind the evolution of the alpheid snapping claw…

Further reading

Hyžný, M., Kroh, A., Ziegler, A., Anker, A., Košťák, M., Schlögl, J., Culka, A., Jagt, J.W.M., Fraaije, R.H.B., Harzhauser, M., Van Bakel, B.W.M. & Ruman, A. 2017: Comprehensive analysis and reinterpretation of Cenozoic mesofossils reveals ancient origin of the snapping claw of alpheid shrimps. Scientific Reports 7: 4076.

Lohse, D., Schmitz, B., Versluis, M. 2001: Snapping shrimp make flashing bubbles. Nature 413: 477–478.

Versluis, M., Schmitz, B., von der Heydt, A., Lohse, D. 2000: How snapping shrimp snap: through cavitating bubbles. Science 289: 2114–2117.

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