An international team of scientists has found fossil evidence for the evolution of the snapping claw
Snapping shrimps, also known as pistol shrimps, exhibit a remarkable anatomical adaptation: these marine crustaceans use a snapping claw to “shoot” at their prey. Although snapping shrimps have been studied extensively, their fossil ancestors have so far received little attention. A new study by an international team of researchers supported by the Austrian Science Fund now fills an important gap in our understanding of the evolution of snapping shrimps. Using a set of invasive and non-invasive analytical techniques, the team presents the first comprehensive identification of snapping shrimp fossils. The oldest record is documented from about 28 million years old rocks, which establishes a novel minimum age for the entire group.
By: CU Public Relations Office
Snapping shrimps (Malacostraca: Alpheidae) represent a super-diverse group of benthic marine crustaceans. Due to their extraordinary morphological adaptations and remarkable diversity, alpheids constitute one of the model groups for studies on the evolution of complex structural, ecological, and behavioural traits among marine organisms. The most characteristic feature of alpheids is the snapping claw, a specialised appendage (see Figure). “The chelipeds of alpheids exhibit a unique combination of morphological characters not present in any other decapod group”, explained Dr. Matúš Hyžný from the Comenius University in Bratislava. This extraordinary organ is a multifunctional tool used for various behaviours such as aggression, warning, or defence, as well as for stunning and killing prey. The process of snapping involves a loud cracking sound originating from the collapse of a cavitation bubble and a short flash of light, a phenomenon known as shrimpoluminescence. However, reliable fossil material that would permit studying the evolution of the alpheid snapping claw more comprehensively had previously not been identified. The main question of the international research team therefore was: when exactly did snapping claws evolve?
As in several other crustacean groups, the only structures of the body to fossilise are the strongly calcified fingertips of the claws. “The poor fossil record of snapping shrimps is a direct consequence of low fossilisation potential of small-sized decapods and difficulties in attributing fossil remains to alpheids and not to other shrimps”, opined Dr. Hyžný. However, unless unequivocally identified, the precise dating of the evolutionary origin of the snapping claw remained difficult. The present study demonstrates that the well-calcified fingertips of alpheid snapping claws are not uncommon in the fossil record and should be considered a novel type of mesofossil. More precisely, the novel data show that numerous remains previously assigned to various other groups of marine invertebrates, including cephalopods and crabs, actually represent a compelling match with modern alpheid species. “Using a set of state-of-the-art techniques, our study provides the first comprehensive morphological and chemical examination of these enigmatic remains”, clarified Dr. Alexander Ziegler from the Bonn University, and further explained: “We observed a fundamental difference in the microstructure of 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 preservation of the claw fingertips.” Based on their results, the team of scientists can show that the oldest fossil record of snapping shrimps is 27–28 million years old, thus providing a novel minimum age for the entire group.
Within the studied fossils scientists identified several distinct morphologies. Based on this variation, the team concluded that remains of more than one species are present in studied samples. “It can be stated with confidence that a substantial part of the snapping claw diversity we observe today had already evolved 20–30 million years ago”, concluded Dr. Andreas Kroh from the Natural History Museum in Vienna.
Hyžný M, Kroh A, Ziegler A, Anker A, Košťák M, Schlögl J, Culka A, Jagt JWM, Fraaije RHB, Harzhauser M, van Bakel BWM, Ruman A (2017) Comprehensive analysis and reinterpretation of Cenozoic mesofossils reveals ancient origin of the snapping claw of alpheid shrimps. Scientific Reports 7: 4076, DOI:10.1038/s41598-017-02603-5
Publicly available resources:
Project 2524. MorphoBank, DOI:10.7934/P2524