Neogonodactlyus bredini with damage on its predatory appendage from another mantis shrimp's strikes! Photo by Roy Caldwell.
Ask most anyone what butterflies use their wings for or what fish do with their fins and you will undoubtedly hear an answer like, "Wings are used for flying and fins are used for swimming!" Some body parts just seem so well-adapted to perform certain functions; this is why there is a paradigm in biology that "specialized" body parts correspond to specific ways in which animals go about their daily business. In other words, specialization in morphology corresponds to specialization in ecology. A classic example of this concept is variation in the beaks of the Galapagos finches. Some finches have beaks adapted to crush hard seeds, while others have beaks specialized for eating insects.
However, not all animals seem to exhibit this pattern. The marine crustacean known as the mantis shrimp has legs, called predatory or raptorial appendages, which can produce one of the fastest movements in the animal kingdom. These raptorial appendages come in many shapes ranging from sharp spear-like appendages to hammer-like appendages. Mantis shrimp use their fast-moving appendages to crush open snails and other hard-shelled marine organisms, so they can eat the soft bodies inside. However, mantis shrimp also appear to eat other foods, like fish, which probably do not need to be smashed to bits before they are consumed. Even though they have specialized legs well adapted to smashing or spearing prey, some species may not use their raptorial appendages for this purpose. The goal of my research is to determine if mantis shrimp have diverse diets. Then if so, I will see how diet diversity correlates with raptorial appendage morphology across the mantis shrimp family.
First, a little background about mantis shrimp. Mantis shrimp are closely related to decapods, such as lobsters, crabs, and true shrimp. Even though mantis shrimp look like decapods, they actually branched off and became their own group 400 million years ago. Mantis shrimp have the most complex visual system ever reported in the animal kingdom. They are also one of the fastest swimmers in the sea, swimming at speeds of up to 30 body lengths per second — comparable to speeds measured in squid, which previously held the record.
But my favorite characteristic of mantis shrimp is of course their lightning fast raptorial appendages. Researchers in the Patek Lab at the University of Massachusetts and Caldwell Lab at Berkeley have found that a mantis shrimp’s predatory strike can move 23 meters per second (50 miles per hour) and produces accelerations that are comparable to a flying bullet! So it would be surprising if some mantis shrimp species were capable of this rapid movement, but didn't use it to catch prey. Hence, my study of mantis shrimp diets! I am using two techniques, stable isotopes and behavioral studies, to figure out which food items mantis shrimp eat.
Before I could study their diets, I first had to collect several different species of mantis shrimp and their possible prey. Most mantis shrimp live in the tropics, so I have traveled to Lizard Island, Australia and Mo’orea, French Polynesia to collect the animals. However, my main field site is in Colon, Panama where I collect at the Smithsonian Tropical Research Institute’s Galeta Marine Laboratory. After collecting, I transport all of the specimens back to the UC Berkeley Center for Stable Isotope Biogeochemistry, where I analyze the carbon and nitrogen stable isotopes of mantis shrimp and their prey.
What is a stable isotope? Let's go back to high school chemistry for a moment! A normal atom has the same number of neutrons and protons in the nucleus, but a stable isotope has more neutrons than protons in the nucleus. For example, a normal carbon atom has 12 neutrons in the nucleus, but its stable isotope has 13 neutrons. These isotopes are stable, because they do not exhibit radioactive decay over time — they won't lose that extra neutron — which means that the isotope will always have 13 neutrons in the nucleus. Researchers look at the ratio of normal atoms to stable isotopes to track diet, because the ratio of normal atoms to stable isotopes in the body of a predator can reflect the type of prey it has eaten. For example, if the mantis shrimp has a ratio of 10 carbon-13 atoms to carbon-12 atoms and the crab that you think the mantis shrimp eats has a ratio of 8, then there is a good chance that the mantis shrimp eats this species of crab. The reason why the mantis shrimp’s ratio is not exactly 8 is that there is an expected change in the predator’s ratio that occurs when the predator metabolizes the prey. You are what you eat (plus a little bit!), and stable isotopes allow us to track this pretty accurately.
Back In the laboratory, my assistants and I identify all of the prey items and stomatopods that we collected. We then take muscle tissue samples from the mantis shrimp and from the prey. We use a mass spectrometer to analyze the carbon and nitrogen stable isotopes in both the mantis shrimp and prey tissue. Finally, we compare the isotope ratios of the mantis shrimp and prey to determine who ate what. Since the mantis shrimp is what it eats, all prey items that have isotope ratios similar to the mantis shrimp’s ratios are likely a part of the mantis shrimp diet.
To confirm the accuracy of the stable isotope analyses, I also conduct behavior experiments that help me to determine which animals mantis shrimp are physically capable of eating. To do this, I stock aquaria with mantis shrimp and potential prey, and I wait to see which prey the mantis shrimp eat. So far, I have performed this experiment on only one species, but eventually I will look at many species of mantis shrimp, with different appendage morphologies, to see if mantis shrimp with different appendage shapes have different diets. Together with the isotope analyses, these experiments will give me a good picture of mantis shrimp diet and ultimately lead to an in-depth understanding of the relationship between raptorial appendage morphology and diet across the mantis shrimp family. This fall, I’ll return to Panama to complete my field experiments, so stay tuned for updates in future blog posts!
To learn more about Maya's research, watch the Paleo Video Field notes: Collecting stomatopods on the Great Barrier Reef.