The Feeding Morphology and Ecology of Stomatopod Crustaceans
The paradigm that animals with specialized feeding morphology consume specific prey types is central to current understanding of ecological and evolutionary processes seen in nature. Mantis shrimp, or stomatopod crustaceans, are often hailed as having highly specialized feeding morphology; their raptorial appendages produce among the fastest, most powerful strikes ever reported in the animal kingdom, allowing species to capture fast-moving prey or to crush hard-shelled prey. While all stomatopods have appendages that produce fast movements, their appendage forms differ dramatically, which has led researchers to divide stomatopods into two groups: spearers that unfurl streamlined appendages to capture soft-bodied, evasive prey, and smashers that generate enough force to crush hard-shelled prey with hammer-like appendages. Smashing appendages are thought to be more specialized for generating high speeds and accelerations, but some smasher species have been observed consuming everything from gastropods to evasive fish. Counter to expectation, this observation suggests that morphological specialization allows smashers to consume a wider range of prey compared to spearers. Thus, the classic notion of a one-to-one relationship between "specialized morphology" and diet may not apply to stomatopods.
This dissertation lays the foundation for testing the hypothesis that stomatopods with appendages specialized for speed and acceleration have broad rather than narrow diets. Specifically, this dissertation addresses three fundamental questions in three chapters: 1) what are the kinematics of the spearing mantis shrimp, 2) do different stomatopod tissues integrate diet over a variety of timescales, and 3) does morphological specialization for speed and acceleration correspond with a broad or narrow diet in a smashing mantis shrimp?
The first question addresses a key gap in current knowledge of stomatopod mechanics by providing the first in depth analysis of strike kinematics in a spearer. High speed and field videos of prey capture events were used to quantify appendage movements and behavior. Morphology was analyzed with Computed Tomography scans of the appendage. The results from these analyses were then compared to previous research on a smashing species. I found that the spearer exhibited greater reach but lower speeds and accelerations than the smasher, which implies a trade-off between reach and speed.
The second question aims to develop methods in stable isotope ecology that determine the diet breadth of smashing mantis shrimp, because stable isotope analysis measures and compares intra- and interspecific patterns of diet breadth. The isotopic incorporation rates and isotopic discriminations of carbon and nitrogen in tissues are essential for correctly interpreting stable isotope data. I measured these variables in the smashing mantis shrimp, Neogonodactylus bredini, by feeding individuals a single species of prey and periodically sampling their muscle and hemolymph. I found variation in incorporation rates between and within tissues, and discrimination factors that were different than expected based on published literature values. N. bredini's rate of carbon incorporation was consistent with rates predicted by an allometric equation correlating incorporation rate to body mass for teleost fishes and sharks.
The third question builds on the previous study and seeks to determine the diet breadth of N. bredini. Specifically, I combined abundance studies of prey items, a laboratory feeding experiment that examined which prey N. bredini would consume, a stable isotope analysis of diet, and field observations of feeding behavior. I conducted these studies in a sea grass habitat and a coral rubble habitat over two different seasons to determine whether diet changed spatially and temporally. The abundance study revealed that prey abundances vary between habitats. The feeding experiment showed that N. bredini was capable of consuming both hard- and soft-bodied prey. The stable isotope analysis showed that N. bredini consumed a wide range of different prey in the field, including snapping shrimp and soft-bodied worms. Thus, in contrast with the hypothesis that specialized feeding morphology corresponds to a narrow diet, this suite of observations demonstrates that N. bredini's smashing appendage does not limit this predator to a diet of hard-shelled prey.
Together, the answers to these questions provide novel insight into the relationship between appendage morphology and diet specialization in mantis shrimp and lay the foundation for examining the ecomorphology of this diverse group of animals.