Levinton - Marine Ecology Lab

Current Research

Latest Research 2023: Superior Performance of a Trailing Edge Population of the Sand Fiddler Crab Leptuca pugilator, recently published.

Behavior, biomechanics, evolution of fiddler crabs. The overall objective of this study is to link the origin of an evolutionary innovation, namely the giant major claw of fiddler crabs, to reproductive function, behavioral evolution and biogeography of a widespread group. The fiddlers are pantropical and the major claw is nearly half the weight of the animal. It is used by males in threat display and combat. We are investigating the following questions: (1) Does the claw function as mainly display, or does its biomechanical features reflect a fully functioning closing structure? (2) Is sexual selection a cause of variability in this structure, much as the process works to increase variability of other structures and colors controlled by sexual selection? (3) Did behavioral complexity increase in this group, or did other factors allow the evolution of complex reproductive behavior? (3) Did this group arise in the IndoPacific as previously believed? Our work combines field studies of behavior, morphometrics, molecular phylogenetic tools, and biomechanics to answer these questions.

Recent research has focused on the functional morphology of the sexually selected major claw, which is about 1/3 the mass of a male fiddler crab. The claw is used in display and combat. Our previous work demonstrated that the major claw is fully functional and that variation of closing force with body size can be predicted successfully from simple measurements of claw dimensions, related to a simple lever system. However, there is a paradoxical result: As body size increases the closing force of the smaller feeding claw increases with the square of any linear measurement, as expected from a proportional increase to muscle cross-sectional area. But the closing force of the major claw proportionally decreases with increasing body size, mainly because the mechanical advantage decreases. Thus a fighting male is weaker than it would otherwise be if it maintained the same claw mechanical proportions of smaller crabs. I have been working on explaining this paradox - why be weaker than you were going to be? Is the claw used more for display? Is some other function being served? Thus far, we have concluded that there is a tradeoff between strength and closing speed and have tested this successfully. This tradeoff is also found in fish jaws and lizard jaws. Thus larger crabs may require speed of clasping more than strength. This fits in with observations of combat and the usual lack of severe damage that one male might inflict upon another. I believe that this general result applies to many beetles bearing pincers as well. The race is to the swift, not necessarily to the strong. A contrary argument is that closure at an interior tooth can compensate for the tradeoff, but our recently acquired data shows that this does not work for several species of fiddler crabs, and not for Leptuca pugilator. 

We are also working on measuring the costs encumbered by having such a large structure as the major claw. In males of Uca pugilator and other species, this size is so large that the walking limbs beneath the cheliped are more robust than the legs beneath the much smaller feeding claw. Do males pay a price by carrying this large structure? Do they escape more poorly from predators? Do they have high metabolic loads, since this claw as a high proportion of the total striated muscle of the whole crab? We have been investigating the cost by measurements of metabolic rate, subjecting crabs to stress on a treadmill, and measuring running speeds of males in the field, as compared with males not bearing the large major claw. A counterintuitive result is that running speed is the same for crabs with and without the major cheliped. In sprinting, the mass of the claw does not impede sprint speed, so males and females of similar carapace size run at the same speed. But this result does not apply to longer term running on a treadmill. The major cheliped, or a similar inanimate weight planted on the clawless male causes stress and the male loses the ability to continue to run much earlier than a clawless male, not bearing the mass.

Thermal Ecology of Fiddler Crabs and the Effect and Role of the Giant Major Cheliped.

We have investigated a hypothesis that suggests a role for the major claw in thermal regulation. Does the major cheliped help act as a heat sink, absorbing and perhaps radiating heat to the air? Our evidence suggests that the claw can absorb heat, but there is no evidence of significant heat transfer between body and cheliped. Indeed, heating from above appears to have a small negative effect on the body as a result of bearing the major claw. This is an active area of research and not yet resolved.

Latitude, Thermal Adaptation, and Climate Change.

A different emphasis is on the thermal component of fiddler crab performance. Fiddler crabs are an excellent model for the study of thermal stress on a mobile species in the intertidal zone and climate change may have strong impacts on behavior, mating strategies and survival. We have found that male size is related to vulnerability to thermal stress and this may contribute to larger-male mating success. We are investigating tradeoffs between sexual display and physiological stress. This study is embedded in a current National Science Foundation research grant devoted to latitudinal thermal adaptations of Leptuca pugilator, in collaboration with Zachary Darnell (University of Southern Mississippi) and Warren Porter (University of Wisconsin). Current research has compared several measures of performance of males of Leptuca pugilator collected from different latitudes.

Heartbeat and Performance in Response to Temperature

How do fiddler crabs respond to changing temperature in terms of performance. We are taking two approaches, focusing on performance in air and water, since fiddlers are biphasic creatures. Heartbeat was measured using an infrared detector. We have completed a study of heartbeat of fiddlers as a function of temperature. The results in air and water were strikingly different. In water heartbeat increases with increasing temperature to 40, with a decline at higher temperatures. But in air no increasing trend was seen with increasing temperature. This may be due to continual availability of oxygen in air, but a much slower delivery when dissolved in water. This suggests perhaps a very different metabolic regime under thermal stress in water and air. We are now following this up by measuring oxygen consumption in air and water using an optode method.

Global Analysis of Major Claw Length in Fiddler Crabs - published Dec. 2021

This project was done in collaboration with Marc Weissburg of GA Tech. Sexual selection is famous for producing diversified structures, as has been found for horns of scarab beetles that show diversified horn size and shape and many instances of gains and losses in phylogeny. Our previous work suggests that fiddler crabs arose over 23 million years ago and diversified from an ancestral group in the Americas, producing diverse groups throughout the Indo-Pacific, Eastern Pacific, Western Atlantic, and Eastern Atlantic. But our analysis shows that major cheliped length is amazingly in a constant linear relationship to carapace length. This strongly suggests that some functional restraint guides the constancy but we don't have complete evidence of this control. Of course, species deviate from the overall relationship and there is even a signature of sexual selection - variance of the major cheliped length is greater than that of the minor cheliped, which is much more under the control of natural selection especially feeding ecology. Do you have any ideas? See our paper and some suggestions. We look forward to discussion on this.

For more information on past research.