A closer look at our research on gut ecomorphology |
|
Using ecomorphology as a predictive tool |
|
Rapid environmental changes and pressing human needs to forecast the consequences of those changes are increasingly driving ecology to become more predictive. Given the number of species and habitats existing on our planet, we can’t hope to possibly study each one. Therefore, understanding the ramifications of a changing planet requires that we develop tools that allow prediction across species and systems. This requires both the development of new tools and the honing of existing tools to make them applicable to systems of interest. One existing tool that historically has been more descriptive in nature is ecomorphology (the study of relationships between ecological roles and morphological adaptations of species and individuals). Ecomorphology is useful for investigating a variety of ecological relationships, among which is trophic relationships. Given the central importance of trophic interactions in many ecological processes, thousands of studies have examined diets and consumption rates of various species. However, despite the vast number of species examined, this still represents only a small minority of the consumer species that exist. |
|
Using gut ecomorphology to predict percent herbivory in crab diets |
|
One important group of consumers in marine systems are the brachyuran crabs. There are more than 10,500 species of brachyuran crabs that have been described by science. These crabs live in a variety of habitats including marine, estuarine, freshwater, semiterrestrial and terrestrial environments. Crabs are notoriously omnivorous (consume both plant and animal foods) and often exert strong top down control in food webs. Further, crabs are becoming even more important within food webs in many marine systems, largely for two reason. The first reason is the loss of large predators that previously kept crab numbers in check. These large predators are being lost, for example, because of overfishing. And secondly, the importance of crabs in many food webs is changing because of the continual movement of this highly invasive group of species into new habitats. Given the vast number of crab species, their diverse trophic strategies and their ecological importance across a variety of habitats, developingmethods to predict the trophic impacts of these consumers (what they eat, how much they eat, etc.) could have several benefits. |
 |
|
For trophic interactions then a logical place to start is metabolic theory. An early prediction of metabolic theory derived by Sibly from assumptions of optimal digestion is that animal gut sizes should increase with decreasing diet quality because organisms that eat low quality food must eat large volumes in order to attain sufficient nutrition. |
Gut morphology of Brachyuran crabs is relatively conserved across species. Consumed food is passed first to the large cardiac stomach where it is stored for eventual processing, which takes place a bit at a time in the much smaller pyloric stomach. This arrangement allows food to be taken in quickly in bulk and then processed later. This is ideally suited to the general crab strategy of active foraging bouts only during specific times of the day, such as night-time high tides or day-time low tides, depending on the species. One implication of this combined foraging strategy and gut morphology is that food consumption by crabs is ultimately limited by the volume of the cardiac stomach. Also, crab guts can be approximated quite well by a triangle tetrahedron. This means that the volume of the gut can be easily calculated using simply the measured gut width. |
 |
 |
Recall that crabs are omnivorous, consuming both plant and animal foods. However, some fall closer to the herbivore end of the continuum and some fall closer to the carnivore end of the continuum. Because animal tissue is commonly more nitrogen-rich than plant material, crabs that are primarily herbivorous are generally nitrogen-limited. Thus, we can take Sibly’s general prediction and apply it to crabs to get the expectation that cardiac stomach size should increase with percent herbivory, so that herbivorous crabs can consume enough food to meet nitrogen demands. |
The figure on the right comes from Griffen and Mosblack 2011 where we first determined the phylogenetic relationships between 15 different species of crab using overlapping regions of 16s ribosomal RNA (part A, top). We then determined the percent herbivory in the diet of each species using both gut content analysis as well as all available published literature on gut contents. In addition we measured the gut width of ~100 individuals of each species and used this to calculate gut volume. We then determined the residual gut volume for each individual from a regression across all 15 species. A positive residual means that the crab has a larger gut than expected for a give sized crab, while a negative residual means the crab has a smaller stomach than expected given crab size. Using residuals facilitates the comparison of guts across different sized crabs. Finally, we used the phylogenetic relationships shown in part A to derive phylogenetically independent contrasts for both percent herbivory and for gut volume (part B, bottom). With the exception of a single outlier (which subsequent analyses have demonstrated that we overestimated its percent herbivory), crabs that eat more plant material have larger cardiac stomachs. And in fact, even after accounting for phylogenetic relationships, percent herbivory explains 80% of the variation in gut size across crab species! |
 |
 |
Finally, we can make this a little less rigorous, simply to facilitate quick and easy predictions. If instead of worrying about phylogenetic relationships and converting to gut volume, if we simply measure the gut width and divide this by the carapace width, this gives us a simple measure of standardized gut size (or gut size standardized for different sized crabs) that can be easily measured, even in the field, and can be readily used to compare across species. And when we do this for the species studied by Griffen and Mosblack 2011 (after removing fiddler crabs that eat lots of sediment), we find that there is a very good relationship (figure on left) which can be used to derive rough approximations of the average percent herbivory that we should expect for any given crab species. Thus, for example, we would predict that a species that has a standardized gut width of 0.35 would have a diet that includes approximately 45% herbivory. Other work presented by Griffen and Mosblack 2011 demonstrates that gut size can also be used to examine individual diet specialization within a single species, and that it can also be used to approximate relative consumption rates of different species or individuals. |
Current work in this area |
|
Additional work in our lab suggests that, at least for some species, gut morphology is a plastic feature, changing as diets shift seasonally and/or ontogenetically. Current work is therefore examining the utility of using gut ecomorphology to examine these diet shifts within a population. In addition, we are also investigating the time scale on which gut size changes to track changing diets (days, weeks, months, etc.). |
|
