Photo credit: inuc0r0 @blogs.yahoo.co.jpA few weeks ago, I got back tissue stable isotope data from 30 Thalamita crabs that I caught in the fishpond. Muscle tissue from this many crabs is not enough to make any sound conclusions about their feeding habits but I noticed one interesting thing. In order to describe it, however, I need to explain some chemistry.
Nitrogen isotopes are generally a signal of trophic level, or the position of an organism in the food web. Organisms at lower trophic levels, such as algae, have lower δ15N values than organisms at higher trophic levels, like shark or ulua. This is because the heavy nitrogen isotope (15N) accumulates in animal tissues, while the lighter one is excreted. In brief: when an animal eats something, it can either absorb or excrete molecules. Chemical reactions occur more often with light isotopes than heavy isotopes. The chemical reactions that are involved in excretion (transamination and deamination) are no exception: they occur far more frequently with 14N. Therefore, when the animal produces waste, 14N is removed from the "pool" of available nitrogen in the animal. What remains, and what is absorbed, is enriched in 15N, meaning there is more heavy N isotope in the animal's tissue than there was in the thing it ate (Gannes et al. 1998). The general rule is that δ15N increases by 3‰ with every trophic level.
Why the chemical digression? My description above paints a picture of a tidy trophic ladder, with producers at the bottom and a big predator at the top. Unfortunately for ecologists, food webs are almost never this simple. Here's what I noticed in the T. crenata data from the fishpond: the crabs I sampled have a large (~3-8‰) range in δ15N values. This suggests that crabs are eating at a number of different trophic levels. Since juvenile T. crenata have the same diet as adults (Cannicci et al. 1996), these differences are not likely to be due to differences in crab maturity (young crabs aren't nibbling on algae while larger ones feast on old fish carcasses).
T. crenata is an opportunistic, detritivorous cannibal. We don't see everything these crabs eat, we can only look at the chemical clues in their tissues. As you can imagine, it is hard it is to determine what exactly a crab has been snacking on if its diet and stable isotope signatures are both so variable. Questions like this come up often in stable isotope ecology, and require us to look further, either physically by looking at the animal's stomach contents*, or mathematically by using models to predict proportions of certain foods in the animal's diet (more on this later). The variability in my crab SI values tells me already that I will need some of these extra tools in order to make sense of my data. And I thought crabs were the biggest pain about working in the fishpond...
*This is a messy but often necessary process. The polite word for it in the sciences is "gut content analysis."
Gannes LZ, Martínez del Rio C, & Koch P (1998). Natural abundance variations in stable isotopes and their potential uses in animal physiological ecology. Comparative biochemistry and physiology. Part A, Molecular & integrative physiology, 119 (3), 725-37 PMID: 9683412
