Tag Archives: Crustaceans

A start to Zane’s Crab predation experiment

As I previously mentioned, one of my students this summer, Zane, was interested in the two freshwater crabs that occur here, Allacanthos pittieri and Ptychophallus paraxanthusi, and how they may partition habitats. We find both species throughout the watershed, but it seems that the larger, P. paraxanthusi occurs in higher abundance in larger streams (i.e., the Rio Java proper), whereas A. pittieri occurs more abundantly in the smaller tributaries. This observation is a bit confounded by the lack of success we’ve had in doing mark-recapture studies (or, more generally, accurately estimating population densities). In any case, there may be some partitioning, and it could be due to a variety of factors… including predation.

While Zane started out interested in measuring crab predation risk within, at the bank, and outside of the streams to assess whether crabs may evaluate predation risk and use different parts of the stream or forest to reduce their risk (we’ve observed several individuals in the forest and on the trails!), we collected about 45 individuals—divided about equally between species. Thus, we modified the original plan: Zane would assess predation risk by species and by stream order (i.e., small, second-order streams, and a larger, third-order stream).

Crabs were tethered to monofilament line in the lab, numbered, and transported to the stream.

Some individuals had to be tethered in the field—super glue was used to tether them.

Zane and Ahmi stake the crabs into the streams with labeled flags. Crab species were pair in tethering sites.

Some habitat variables were measured, including substrate and depth of the tethering sites.

Crabs remained tethered for about nine days and were observed once a day.

And some results immediately surfaced: some crabs likely escaped by chewing the line, other lines were cleanly cut, and some damaged carapaces remained.

In other cases, parts of a missing crab were found near the tethering sites!

Some Crab predation!

A few years ago, a NAPIRE student of mine, Jerry, investigated predation risk of crabs in and away from leaf litter. He found substantial predation in the assay, and, this year, Zane is picking up the idea again (more to come).

The crabs are fairly abundant in the streams, but their densities can vary greatly, and, overall, the density is lower in the larger reaches of Rio Java (greater discharge) and those crabs that we do find in the river have a larger carapace width—big crabs in the big river. Perhaps predation is driving this pattern! And look, here’s a pile of dead crabs, partially eaten, probably by a mammal adjacent to a small backwater pool within the river.

Marking crabs

June, one of the NAPIRE students, measures and marks crabs (Allacanthos pitteiri) she’s caught with baited minnow traps. ┬áHer project has a lab focus too: she is investigating size and sex differences in coarse particulate organic matter (leaf) shredding behavior by presenting male and female crabs that vary in size with stream conditioned leaves and measuring how much they consume, or shred. This parallels a project another student performed in 2013, Joseph Jack, although Joseph varied leaf species and conditioning time.

Last year, Jerry studied predation risk in crabs and measured crab densities in stream pools. While he marked the crabs with nail polish as a mark-recapture method, the small labels June used have individual numbers (her mentor, Anne Brasher, brought them and has used them extensively with snails), which is a powerful tool.

An interesting observation: some of the crabs seemed to have rust-colored lesions on their carapace that look similar to lesions we often found on crayfish (Orconectes obscurus) caused by a chitin-eating bacteria, isolated (or at least studied) by Adam Leff’s lab at Kent State.