Tag Archives: Arthropods

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!

Army ants

Several species of army ants, Ecitoninae, are common in Las Cruces, and there is a convenient picture guide here: http://ants.biology.utah.edu/genusguide/genusguide.html

I think these smaller one are Neivamyrmex—it’s not possible to see their tarsi, which is a critical characteristic in determining genus, but they are certainly smaller and less conspicuous than others I’ve seen. These were moving along a log over Quebrada Culvert.

I’ve observed other Eciton in La Selva and Palo Verde, and these could also be Eciton from the Wilson Botanical Garden.

An insanely huge fly

I don’t know what this fly is or does, and it’s maggot must be phenomenally large (flies, like many other insects with complete or holometabolous metamorphosis, don’t “grow” as adult—little flies aren’t babies, they’re likely just different species). On top of it’s size, there were pseudoscorpians hitching a ride on its legs (phoretic dispersal), so when it landed on our current, they began to climb down and investigate their surroundings!

I’ve seen some phoretically dispersed mites on dung beetles (an Instagram post I can’t link) and tabanid flies before too.

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.