Tag Archives: Research

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.

Introduction to Ahmi’s project – Fish communities at Las Cruces

One of my two students for this year’s LSAMP REU is sampling and documenting the fish communities within the rivers at Las Cruces as an extension of a previous student of mine’s project (David’s). David had discovered four species, Bryconamericanus terrabensis, Brachyrhaphis terrabensis, Rhamdia laticauda, and Trichomycterus striatus, and he had presented his work at AFS. Interestingly, at Bry. terrabensis and T. striatus, have not been reported as occurring above 1,000 meter above sea level in the literature, and he thought this could be due to (1) undersampling and reporting and/or (2) recent changes in the distribution of these fishes, possibly due to climate change. One way to find out—continued monitoring. And, hence, Ahmi’s interest.

Ahmi may be employing three survey methods (visual surveys via snorkeling, seining, and minnow trapping) and sample along the entire elevational gradient within the Las Cruces property (1,000 – 1,400 masl).

Here, Ahmi practices visual surveys—a 5-10 minute zig-zag, calling out letters that represent the most likely fishes. The water is cool and turbid, making it difficult to complete, unlike some of the rivers, like Rio Madrígal, Rio Claro and Rio Nuevo, I surveyed using similar methods years ago on the Osa Penninsula.

Next, Ahmi and Zane practice seining the same pool; the current and steep, slippery banks present some difficulty, but they do catch a few.

Seining is hard work!

Some catch from the sample, including Bry. terrabensis.

Land-use impacts on macroinvertebrate predators

Darryl, one of the students mentored by Patricia, sampled his first macroinvertebates yesterday using a Surber sampler. He’s interested in functional feeding groups—groups of organisms that are categorized with one another based on what they do in the stream (their role)—and how the composition of those groups may differ with the land-use of the stream catchment. There’s a lot of literature on this, and the predictions are related to the river continuum concept—a set of theories about how the ecology of rivers changed from up- to downstream. Jacklyn, a former student of mine in the NAPIRE program, investigated the RCC.

Darryl is particularly interested in the predator functional feeding group, of which there are many: from Naucorids and Belastomatids, to some mayflies and megalopterans. Here, Darryl and Patricia take a few stream samples.

The crabs of Las Cruces

To some degree, I have been working with crabs at Las Cruces since 2013, and I have identified only a handful of males using various keys (the latest by Magalhaus et al 2015). This year, I aimed to identify all the crabs we caught and studied if we could, so Gabby, Juliet and I sat down to identify two males, randomly chosen from a set we had discovered trematode metacercariae in, using the key.

As we slowly moved through the key towards Allacanthos pittieri which I had previously identifed in other summers and a few days earlier as a brush up, Gabby expressed some skepticism and challenged my guidance through the key. I dismissed her concerns to some degree (and I hope she’ll forgive me) as we worked through defining the terms in the first couplet, but eventually had a look at her crab’s gonapods (the structures used to transfer sperm in decapods and commonly used to identify them to species). I immediately and excitedly knew she had something else.

The three of us spent the rest of the day keying the new crab and confirming the identity of the other, and it was glorious. There is nothing cooler than working with engaged, enthusiastic, and smart students… especially on something as cool as a second crab species!

While I think we stumbled on a couplet (11) and eventually mis-identified the crab as Ptychophallus montanus, I think we’re confident that the other species of crab is P. paraxanthusi. At this point, we’re working on markers of the crab that don’t involve microscopy and allow us to identify the females.

A preliminary summary of crab projects

There are three students in this year’s LSAMP-OTS REU working with freshwater, pseudothelphusid crabs: Kainalu (Patricia’s student), Gabby, and Juliet.

Kainalu’s focus: population structure and habitat preferences of the crabs. He’s been using minnow traps to collect crabs from riffles and pools of several streams so far, including Río Java, Quebrada Culvert, and Quebrada Culebra.

Juliet and Gabby, my students, have related questions, so we’ve been keeping the crabs Kainalu has collected and bringing them into the lab.

Juliet is documenting the presence and prevalence of trematode worms (Paragonimus sp.) parasitizing the crabs. There are two known species of trematode to occur in Costa Rican freshwater crabs—P. mexicanus, which can cause paragonimiasis in humans, and P. caliensis (Hernández-Chea et al 2017). While various crab host species have been identified in Costa Rica, including those in genus Ptychophallus and Allacanthos, there doesn’t appear to be good literature identifying the specific species at Las Cruces as hosts—Ptychophallus paraxanthusi and Allacanthos pittieri.

We’ve already found some metacercariae in the hepatopancreas of both crab species here already, and Juliet’s exploring how stream characteristics, carapace length, and sex may impact the parasitization, so she has worked with Kainalu on most field collections.

As Juliet and Kainalu bring in crabs, Gabby has been sorting, helping to measure them, and caring for them in individual containers as she prepares assessing sex-dependent, and claw-dependent agonistic behaviors. While agonistic behaviors are well documented in decapods, these behaviors are often species-specific, and there doesn’t seem to be much literature on Pseudothelphusid behavior. One of the other cool ideas she’s exploring—how does handedness affect behavioral outcomes? These crabs, along with many others, have asymmetrical chelae (claws) and some new research (according to Patricia) suggest different outcomes for bouts when the asymmetry is matched (both crabs have their right chelae larger than the other) versus when the asymmetry differs between crabs (one crab has the right chelae larger, the other has the left).

Assessing morphology of chelae


A minnow trap in Quebrada Culebra

Sherman traps for crabs

Some authors have evidently used Sherman traps, usually used for small mammals such as rodents, to catch semi-terrestrial crabs. Given that we don’t really know the terrestrial activity of the stream crabs in Las Cruces, we decided to set a few baited traps up, without luck. The crabs are small, so may not be large enough to trigger the trap, but all the bait was present after the test.

Initial dissection of crabs

The first project ideas Gabby and Juliet found interesting involved dissecting the pseudothelphusid crabs found in the streams in Las Cruces. While Gabby’s project has since changed dramatically from diet composition of the crabs to testing sex and heterochelae effects on agonistic behavior, Juliet is exploring trematode (Paragonomus spp.) parasite prevalence in the hepatopancreas (the yellowish tissue) in the crabs.