Tag Archives: Research

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.

Pseudothelphusid crab predation risk: Part 1

Jerry, one of my NAPIRE students from Pohnpei, Micronesia, is investigating pseudothelphusidae crab (Allacanthos pittieri)  populations at the Las Cruces Biological Station this summer. His study has two major components: (1) measuring crab densities in areas that vary in the amount of leaf litter present (i.e., high and low leaf litter densities), and (2) assessing predation risk in high and low leaf litter areas (after Jenn Clark’s crayfish study; Clark et al. 2013 Hydrobiologia).  Jerry hopes to demonstrated higher crab densities and lower predation risk in areas of high leaf litter because crabs are able to use the leaves as refuge and a source of food, both directly, as they are shredders, and indirectly by preying on other invertebrates in the leaf litter.

Here, Jerry has tethered crabs using monofilament fishing line to flagging in about 20 stream pools. The crabs were paired in pools and restricted to either high leaf litter sections or low leaf litter sections of each pool site.

Predation risk of roosting Ithomiin butterflies

Adrea, a recent PhD graduate from UCLA, is mentoring three students in the NAPIRE program with projects on Ithomiin butterflies—a diverse group of clear-winged, neotropical butterflies that form breeding aggregations and tend to roost together.  One of Adrea’s students, Katie, is investigating predation risk of roosting butterflies using models of two species that she’s constructed. Here, Katie checks her model butterflies for damage inflicted by predation attempts on the models.

West Branch of the Rio Java

Some images of the West Branch of Rio Java.

This branch drains primary forest, and is rich in macroinvertebrate diversity, but doesn’t appear to have fish. In 2013, David surveyed this stream for fish, but did not discover any, and this year, Jackalyn collected macroinvertebrates using a D-net and immediately found a couple of new taxonomic groups she had previously not sampled.

The stream is steep, which likely prevents fish dispersal, and it’s bedrock substrate isn’t forgiving when the near-daily rains fill the banks.

Collecting fish from the West Branch of Rio Java

David and recruited field help sample fish the west branch of Rio Java, a tributary that drain primary wet forest.