Tag Archives: Streams

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.

Quebrada Cerro

One of my favorite streams within the Las Cruces preserve is Quebrada Cerro, which drains nearly exclusively primary forest. The water is clear and filled with tadpoles and belostomatids, but surprisingly few crabs, to Kainalu’s disappointment.

This summer, I found a tile hot glued to a PVC pipe within the stream—a remnant of a 2015 primary productivity study one of Frank Camacho’s students was performing that was washed out in most sites.

Voracious click beetle larvae

These elateridae larvae roam moss-covered boulders and banks of the streams in Las Cruces, mandibles wide-open and hunting prey. Not pictured, because I don’t have an appropriate camera with shutter control, is their glow. If you’re in the stream at night and turn off your headlamp, you’ll soon notice small green “stars” lining the banks—hundreds of them. These guys are bioluminescent!

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.