Tag Archives: mud crabs

A mud crab ready for his hearing test.

Can crabs hear? (A testament to the benefits of collaboration)

Over the last few weeks, we’ve explored the ecology of fear in oyster reefs. What makes oysters too scared to eat, potentially keeping them from reaching market size or filtering water? What makes mud crabs too scared to eat oysters, giving the oysters a better chance to succeed? New research by Dr. Randall Hughes and Dr. David Kimbro might change the way we understand fear in mud crabs.
Dr. Randall Hughes FSU Coastal & Marine Lab

IGOR chip_ predators_NCE 150When we started the In the Grass, On the Reef project, Rob (WFSU-TV Producer Rob Diaz de Villegas) embarked on a crash course learning about oyster reefs and salt marshes, biodiversity, and non-consumptive predator effects.  While you’re most likely familiar with those first few terms, the last one – non-consumptive effects – is a bit of a mouthful and hasn’t exactly made the list of new slang words of 2013.  The term refers to the ability of predators to SCARE their prey even when they don’t EAT them, causing the prey to hide, or eat less, or change their size/shape to make it less likely that they will be eaten.  Of course, these changes are only possible if the prey realizes the predator is there before getting eaten!  There are several “cues” that prey can use: (1) they can see them (visual cues); (2) they can feel them (physical cues); or (3) they can “smell” them (chemical cues).  This last category is really common in the ocean, especially with slimy snail or fish predators that give off lots of chemicals into the water!

As Rob was learning more about the fish predators that we find on our oyster reefs, he discovered audio clips of the sounds that several of these fish make.  Putting 2 and 2 together, he posed a simple question to David and me: Can mud crabs use fish sounds as a cue that their predators are near?

Housam collecting juvenile clams attached to oyster shells for use in the experiment.

To be quite honest, David and I didn’t have an answer.  But, we knew how to find out – do the experiment(s)!  We enlisted Housam Tahboub, an undergraduate at the University of Michigan Flint, who wanted to do his summer Honors project in our labs.  (Little did he know what he was getting into.)  And then we set off on a crash course in bioacoustics, underwater speakers, and crab torture chambers (more on that in a minute).

Rob’s question really has 2 parts:
(1) Can crabs hear (anything)? (They don’t have ears.)
(2) Do crabs respond to the sounds of their fish predators?

A mud crab ready for his hearing test.

A mud crab ready for his hearing test.

To answer #1, we paired up with Dr. David Mann at the University of South Florida. Dr. Mann is an expert in bioacoustics, and particularly in evaluating whether marine critters (primarily fish) can hear different sounds. We modified his methods slightly to accommodate our crabs – basically, we needed to immobilize the crabs on a ‘stretcher’ so that we could insert one electrode near the crab’s antennae, and another in the body cavity to pick up any background “noise” the crab may be produce that was not in response to the acoustic stimuli. Although I know it looks like crab torture, all the crabs survived the experiment!

A mud crab submerged in the acoustic chamber

A crab submerged in the acoustic chamber.

Once the crab was immobilized and the electrodes were in place, we submerged the crab in a tank full of seawater that had an underwater speaker in it. We then played a series of acoustic stimuli of different volumes and frequencies and quantified the response recorded by the electrode. The really nice thing about this method is that we don’t have to train the crabs to tell us when they hear the noise like in the hearing tests that you and I take!

A marked oyster shell with juvenile clams glued on it as a crab buffet.

To tackle question #2, we set up a mesocosm experiment at FSUCML. Each mesocosm (aka, bucket) had sediment, a layer of loose oyster shell to serve as habitat for the crabs, and 5 mud crabs that we collected from nearby oyster reefs. We also added some juvenile clams glued to a few marked oyster shells in each mesocosm – this way, we could count the number of clams eaten over time and determine whether crabs were eating more or less in response to the predator sounds.

To run the experiment, we downloaded sound clips of several different crab predators – toadfish, black drum, and hardhead catfish – as well as 2 non-predators to serve as controls – snapping shrimp and a silent recording. Housam put these on his iPod, connected it to an amplifier and underwater speaker, and we were ready to begin.

(Well, let’s be honest, it wasn’t quite that simple. Housam read a lot of papers to figure out the best methods, spent lots of time collecting crabs, and logged lots of hours (both day and night, in the company of mosquitoes and biting flies) moving the speaker from tank to tank before we finally settled on a good protocol. He even tried all of this in the field! But when his summer ended, Tanya, Phil, and Ryan kindly stepped in to run the rest of the trials we needed.)

But we didn’t stop there. We know from our earlier experiments with Kelly Rooker (another undergraduate researcher) that the crabs don’t eat as much when exposed to water that hardhead catfish have been swimming in, most likely because they can detect chemicals in the water that the fish give off. So which cue generates a stronger response – chemical cues or sound cues? Time for another experiment!

Phil checks on the mesocosm experiment at FSUCML

In this version, the mesocosms were assigned to one of 4 combinations: (1) a silent recording, paired with water pumped from a tank holding 2 hardhead catfish into the mesocosm; (2) a recording of a hardhead catfish, paired with water that did not go through the catfish tank; (3) a recording of a hardhead catfish, paired with water from the catfish tank; (4) a silent recording, paired with water that did not go through the catfish tank. We again looked at the number of clams eaten over time to see how the crabs change their behavior.

This project has been a lot of fun, and it never would have happened were it not for Rob’s curiosity. We gave a preview of our results at the Benthic Ecology conference in Savannah, GA, last weekend. But we’ll have to wait until everything is reviewed by other scientists and published in a scientific journal before we can share all of the details here. So stay tuned!

Music in the piece by zikweb.

Black Drum recording used in the video courtesy of James Locascio and David Mann, University of South Florida College of Marine Science.

Catfish and toadfish recordings copyright University of Rhode Island.  They were obtained from dosits.org, under these terms:

Copyright 2002-2007, University of Rhode Island, Office of Marine Programs. All Rights Reserved. No material from this Web site may be copied, reproduced, re-published, uploaded, posted, transmitted, or distributed in any way except that you may download one copy of the materials on any single computer for non-commercial, personal, or educational purposes only, provided that you (1) do not modify such information and (2) include both this notice and any copyright notice originally included with such information. If material is used for other purposes, you must obtain permission from the University of Rhode Island. Office of Marine Programs to use the copyrighted material prior to its use.

In the Grass, On the Reef is funded by a grant from the National Science Foundation.

Spat on a Platter

Tanya Rogers FSU Coastal & Marine Lab

IGOR chip_ predators_NCE 150“Spat tiles” are a tool our lab commonly uses to measure the growth and survivorship of juvenile oysters under different conditions, and we’ve used them with varying degrees of success in many of the experiments chronicled in this blog. What these are essentially (in their final form, after a good degree of troubleshooting), are little oysters glued to a tile, which is glued to a brick, which is glued to a mesh backing, which is zip tied vertically to a post. Rob and I have put together a couple interesting slideshows chronicling the growth of these spat over time from two of those experiments. Ever wonder how fast oysters grow? Observe…

This is a time series from our first spat tile experiment, which you can read about in this post. As you may recall, this experiment was largely a failure because the adhesive we used to adhere the spat was inadequate. However, we decided to keep the fully caged tiles out on the reefs to see how they fared over time in different locations. I photographed the tiles every 6 weeks or so, so that we now have a series showing their growth over time. The slideshow shows one of the tiles from Jacksonville. It starts in October of 2010. You’ll notice that not much growth occurs though the late fall and winter, but the spat start to grow noticeably from April-June 2011. From June-September the spat grow explosively and many new spat settle on the tile from the water column and grow equally rapidly. Just as plants (and algae) have a summer growing season, so too do the oysters that feed on them, when conditions are warm and there is abundant phytoplankton in the water to eat.

Next is a series of images from our caging experiment last summer, which you can read about here. Our large cages contained either:


no predators (bivalves only),


spat-consuming mud crabs and oyster drills (consumers),


or mud crabs and oyster drills plus blue crabs and toadfish (predators).

The spat tiles within the larger cages were placed either exposed to potential predators or protected from them in a smaller subcage. Here are typical examples of what tiles looked like at the end of the experiment (about 2 months after starting). You can see how all the spat on the unprotected tiles were wiped out in the consumer treatments, but a good number survived in the treatments with no predators, as we would predict. In the predator treatments, most of the spat on unprotected tiles were removed, but not as fully or quickly as in the consumer treatments, which we would predict if the predators are inhibiting consumption of spat by the mud crabs and drills through consumptive or non-consumptive effects. You’ll see one tiny spat holding on in the predator tile shown. On the protected tiles, most of the spat survived in all treatments, as expected. We plan to further analyze the photographs from the protected tiles though, to see whether spat growth rates differed between them. We may find that protected spat in the consumer treatments grew slower than in the other treatments because of non-consumptive predator effects.

Currently, we’ve recovered most of our arsenal of spat tiles from the field, and I say we have probably amassed enough bricks to pave an entire driveway! Good thing we can reuse them!

The Biogeographic Oyster Study is funded by the National Science Foundation.


Scared hungry?

Dr. Randall Hughes FSU Coastal & Marine Lab

A hardhead catfish, one of a mud crab's primary predators on North Florida oyster reefs.

IGOR chip_ predators_NCE 150As David has mentioned previously, predators can affect their prey by eating them (a very large effect to the prey individual concerned!) or by changing their behavior. And exactly how the prey change their behavior can have large consequences for the things that they eat. For instance, if you’re out camping and hear a bear lumbering around, do you quickly pack up all your food and put it out of reach of the bear and yourself? Or do you quickly eat as much as you can?

This summer we worked with Kelly, an undergraduate from Bridgewater College, to document how mud crabs deal with this dilemma of getting enough to eat but not getting eaten themselves.


Kelly with the broken down truck on an ill-fated return trip from St. Augustine.

Specifically, we wanted to know how they respond to the presence or absence of catfish, and how this response affects the survival of juvenile oysters. Sounds straightforward, right? Well, yes, in concept, but as Kelly quickly discovered, putting that “on paper” concept into reality at the lab took a lot of time and effort!


First, she had to get the “mesocosms” (aka large tubs) ready to serve as adequate habitat for the crabs, with plenty of sand and dead oyster shell for them to hide in.


Next, Kelly took individual juvenile oysters, or “spat”, and used a marine adhesive to attach them to small tiles that we could distribute among all of the mesocosms.


Juvenile oysters attached with Zspar (a marine adhesive) to a tile so we could assess mud crab predation.


You may have noticed that I mentioned catfish, and that these mesocosms are not particularly large relative to the size of a catfish. Never fear – because we wanted to separate the effects of catfish cues from the effects of catfish actually eating mudcrabs, the catfish were kept in a much larger tank, and then water from this tank was pumped into the mesocosms receiving catfish cues. (Setting up the pump and tubing to 60+ tanks was a several-day effort in itself!)


The catfish tank, with tubing carrying catfish "cues" to individual mesocosms.

Once everything was in place, it was time to collect the mud crabs. We couldn’t collect the crabs gradually, because they like to eat each other when confined in small spaces in the lab, so we garnered as much help as we could and held our own little mud crab rodeo. (And got caught in quite a thunderstorm in Alligator Harbor, but that’s another story).

Finally, it was time to start the experiment! We measured the size of each of the mud crabs, added them to the mesocosms, and let them eat (or not). Each day, Kelly would count the number of live oysters remaining, and she would remove a few mud crabs from some of the mesocosms to simulate catfish predation. There were a lot of moving parts to this experiment, and Kelly did a great job managing it!

And what did we find? Turns out that individual mud crabs actually eat more juvenile oysters when they are exposed to catfish cues and the removal / disappearance of some of their neighboring mud crabs, compared to just the removal of neighboring mud crabs or the absence of catfish cues. But overall, the the removal of mud crabs have a positive effect on oyster survival. (Even though individual crabs may eat more, there are not as many crabs around, so it’s a net positive for oysters.)


Mud crabs ate more oysters per individual in buckets with exposure to catfish cues and high rates of manual removal of mud crabs (to simulate predation).

Kelly has returned to classes, so we’ve now recruited a new assistant, Meagan, to help us with an experiment to address the additional questions that inevitably arise as you learn more about a system – for example, do mud crabs behave differently if catfish are around all the time versus only some of the time? We’ll keep you posted…

Randall and David’s research is funded by the National Science Foundation.

Oyster Study: Year Two, Under Way in a Big Way

Rob Diaz de Villegas WFSU-TV

IGOR_chip_predators_NCE_100IGOR chip- biogeographic 150I’ve come to Saint Augustine to get the last of the footage I need to finish the In the Grass, On the Reef documentary, and we’ve come a long way from where we started from on this blog.  One year ago today, this site went live and Randall and David introduced you to their research.  The oyster study had just gotten its grant from NSF and we went out with David as he walked out into Alligator Harbor in search of study sites.  It was a slow, messy day- but a necessary first step. Continue reading

In the Grass, On the Reef, Over the Airwaves

In the Grass, On the Reef

June 29, 2011 at 7:30 PM/ ET


Rob Diaz de Villegas WFSU-TV

A little over a year ago, when the FSU Coastal & Marine Laboratory and WFSU-TV – a TV station – started this online enterprise, the understanding was that at some point this would end up being a show.  And so here we are.  As you may have gathered from that video up there, this will be about predators and prey: who’s eating whom, and who’s scaring whom.  We will of course be doing this through the prism of David and Randall’s studies: the consumptive and non-consumptive effects of predators in salt marshes and oyster reefs, and the methods used to shine a light on these interactions. Continue reading