Tag Archives: consumptive_nonconsumptive

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Can crabs hear? (Revisited, with answers!)

P1050260Four years ago, we traveled out into the oyster reefs of Alligator Harbor with Dr. David Kimbro.  It was both the start of an ambitious new study and of our In the Grass, On the Reef project.  Last June, we went back to those reefs with Dr. Randall Hughes as she, David, and their colleagues revisited study sites from North Carolina to the Florida Gulf.  In 2010, they sampled the reefs with nets and crab traps, and harvested small sections of reef.  This more recent sampling, which unfolds in the opening scenes of our recent documentary, Oyster Doctors, was conducted with underwater microphones.  Randall explains how sound became a tool in further understanding fear on oyster reefs.

The research in the following post was conducted while Randall and David worked at the FSU Coastal and Marine Laboratory.

Dr. Randall Hughes Northeastern University

A little over a year ago, I wrote about our research project, motivated by a question from WFSU producer Rob Diaz de Villegas, to test whether crabs can hear the “songs” made by their fish predators. At the time, the work had not been published, and so I was not able to share all of the juicy details. But now it has, in the Proceedings of the Royal Society B, so I can finally answer with a resounding YES!

To review a little bit, Rob’s question really had 2 parts:

  1. Can crabs hear (anything)? (They don’t have ears.)
  2. Do crabs respond to the sounds of their fish predators?

To answer #1, we paired up with Dr. David Mann. 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 mud 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 it looks like mud crab torture, all the crabs survived the experiment!

Mud Crab Hearing TestWhat did we find? The crabs had a neurological response (i.e., they “heard”) a range of frequencies. They certainly wouldn’t ace any hearing tests, but if a sound is low- to mid- frequency and relatively close by, they can likely hear it. They do this using their statocyst, a structure containing sensory hairs that can detect changes in orientation and balance, and in this case, can detect changes in particle acceleration associated resulting from the acoustic stimuli.

Although cool to someone like me who is fascinated by marine biology, many of you are probably thinking “So what?”. And for that, we turn to the second part of our study, where we tested whether mud crabs change their eating habits in response to the songs made by their fish predators. We compared the number of juvenile clams that crabs ate when we played them either a silent recording or a recording of snapping shrimp (a common organism on oyster reefs that doesn’t eat crabs) to the number of clams that they ate when we played them recordings of songs from 3 fish that DO eat mud crabs – hardhead catfish, black drum, and toadfish. Apparently catfish and black drum songs are the same to a crab as the Jaws theme song is to me, because they hunkered down and did not eat nearly as many clams when they heard the calls of those two predators.

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Phil Langdon feeds a catfish in an iteration of a mud crab hearing experiment.  They had already noticed that mud crabs were eating less when they heard sounds made by catfish and other predatory fish.  Here, they sought to measure whether the response was more intense with chemical cues (pumped via those tubes into tubs), or predator sounds (played from underwater speakers).

So, now we know that mud crabs can hear, and that they don’t eat as much when they hear some of their predators. But we also know from our earlier experiments that these same crabs don’t eat as much when exposed to water that hardhead catfish have been swimming in, most likely because they can “smell” chemicals in the water that the fish give off. So which catfish cue generates a stronger response – sound or smell? Turns out that both cues reduce crab foraging and to about the same degree, although in our experiment the effects of catfish songs were slightly stronger than the effects of catfish smell.

So what’s the take-home message from this work? For one, it highlights that we still have a lot to learn about the ocean and the animals that live in it – we (and others) have been studying these mud crabs for years and never thought to consider their ability to use one of the 5 major senses! In addition, it’s a reminder that in studying the “ecology of fear”, or the effects that predators have on their prey even when they don’t eat them, we need to remember that few predators are silent, and the sounds that they make could be important cues that prey use to escape being eaten. And finally, it demonstrates that science can be really fun!

This material is based upon work supported by the National Science Foundation under Grant Number 1161194.  Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

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Oyster Research Needs Your Help In Apalachicola Bay

Oyster drills infest one of David Kimbro's Apalachicola Bay experimental spat tile cages.

In January, David Kimbro’s lab did a preliminary survey of Apalachicola Bay oyster reefs, looking at the overall health of oysters and the presence of predators. They followed this up with an experiment meant to monitor oyster health and predator effects over time. Many of their experimental cages were displaced, likely due to the buoys marking them breaking off. But what they found in the cages that remained intact was that oyster drill numbers appear to be exploding in warmer waters.  David is looking for help keeping tabs on them.

Dr. David Kimbro Northeastern University/ FSU Coastal & Marine Lab

Wishing that you were wrong is not something that comes naturally to anyone. But that is how I felt at the most recent oyster task force meeting in April. There, I shared some early research results about the condition of the oyster reefs. In our surveys, we found that the oyster reefs in Apalachicola Bay were in really bad shape and that there were not any big bad predators hanging around the reefs to blame. Even though I had originally shot off my big mouth about the oyster fishery problem being caused by an oyster-eating snail, I hoped that our first bit of data meant the snails were never there. Or better…that they were gone. The story of the boy who cried wolf comes to mind.

But an alternative of this David-cries-wolf story is that our January sampling didn’t turn up many predators because it’s cold in January, and because they were hunkered down for a long winters nap. Unfortunately, this option is looking stronger.

Experimental cages to be deployed in Apalachicola Bay.

Experimental cages to be deployed in Apalachicola Bay.

Since the task force meeting, we have been figuring out how conduct field experiments in Apalachicola. To be honest, an underwater environment without any visibility is an experimentalist’s worst nightmare. Still, we deployed fancy equipment, big cages, and then little mini experiments inside each big cage to figure out how much of the oyster problem is due to the environment, to disease, or to predators.

Even though we lost over half of our experiment and instrumentation, we recovered just enough data to show that the problem could be predation and that the culprit is a voracious snail.  So, after learning some lessons on how to not lose your equipment, we decided to take another crack at it. In fact, Hanna and crew just finished sampling half of our second experiment today. We got the same results….lots of snails quickly gobbled up all of the oysters that were deployed without protective cages. But the oysters that were protected with cages did just fine.

This photo illustrates what Apalachicola oyster reefs are dealing with. This is one clutch of eggs laid by one adult snail. Within each little capsule, there are probably 10-20 baby snails. After a long winter’s nap, these snails are hungry.

We are going to keep at this, because one week long experiment doesn’t really tell us that much. But if we keep getting the same answer from multiple experiments, then we are getting somewhere.

In addition to updating y’all, I wanted to ask for your help. Because my small lab can’t be everywhere throughout the bay at all times, there are two things you could do if you are on the water.

Click the link to the right for GPS coordinates.

First, if you come upon our experiment, can you let me know when you happened upon them and how many buoys you saw? If you report that all buoys are present, then I’ll sleep really well. And if you alert us that some buoys are missing, then I’ll be grateful because we will stand a better of chance of quickly getting out there before the cages are inadvertently knocked around, so that we can recover the data.  Click here for GPS coordinates and further instructions.

Second, if you are tonging oysters, then you are probably tonging up snails. It would really help us to know when, where, and how many snails you caught.  Take a photo on your phone (Instagram hashtag #apalachcatch – Instagram instructions here) or e-mail them to robdv@wfsu.org.  We’ll be posting the photos and the information you provide on this blog.

This is kind of a new thing for us, attempting to use technology and community support this way.  There may be some bumps along the way.  If you’re having trouble trying to get photos to us, contact us at robdv@wfsu.org.

Thanks a bunch!

David

David’s Apalachicola Research is funded by Florida Sea Grant

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

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.

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Crown Conchs- Friend or Foe?

For today’s post, we shift our look at the ecology of fear from oyster reefs to the (often) neighboring salt marsh.  We know crown conchs are villains on oyster reefs, but might they redeem themselves “in the grass?”  If they live on the Forgotten Coast, it depends on what side of Apalachicola they live.
Dr. Randall Hughes FSU Coastal & Marine Lab
The Crown Conch (Melongena corona).

The Crown Conch (Melongena corona).

IGOR chip_ predators_NCE 150If you’re a fan of oysters and you read David’s previous post, then you probably don’t like crown conchs very much. Why? Because David and Hanna’s work shows that crown conchs may be responsible for eating lots of oysters, turning previously healthy reefs into barren outcrops of dead shell.  And we generally prefer that those oysters be left alive to filter water and make more oysters.  And, let’s be honest, we would rather eat them ourselves!

But, in something of a Dr. Jekyll and Mr. Hyde act, crown conchs can take on a different persona in the salt marsh. Here, the exact same species acts as the good guy, increasing the abundance of marsh cordgrass.  And more abundant marsh plants generally means more benefits for we humans in the form of erosion control, water filtration, and habitat for the fishes and crabs we like to eat.  How exactly does that work?

Periwinkle in Spartina predator experiment

The Marsh Periwinkle (Littoraria irrotata).

If you look out in a salt marsh in much of the Gulf and Southeast Atlantic, I can nearly guarantee that you’ll see a marsh periwinkle snail. Usually, you’ll see lots and lots of them. These marine snails actually don’t like to get wet – they climb up the stems of the marsh grass as the tide comes in. While they are up there, they sometimes decide to nibble on a little live cordgrass, creating a razor blade-like scar on the plant that is then colonized by fungus. The periwinkles really prefer to eat this fungus instead of the cordgrass, but the damage is done – the fungus can kill the entire cordgrass plant! So these seemingly benign and harmless periwinkles can sometimes wreak havoc on a marsh.

But wait a minute – if periwinkles cause all the cordgrass to die, then why do you still see so much cordgrass (and so many snails) in the marsh? That’s where the crown conch comes in.

Crown conch pursuing periwinkle snail

At the edge of a marsh at high tide, a crown conch approaches a periwinkle snail. As shown in the video above, the conch was soon to make contact with the smaller snail and send it racing (relative term- the video is of course sped up) up a Spartina shoot.

In marshes along the Gulf coast, there are also lots of crown conchs cruising around in the marsh (albeit slowly), and they like to eat periwinkles. Unlike other periwinkle predators such as blue crabs, the crown conchs stick around even at low tide. So when the periwinkles come down for a snack of benthic algae or dead plant material at low tide, the crown conchs are able to nab a few, reducing snail numbers. And fewer snails generally means more cordgrass.

Of course, the periwinkles aren’t dumb, and they often try to “race” away (again, these are snails!) when they realize a crown conch is in the neighborhood. One escape route is back up the cordgrass stems, or even better, up the stems of the taller needlerush that is often nearby. By causing periwinkles to spend time on the needlerush instead of grazing on cordgrass, or by making the periwinkles too scared to eat regardless of where they are sitting, the crown conch offers a second “non-consumptive” benefit for cordgrass. One of our recent experiments found that cordgrass biomass is much higher when crown conchs and periwinkles are present compared to when just periwinkles are present, even though not many periwinkles were actually eaten.

Periwinkle in Spartina predator experimentOn the other hand, if the periwinkles decide to climb up on the cordgrass when they sense a crown conch, and if they aren’t too scared to eat, then crown conchs can actually have a negative effect on the plants. This is exactly what David found in one of his experiments.  In this case, the tides play an important role – west of Apalachicola, where there is 1 high and 1 low tide per day, each tide naturally lasts longer than east of Apalachicola, where there are 2 high tides and 2 low tides per day.  The longer tides west of Apalach appear to encourage the snails not only to stay on the cordgrass, but also to eat like crazy, and the plants bear the brunt of this particular case of the munchies.

So even in the marsh, it turns out that crown conchs can be both a friend and a foe to marsh cordgrass, depending on how the periwinkles respond to them. And figuring out what makes periwinkles respond differently in different situations just gives us more work to do!

Music in the piece by Revolution Void.

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

Mud crabs (like the one pictured here), oyster drills, and crown conchs are the primary consumers of oysters on the reef.

How Do Predators Use Fear to Benefit Oysters?

Over the last few weeks, we’ve explored the concept of the ecology of fear on oyster reefs. But, as David asks in the video, “does it matter?” Exactly how much does fear affect oyster filtration, or their ability to support commercially and ecologically important species? And how does fear affect the benefits we receive from ecosystems such as salt marshes and seagrass beds? Coming up, we see how David and Randall took these big questions and broke them down into a series of experiments and investigations geared at creating a clearer picture of fear in the intertidal zone.
Dr. David Kimbro FSU Coastal & Marine Lab

IGOR chip_ predators_NCE 150A few weeks ago, we had a bayside conversation about the important link between nutrients and oysters. But there is something else that may dictate whether a reef thrives: predators.

Academically, the importance of predators dates back to the 1960s. Some smart people proposed that the world is green because we have lots of big animals, which eat all of the smaller animals that would otherwise consume all the plants…hence the green world.

busycon eating moon snail

Busycon spiratum eating an Atlantic Moon snail on Bay Mouth Bar. These seagrass beds off of Alligator Point are home to the greatest diversity of predatory snails in the world. In the late 1950s and early 1960s, Dr. Robert Paine investigated the effect of the horse conch, the most dominant predator among the snails, on the habitat. David and his crew have similarly used the dynamic invertebrate population to test their theories on the ecology of fear. (click the photo for more on Bay Mouth Bar).

Now, that’s a pretty simple yet powerful concept.  Since then, lots of studies have tested the importance of predators and how they keep our world spinning. For example, Bob Paine relentlessly braved the icy waters of the NW Pacific for a decade in order to chunk ravenous sea stars from one rocky cliff, but not the other. After several years, the cliff with sea stars still had a tremendous diversity of sea creatures (algae, anemones etc.) and the cliff without predatory sea stars did not. The absence of sea stars allowed pushy, bullying mussels to outcompete all other animals for space and this gave the rocky cliff a uniform and boring mussel complexion.

The same concept has been tested on land. Ripple and Beschetta showed us why the national parks out west no longer have the really important and woody trees (aspen, willow, and cottonwood) that they historically had. By suppressing wolves for the last 50 years, we allowed elk numbers to explode and the elk have overrun the really important woody species.

But predators don’t just eat.  Enter my vivid memory of trying out for the Nash Central 8th grade football team in rural North Carolina. Contrary to my father in-law’s belief (who is a hall of fame football coach in Georgia), I wanted to play football instead of soccer.  But when it came time for try-outs, fear prevented me from pursuing this line of work.  To practice breaking tackles, each player had to lie on the ground and the rest of the team formed a circle around this player.  Unbeknownst to the guy on the ground, the coach secretly selected three players to tackle the football player at the sound of the whistle.  For twenty minutes, I watched physically un-developed friend after late-blooming friend get crushed by other guys who were definitely not late bloomers. The sights and sounds of this drill kept me nauseous until it was my turn. When my turn came, I couldn’t deal with the fear, didn’t perform well, and consequently became a soccer player.

My point is that fear is very powerful. Of course, I knew the charging football players were not going to eat me. But if I was paralyzed with fear from football, then imagine what it’s like for something that has to worry about being eaten. Going to back “the world is green” story: what if we overlay the concept of fear on that? How does the story change?

Well, the next generation of predator studies has examined how the fear of predators can be just as important as the appetite of predators. In addition, because predators can only eat only one animal at a time but can simultaneously frighten many more, fear can create powerful “remote-control effects”. In Australia, the fear of tiger sharks causes dugongs to avoid certain depths in a bay. As a result, only a small portion of the seagrass beds get grazed down by dugongs, possibly being one of the main reasons why areas like Shark Bay still have huge and lush seagrass meadows.

Mud crabs (like the one pictured here), oyster drills, and crown conchs are the primary consumers of oysters on the reef.

For the next few weeks, we will look at some work that my friends and I have conducted for the past three years on how predators and the fear of predators influence oyster reefs and the services that they provide us throughtout the southeast. Although we have the same predators and things that like to eat oysters from North Carolina to Florida, we suspect that differences in the environment will cause the effect of predators to play out differently.

In parting, I just want to say that this predator stuff is really interesting and I think it’s very important for oyster reefs. But of course, when you are dealing with an ecosystem that may be on the verge of collapse like Apalachicola Bay, the distinction between the appetite and fear of predators may not matter that much. But, we will soon see because we are now investigating this important system too.

We'll be following the Apalach study as well. Here, Stephanie Buhler, who we had previously seen diving in Apalachicola Bay, welds a cage to house an upcoming experiment in that research. It's a variation of the tile experiments that became such a staple of the NSF oyster study. In a few days, we break down the tile experiment, and David's collaborator, Dr. Randall Hughes, talks about what the results are telling them so far.

Music in the video by Revolution Void.

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