Tag Archives: experiments

Tricks or Treats? And more on the effects of predators in marshes.

Dr. David Kimbro FSU Coastal & Marine Lab

IGOR chip_ predators_NCE 150Unlike most of the experiments that I’ve conducted up to this point in my career, the oyster experiment from this past summer does not contain a lot of data that can be analyzed quickly.

For example, predator effects on the survivorship of oysters can be quickly determined by simply counting the number of living as well as dead oysters and then by analyzing how survivorship changes across our 3 experimental treatments (i.e., cages with oysters only; cages with mudcrabs and oysters; cages with predators, mudcrabs, and oysters).  But this simple type of data tells us an incomplete story, because we are also interested in whether predators affected oyster filtration behavior and whether these behavioral effects led to differences in oyster traits (e.g., muscle mass) and ultimately the oyster’s influence on sediment characteristics.  If you recall, oyster filter-feeding and waste excretion can sometimes create sediment conditions that promote the removal of excess nitrogen from the system (i.e., denitrification)

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As we are currently learning, getting the latter type of data after the experiment involves multiple time-consuming and tedious steps such as measuring the length and weight of each oyster, shucking it, scooping out and weighing the muscle tissue, drying the muscle tissue for 48 hours, and re-weighing the muscle tissue (read more about this process here).

After repeating all of these steps for nearly 4,000 individual oysters, we can subtract the wet and dry tissue masses to assess whether oysters were generally:

(a) all shell…“Yikes! Lot’s of predators around so I’ll devote all of my energy into thickening my shell”

(b) all meat…“Smells relaxing here, so why bother thickening my shell”

(c) or a mix of the two.

For the next two months, I will resemble a kid with a full Halloween bag of candy who cannot wait to look inside his bag to see whether it’s full of tricks (nonsensical data) or some tasty treats (nice clean and interesting data patterns)!  I’ll happily share the answer with you as soon as we get all the data in order.

Because of this delay, let’s explore some new research of mine that examined how predators affect prey traits in local marshes and why it matters.

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There are two main ingredients to this story:

(a) tides (high versus low) dictate how often and how long predators like blue crabs visit marshes to feast on tasty prey.

(b) prey are not hapless victims; like you and me, they will avoid risky situations.

attach.msc1In Spartina alterniflora systems, periwinkle snails (prey) munch on dead plant material (detritus) lying on the ground or fungus growing on the Spartina leaves that hover over the ground.  Actually, according to Dr. B. Silliman at the University of Florida, these snails farm fungus by slicing open the Spartina leaves, which are then colonized by a fungal infection.  If snails fungal farm too much, then the plant will eventually become stressed and die.

So, I wondered if the fear of predators might control the intensity of this fungal farming and plant damage.

For instance, when the tide floods the marsh, snails race (pretty darn fast for a snail!) up plants to avoid the influx of hungry predators such as the blue crab.

After thinking about this image for a while, I wondered whether water full of predator cues might enhance fungal farming by causing the snail to remain away from the risky ground even during low tide.  Eventually, the snail would get hungry and need to eat, right?  Hence, my hypothesis about enhanced fungal farming due to predator cues.   I also wondered how much of this dynamic might depend on the schedule of the tide.

Before delving into how I answered these questions, you are probably wondering whether this nuance really matters in such a complicated world.  Fair enough, and so did I.

Addressing this doubt, I looked all around our coastline for any confirmatory signs and found that Spartina was less productive and had a lot more snail-farming scars along shorelines subjected to a diurnal tidal schedule (12 hours flood and 12 hours ebb each day) when compared to shorelines subjected to a mixed semidiurnal schedule (2 low tides interspersed among 2 high tides that are each 6 hours).  Even cooler, this pattern occurred despite there being equal numbers of snails and predators along both shorelines; obviously density or consumption effects are not driving this pattern.

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Ok, with this observation, I felt more confident in carrying out a pretty crazy laboratory experiment to see if my hypothesis might provide an explanation.

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Enter Bobby Henderson.  This skilled wizard constructed a system that allowed me to manipulate tides within tanks and therefore mimic natural marsh systems; well, at least more so than does a system of buckets that ignore the tides.

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Within each row of tide (blue or red), I randomly assigned each tank a particular predator treatment.  These treatments allowed me to dictate not only whether predators were present but whether they could consume & frighten snails versus just frightening them:

-Spartina only

-Spartina and snails

-Spartina, snails, and crown conch (predator)

-Spartina, snails, blue crab (predator)

-Spartina, snails, crown conch and blue crab (multiple predators)

-Spartina, snails, cue of crown conch (non-lethal predator)

-Spartina, snails, cue of blue crab (non-lethal predator)

-Spartina, snails, cues of crown conch and blue crab (non-lethal multiple predators)

attach.msc6After a few weeks, I found out the following:

(1) Predators caused snails to ascend Spartina regardless of tide and predator identity.  In other words, any predator cue and tide did the job in terms of scaring the dickens out of snails.

(2) Regardless of tide, blue crabs ate a lot more snails than did the slow moving crown conch and together they ate even more.  This ain’t rocket science!

(3) In this refuge from the predators, snails in the diurnal tide wacked away at the marsh while snails in the mixed tide had no effect on the marsh.

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Whoa…the tidal schedule totally dictated whether predator cues indirectly benefitted or harmed Spartina through their direct effects on snail predator-avoidance and farming behavior.  And, this matches the observations in nature… pretty cool story about how the same assemblage of predator and prey can dance to a different tune when put in a slightly different environment.  This study will soon be published in the journal Ecology.  But until its publication, you can check out a more formal summary of this study here.

If this sort of thing happens just along a relatively small portion of our coastline, I can’t wait to see what comes of our data from the oyster experiment, which was conducted over 1,000 km.

Till next time,

David

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

The making of an experiment

Dr. Randall Hughes FSU Coastal & Marine Lab

“Wow, quite the set-up! I am jealous of that space!

“…As a side question, how did you pump the cue water to all your tubs, a peristaltic pump? Was it just gravity? Seems like quite the complicated set-up.”

Excerpts from a comment on Randall’s September 28th post, Scared Hungry.  Read the whole comment here.

IGOR chip- employment 150This recent comment by John Carroll made me realize that there are a lot of unsung heroes at the lab that don’t typically get credit for the essential work that they do to facilitate our research. So here is a ‘behind-the-scenes’ look at setting up an experiment:

1. The idea. This is the main part that I can take credit for, though even then an idea usually stems not simply from my brain, but from a paper I’ve read, a conversation with a colleague or student, or an observation in the field.

2. The infrastructure. Each experiment has its own specifics, but in my research there are generally 3 main requirements:

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The "small" deck, used by David and me for mesocosm experiments with snails, crabs, plants, oysters - you name it!

a. Space. FSUCML has numerous tanks and related facilities for use in research (Visit the Lab site here.). Of course, I often have specific needs or desires, and thus my first step is usually to speak to Mary Balthrop, our Associate Director, and then to Dennis Tinsley, our Facilities Manager. Both Mary and Dennis show a great deal of good humor in receiving my seemingly hair-brained requests (e.g., a deck that can hold 16 plastic kiddie pools full of sand and water!), and they work with me to find (or devise) a suitable space to get the job done. Our incredible carpenter, Dan Overlin, then has the task of modifying or creating that space.

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The newer "large" deck, obscuring the view of the small deck closer to the water's edge

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Another view of the small deck, with the large seawater tanks in the background. (Photo credit: Nancy Smith)

b. Seawater. Since we work with marine critters, access to seawater is critical. FSUCML pumps seawater from the bay in front of the lab into large holding tanks that feed the entire facility.

Mark Daniels and Bobby Henderson then create the plumbing system that gets that water where it needs to go. They know everything there is to know about PVC pipes, water filters, pumps – you name it! As I mentioned in my response to John, it was Bobby who came up with the incredible pump apparatus (and several subsequent revisions) that has enabled us to conduct several experiments examining the effects of predator cues on prey behavior.

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Robyn and Emily working to set up a recent experiment on the large deck. Although the plants love all the light, we decided to erect a tent as a refuge from the sun/heat.

c. Light. When working with plants, light is key. I’m fortunate to have access to a greenhouse, as well as abundant outdoor space at the lab to set up experiments. Or perhaps I should say once-abundant outdoor space, since David and my decks now cover a good chunk of it! Dennis is a pro at thinking of suitable and available spaces to squeeze in a few tanks.

Robyn and Emily releasing grasshoppers into one of our cages. (Photo credit: Nancy Smith)

3. The supplies. Once the infrastructure is in place, it’s time to buy the supplies needed to make each experimental unit. The job then falls to Kathy Houck and Maranda Marxsen to explain to the accountants at FSU why I purchased several large bolts of tulle fabric (grasshopper cages), or 24 pair of knee-high panty hose (they make great filters when filled with gravel), or lots and lots of nail polish (for marking snails). For field experiments, Sharon Thoman is helpful in arranging vehicles and boat reservations, sometimes at the very last minute!

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Robyn and Liz cheerfully using nail polish to mark snails

 

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A thundercloud looms in the distance. Once this summer we were stranded in a storm and Dan came to retrieve us.

4. Set-up. Once things start to come together, there are inevitable surprises the crop up. In our recent predator-prey experiments, we had issues with flow from the pump being greater than that from the regular seawater lines, which required some brain-storming from Bobby, David, Kelly, Meagan, and myself. Or, a plumbing line will clog, and I’ll run to find Mark.  Or, we’ll get stranded in a thunderstorm while collecting mud crabs and Dan will come pick us up.   At least we often provide fodder for funny stories!

5. The experiment. And at last, the actual experiment can begin. When I come up with particularly high-maintenance experiments, it’s useful to utilize the lab dorms for the night. Linda Messer is always understanding of last minute housing requests and changes, making sure the lights (and, more importantly, the A/C) are on! Sometimes, the experiment itself is much shorter than the time required to set it up – duration never seems to equate with complexity. But one of the benefits of consulting with the staff is to ensure that the same space can be used for multiple purposes. And the second experiment is always easier to set up than the first!

Randall and David’s research 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.

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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!

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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.

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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.

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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!)

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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.)

Slide1

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.

Summer Chaos and The Tower of Cards

Throughout this week, Dr. David Kimbro has been updating us about the premature dismantling of his lab’s summer experiment in preparation for Hurricane Irene.   Before this turn of events, David’s lab tech, Tanya Rogers, had written this account detailing how much work went into assembling the experiment and all of its (literally) moving parts.

Tanya Rogers FSU Coastal & Marine Lab

Beautiful, isn't it? But working on oyster reefs in Jacksonville hasn't been as nice as its sunrises.

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For many labs, the summer field season is a period of intensity and madness: a time for tackling far too many projects and cramming as much research as possible into a preciously short window. It’s a demanding flurry of activity occasionally bordering on chaos. The greatest challenge for technicians like myself is to maintain order in this pandemonium of science, and to carry out as much field work as efficiently as possible without going crazy.

Continue reading

Are two friends better than one?

Dr. Randall Hughes FSU Coastal & Marine Lab

IGOR chip- biodiversity 150

Sand fiddler crab.

This summer we’ve been conducting an experiment on our new deck to look at the effects of fiddler crabs and ribbed mussels on Spartina alterniflora (smooth cordgrass).

Past studies by Dr. Mark Bertness have shown that crabs and mussels by themselves can have positive effects on plant growth – most likely because crabs can reduce the stress of low oxygen in the sediments by building their burrows, and mussels can add nutrients to the sediments.

Fig. 3 from Bertness 1984, Ecology 65: 1794-1807

Figure 3 from Mark Bertness's 1984 Ecology study illustrating the positive effects of mussel presence (white bars) on Spartina

Table 3 from Bertness 1985, Ecology 66: 1042-1055

Table 3 from Mark Bertness's 1985 Ecology study. Fiddler removal has a negative effect on Spartina in the marsh flat, but not the marsh edge.

Although both fiddlers and mussels occur together in the field, no studies have looked at how the combination affects the plants. Are the positive effects of each species by itself doubled? Or are they redundant with each other? Do crabs somehow reduce the positive effect of mussels, or vice-versa? How many crabs or mussels do you need to get a positive effect on Spartina? These are some of the questions that we hope to answer with our experiment.

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Our new deck at FSUCML.

But first, we had to get everything set up. There were several long and hot days of shoveling sand into our “mesocosms” (10 gallon buckets) – many thanks to Robyn, Chris, Althea, and all the others who took care of that task! Then there was another day spent transplanting the Spartina.

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Chris, Randall, and Robyn work to transplant Spartina from the greenhouse to the mesocosms.

Finally, it was time to add the fiddlers and mussels, and everything began!

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Mussels nestled among the Spartina stems in one of our experimental mesocosms

Althea and Chris have been leading the charge on this experiment, and they’ve spent a lot of time getting to know (and identify) the fiddler crabs. All in all, a pretty fun study organism!

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Althea working to identify fiddler crab species.

We’ll continue the experiment another month and then measure the height and density of the plants in each treatment to see if there are any differences. Once this experiment is complete, we’ll set up a separate one asking somewhat of the converse question – are two enemies (periwinkle snails and grasshoppers) worse than one? We’ll keep you posted.

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