All posts by Randall

About Randall

Dr. Randall Hughes is an ecologist and marine biologist focusing on the causes and consequences of species and genetic diversity in coastal systems. She has conducted experimental work on plants and animals in seagrasses, salt marshes, oyster reefs, and kelp forests. The common thread throughout these activities is a long-standing interest in generating information that can enhance the effectiveness of conservation and management decisions.

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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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Seagrass Wrack in the Salt Marsh – Blessing or Curse?

2-Minute Video: Seagrass wrack kills part of the marsh, but do its benefits outweigh the destruction?

Our videos to date have centered on biodiversity in the marsh and how it can make a marsh stronger against disturbances. As we see in this video, at least one type of disturbance might actually promote genetic and/ or species diversity.
Dr. Randall Hughes FSU Coastal & Marine Lab/ Northeastern University
This snake was found in a seagrass wrack experiment in the Saint Joseph Bay State Buffer Preserve. Blue crabs were often found taking shelter in their experimental plots as well.

This snake was found in a seagrass wrack experiment in the Saint Joseph Bay State Buffer Preserve. Blue crabs were often found taking shelter in their experimental plots as well.

This time of year if you look around salt marshes in our area, you’ll probably see a strip of dead plant material, or “wrack”, resting on top of the salt marsh plants around the high tide line. Look closer, and you’ll see that it’s mostly made up of seagrass leaves that have either been sloughed off naturally (seagrasses produce lots of new leaves in the summer and shed the old ones) or, occasionally, uprooted by boats driving through shallow seagrass beds. Look even closer (say, by picking it up), and you may just find a harmless marsh snake (or worse, a cottonmouth!) – in our experience, they like to hang out in the cool, moist areas under the wrack.

So is this wrack “good” or “bad” for the salt marsh? As with many things in life, the answer depends on your perspective. If you’re a snake or other critter that likes the habitat provided by the wrack, then it’s probably a good thing. On the other hand, if you’re one of my crew who finds that snake, and particularly if you’re Robyn who REALLY doesn’t like snakes, then it’s most definitely a bad thing. Or, if you happen to be the plant that the wrack settles on top of for long periods of time, then it’s a bad thing, because many of those plants die. But, if you’re a seed that is looking for a good spot to germinate in the marsh, then the bare spot created by the wrack is likely a good thing.

Bare spot left in salt marsh left by seagrass wrack.Last fall, David and I teamed up with Dr. Peter Macreadie from the University of Technology Sydney to find out how the bare “halos” created when wrack mats smother the underlying marsh plants influence the marsh sediments. It turns out, these bare areas store less carbon in the sediments than the nearby vegetated areas, which makes them less valuable as “sinks” for carbon dioxide. But as I mentioned earlier, the bare areas can also serve as a good spot for new plant species (or new genotype of a given species) to start growing, potentially increasing the overall diversity of the salt marsh. And as the seagrass wrack decays, it can provide valuable nutrients to the marsh sediments that support future plant growth. So what is the net outcome of all these good and bad effects?

We decided to do an experiment to answer that very question. As Ryan and Meagan will attest (along with almost everyone else in our labs who we enlisted to help us), this was a very labor-intensive experiment. First, we had to figure out how much wrack is typically in a given area of marsh. Then, we had to collect a lot of wrack, weigh it, assemble it into bags that could be “easily” moved to our experiment, and add it to cages that would help hold it in place. We’re talking ~1.5 tons of wrack picked up and moved to various spots!

FSU Coastal and Marine Lab technician Megan Murdock spin dries seagrass wrack for an experiment at the Saint Joseph Bay State Buffer Preserve.To make matters even more interesting, we had to soak the collected wrack in water to make sure it was all the same wetness, and then spin it around in mesh bags (think salad spinner on a very large scale) for a set amount of time to make sure we could get a consistent weight measurement on each bag. Anyone driving past the SJB Buffer Preserve in early September of last year must have wondered what craziness we were up to! And since we were interested in whether the length of time the wrack sits in one place influences its effects, or whether the number of times that wrack sits in a particular area matters, we moved all of this wrack around in our cages every 2 weeks for 3 months to mimic the movement of natural wrack by the tides. And then we measured everything we could think of to measure about the marsh.

We’re still going through all the data to determine the net outcome, but as expected, whether the wrack is a blessing or curse depends on who you are:

  • Juvenile blue crabs seem to like hanging out in the wrack (which is a much nicer surprise to find than a snake, even when they are feisty!)
  • Fiddler crabs also appear to like the wrack, with greater burrow numbers when wrack is present.
  • Contrary to our expectation that wrack would kill cordgrass and allow other plant species to recruit into the marsh, it looked like cordgrass actually did better in the wrack cages!
  • Sea lavender, a marsh plant with very pretty purple flowers, does not do so well when covered with wrack (Learn more about sea lavender and its relationship with mussels).

More to come once all the data are analyzed…

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.

Music in the piece by Philippe Mangold.

Dr. Randall Hughes inspects a black mangrove growing in the Saint Joseph Bay State Buffer Preserve.

Black Mangroves: Strangers in a St. Joe Bay Marsh

2-Minute Video: Mangroves don’t love the cold, but relatively mild winters have seen them multiply north of their range.  Randall takes a closer look at black mangroves in  the salt marshes of Saint Joseph Bay.

Dr. Randall Hughes FSU Coastal & Marine Lab/ Northeastern University

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A few years ago, I took my colleague Dr. Ed Proffitt to check out the marshes in St. Joseph Bay. He asked to see mangroves, and I thought he was crazy. Mangroves up here? No way! But we had only been in one Buffer Preserve salt marsh together for a few minutes before I realized that the small “shrubs” that I had previously ignored were actually small black mangroves! And the more we looked, the more we found. They aren’t everywhere, but they can be quite abundant in some places.

Shrubby black mangroves (Avicennia germinans) are an increasingly common site in the Saint Joseph Bay State Buffer Preserve.

Shrubby black mangroves (Avicennia germinans) appear to be an increasingly common site in the marshes of the Saint Joseph Bay State Buffer Preserve.

Mangroves typically occur below the “frost line”, or in areas that don’t experience hard freezes. Lore has it that mangroves have become more common in the northern Gulf of Mexico in recent years due to a series of mild winters. I haven’t been monitoring them long enough to say whether or not there are more now than there were, say, 10 or even 20 years ago, but it’s not hard to see that the ones that are here are successfully reproducing, with small seedlings surrounding the adult trees.

There are even red mangroves lingering around – they are less cold-tolerant than the black mangroves and a surprise to find in our marshes!

Dr. Randall Hughes inspects a black mangrove growing in the Saint Joseph Bay State Buffer Preserve.I definitely have not seen any significant dieback in the last 5 winters, even when we have had hard freezes. And I would not be surprised if they become more common and abundant as the climate continues to change.

Mangroves in the marsh raise a number of interesting questions. Will they take over? What will that mean for the services these areas provide to people? Will the fishes and crabs that we like to eat become more or less abundant if mangroves dominate over marsh grasses?

A study conducted in Texas marshes looked at conditions under which mangroves best survived in marshes.

Unfortunately, I don’t have the answer to these questions. But I can say that the mangroves that occur in St. Joseph Bay aren’t necessarily “better” at surviving in the northern Gulf than mangroves from farther down south. And why should they be?  Well, if a group of mangrove propagules arrived in St. Joe Bay, we may expect that only a subset of them would be able to survive the colder temperatures, and when these propagules grew into adult trees and produced propagules of their own, they should pass that “benefit” to their offspring (the process known as natural selection).

Black mangrove propagules.

Black mangrove propagules.

How do we we test whether St. Joe Bay mangroves are better equipped to live here than mangroves from down south? We have 2 ongoing experiments where we’ve planted “propagules” (young mangroves that look a lot like seeds) from different locations throughout FL in St. Joe Bay and followed them through time to see which ones survive and grow the best. There’s a lot of variation, but the St. Joe Bay propagules (which were largely the “runts” of the bunch to begin with) did not do as well as propagules from some of the areas down south such as Cedar Key and Cape Canaveral. These results suggest that it doesn’t take a particularly special propagule to survive in the northern Gulf; instead, there probably aren’t just many propagules that make it up here to begin with.

Of course, we’ve only been monitoring these propagules for 1-2 years; maybe the St. Joe propagules have an advantage when they get old / big enough to reproduce. We don’t want to speed up the mangrove take-over, so we’ll remove the seedlings in our experiment before that happens. But we’ll definitely continue to monitor the ones that already made it here on their own accord to see what they do next!

The Guana Tolomato Matanzas National Estuarine Research Reserve (NERR) south of Saint Augustine is where Randall and David have done a lot of their oyster research.  There, mangroves mingle with marsh cordgrass. Could salt marshes in St. Joseph Bay (or north Florida in general) one day look like something approximating this?

Music in the video by pitx.

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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Diversity- Getting by With a Little Help From (Salt) Marsh Friends

2-Minute Video: Marsh cordgrass, needlerush, sea lavender, mussels, periwinkle snails, and fiddler crabs: diversity in the salt marsh.

In Randall’s last post, she looked at whether genetic diversity within the salt marsh foundation species- smooth cordgrass- made for a stronger marsh (and by stronger, of course, we mean better able to shelter yummy blue crabs for people and sea turtles). In today’s post and video, Randall examines how the combination of plants and animals around cordgrass- the species diversity of a marsh- might play a role as well.
Dr. Randall Hughes FSU Coastal & Marine Lab/ Northeastern University

IGOR chip- biodiversity 150Even though salt marshes often look like one big sea of green in the intertidal, there are plants and animals other than marsh cordgrass around. And even though I devote a lot of effort to understanding the effects of diversity just within cordgrass, these other species are also important – no marsh is an island. (Well, actually they are, but you get the analogy.)

Fiddler crab found in a St. Joseph Bay salt marsh.So who is important, and why? There are at least two animals that can be classified as “friends” of cordgrass – fiddler crabs and mussels. Fiddler crabs create burrows that allow oxygen to get down in the sediment, and cordgrass roots appreciate that oxygen. The fiddler crabs also aerate the sediment during their feeding, and they can excrete nutrients that the plants use to grow.

As an aside, fiddler crabs are also irresistible for kids (and maybe adults too!).

Mussels aren’t quite as charismatic as fiddler crabs, but they like to nestle around stems of cordgrass, and the byssal threads that they use to attach to one another and to the sediment can help prevent erosion. In addition, they excrete nutrients and other organic material as a byproduct of their filter-feeding, and the plants take advantage of these nutrients.

While investigating the relationship between mussels and marsh cordgrass, Randall’s graduate student, Althea Moore, noticed that mussels also seemed to often accompany sea lavender in the marsh. This led to a separate study for Althea.

So who is MORE important, mussels or fiddler crabs? We did an experiment to test that question, or really, to test whether having mussels and fiddler crabs together is better than having just one or another. The answer? As with much in ecology – it depends. For one, it depends on what you measure. If you look at the number of cordgrass stems, then fiddler crabs are the better friend – cordgrass with fiddler crabs does better than cordgrass without fiddler crabs, regardless of whether you have mussels or not. But if you look at how tall the plants are (another important characteristic in the marsh), then mussels are the better friend, but only when fiddlers aren’t around. And if you look at the amount of organic content, mussels increase organic content at the sediment surface, whereas fiddlers increase it belowground. In the end, the take-home message is that the more things you measure about the marsh, the more important it becomes that you have both mussels and fiddler crabs in order to be the “best”.

In the process of doing the experiment I described above, Althea (my graduate student) noticed that when she was out in the marsh, she often found mussels in and around sea lavender (Limonium) plants more often than she found them around cordgrass. She became interested in finding out whether the mussels benefit the sea lavender, the sea lavender benefits the mussels, or a little bit of both. She’s still working on the answer, but it just goes to show that although we often tend to focus on who eats who (think Shark Week) or who can beat who (Octopus vs. Shark, anyone? Or, for kids, there’s always Shark vs. Train – a favorite at my house!), there are just as many instances of species helping one another (not that they always intend to).

Of course, it’s not just animals helping (aka, facilitating) plants – plants can help other plant species to. We’ve shown through a series of experiments that cordgrass benefits from having its tall neighbor needlerush (Juncus roemarianus) around, but only if the snails that like to graze on cordgrass are also present. Nothing is ever as simple as it looks in the marsh…

Music in the piece by Revolution Void.

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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The Many Personalities of Salt Marsh Cordgrass

2-Minute Video: Do sea turtles and fishermen benefit from a genetic diversity in marsh cordgrass?

As Randall mentioned in her last post, biodiversity can mean many things. In this video, she examines diversity within a species, in particular marsh cordgrass (Spartina alterniflora). Each Spartina plant has its own personality. What Randall wants to know is: are more personalities better for a salt marsh (and the sea turtles and blue crabs that use it)?
Dr. Randall Hughes FSU Coastal & Marine Lab / Northeastern University

Its difficult to see the diversity of the cordgrass in this vast sea of green.IGOR chip- biodiversity 150One of the more striking things about a salt marsh at first glance is how uniform it is. A sea of green. Or maybe a sea of green (cordgrass) in the intertidal and brown (needlerush) further back. But definitely not something that screams “diversity”.  And yet, wondering about the importance of diversity in the salt marsh is what I spend a lot of my time doing.

Often, when scientists talk about diversity, they are referring to different species of plants and animals (= species diversity). But there are actually lots of different kinds of diversity – functional diversity, phylogenetic diversity, genetic diversity. To illustrate what these terms mean, let’s shift to the topic I most like to think about when I’m not thinking about science – FOOD. Functional diversity is one of the broader categories, where different species are grouped by how they look or behave. So, think vegetables vs. fruits. Or, even green leafy veggies vs. root vegetables vs. berries vs. melons. Phylogenetic diversity refers to how related the species are – broccoli and cauliflower are more closely related (and thus have less phylogenetic diversity) than broccoli and zucchini – whereas species diversity refers to how many different species there are. Finally, even a single species can have a lot of diversity within it – apples are a perfect example of a fruit with large numbers of varieties to choose from. It’s this last level of diversity, genetic diversity, that I’m really interested in.

Some of a Spartina plant's below ground root structure.  Many of the plants used in Randall's experiment have sent out rhizomes under the sediment which have sprouted new shoots.  If you're at the edge of a salt marsh and see a line of marsh cordgrass plants sticking out into the water, they're likely connected by such a rhizome.

Some of a Spartina plant's below ground root structure. Many of the plants used in Randall's experiment have sent out rhizomes under the sediment which have sprouted new shoots. If you're at the edge of a salt marsh and see a line of marsh cordgrass plants sticking out into the water, they're likely connected by such a rhizome.

Unfortunately, it’s not as easy to tell different ‘varieties’ (aka, genotypes) of cordgrass apart as it is to tell a Granny Smith from a Red Delicious. Most of the plants look really similar, and it’s impossible to tell by looking from above the ground which ones are connected by roots and rhizomes below the ground. I have 2 solutions to this problem:

1. Take small pieces of cordgrass into the lab and use even smaller snippets of DNA to tell who is who. (As my dad says, think CSI with grass.)

2. Take a single cordgrass stem and grow it in a flowerpot in the greenhouse until it starts to produce lots of other stems. By splitting these up and allowing them to continue to grow (and keeping careful track of which pot is which), I can produce a supply of known cordgrass genotypes to do experiments with.

Neither of these techniques happens overnight. In fact, it took us nearly 2 full years to get enough genotypes in the greenhouse to start doing experiments with! (Proof that even someone without a green thumb – like me – can work with plants, but it takes longer than it should.)

Each bar in these graphs represents a different marsh cordgrass genotype. You can see how each plant differs in "personality" from its number of stems and flexibility.

Each bar in these graphs represents a different marsh cordgrass genotype. You can see how each plant differs in "personality" from its number of stems and flexibility.

The cool thing about spending that much time with these plants is that you start to recognize that they have their own flavors or personalities. Some grow a few really tall stems vs. others with lots of average size stems; some flower in July vs. others that flower in October; some have really flexible stems vs. others that are more rigid (which probably matters if you’re a snail climbing up the stem).

It’s these different personalities that may allow a mixture of multiple genotypes to do better over time than a genotype growing by itself. For one, if some genotypes are better at surviving some sort of disturbance, then having a mixture of genotypes increases the chances that one of those “good” genotypes is included in the mix. Or, if genotypes are all a little bit different, they may be complementary to one another, allowing the mixture to do better than any one of them growing alone. (This “complementarity” can also be referred to as facilitation, and it’s an idea we’ll return to in coming weeks.)

We have several experiments that are wrapping up now that test this very idea – do diverse marsh patches perform better than less diverse patches? If so, why does that happen? Early indications are that the answer is like a lot of things in nature – it depends. Sometimes the diverse patches do better, and sometimes they don’t. So now the trick is to figure out how to predict when diversity will matter, and use that information to help conserve existing salt marshes and restore marshes that have been damaged in the past (which can help provide a steady supply of blue crabs for both sea turtles and us!).

Music in the piece by airtone.  Thanks to Mineral Springs Seafood for letting us tag along as they emptied their crab traps.  And thanks to Gulf Specimen Marine Lab for the underwater footage of Allie the sea turtle (good luck out there, Allie!).

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.