I realize that I have not made a blog post in a very long time. My apologies to any followers I still have left. Today marks the opening day of the 42nd Annual Benthic Ecology Meeting, and I figured it was as good a time as any to make a new blog post. Afterall, I have made multiple BEM-related posts in the past, and I am currently waiting for my ride to leave for the meeting.
A lot has changed for me since the last benthic meeting. I completed my dissertation and relocated to UNCW to start my post-doctoral career. It has been pretty hectic. When I first came down, I was trying to finish up some manuscripts from my dissertation, like the chapter on the impacts of Codium fragile on scallop demographics such as growth rate and tissue condition. My conclusions were that the invasive alga might be beneficial for scallop populations, especially in the absence of their native habitat, seagrass. I have made this argument before, and this chapter was recently published in Marine Biology. Other chapters haven’t gone through so smoothly and are still being reviewed, but that is par for the course in this field.
I was balancing those with editing other manuscripts from collaborative efforts with my former lab and one of my committee members. I also made two failed attempts at doing a laboratory study with oyster spat and an ectoparasitic snail. The results were promising, but I kept having high mortality across all treatments, and I need to come up with a better way of maintaining and feeding the oysters in a lab setting. I also have now written 4 proposals to various funding agencies, and am currently working up some old data sets for my current lab. Within all this, I crammed a 2.5 week trip to Jamaica to attempt to do some sponge work, but the weather didn’t cooperate (well, not with me anyway, my stomach is not the biggest fan of the ocean). Suffice it to say, I have been extremely busy, but that isn’t really an excuse to have stopped making regular updates. However, I have only been in “the field” once since I relocated, and it’s not very much fun writing blog posts about writing Sea Grant proposals.
However, this will be a nice little break, and I am excited to be headed down to the meeting. There are a lot of talks this year that promise to be very good and informative, plus there is also the Beneath the Waves Film Festival which is always excellent. And, in general, I like to see former colleagues, friends, potential future collaborators and have a generally good time drinking beer and talking all things marine science.
My talk this year will involve some work from my former lab on multiple predators. Natural communities have multiple predators foraging on shared prey resources, and until the last decade or so, these interactions were largely ignored in lab studies. They are interesting, because the consumption of prey is rarely additive – that is, two predators do not typically consume the same amount of prey you would expect based on how much they can eat when they are alone. More often, the prey either experiences reduced or enhanced risk relative to expected consumption. For crabs interactions, which utilize prey and habitats similarly, we expect that antagonistic interactions increase, resulting in reduced risk on the prey. Check out this video:
What you can see is the smaller crab is like your annoying little sibling who just won’t leave you alone and constantly antagonizes you. It kind of makes you stop what you are doing. In crabs, this means they might stop foraging to deal with each other. This usually means that the prey survive better than would be expected. However, this isn’t always the case when you run the trials and do the statistical analysis:
Proportion of ribbed mussels consumed by Hemigrapsus alone (pink bar), by Carcinus alone (green bar) and the two crabs together (gray bar). The circle denotes the expected consumption.
In this case, our observed consumption was not different than we expected, based on individual consumption rates. We anticipated to see a risk reduction, and based on the video, we know the crabs were interacting. So what gives? Upon further inspection, when we looked at the sizes of mussels consumed, we saw a dramatic shift:
Pink bars are mussels consumed by Hemigrapsus, Green bays by Carcinus and gray bars by both
What we saw was that when foraging along, the green crabs consumed all the size ranges that were offered, but when foraging together, they shifted to selecting smaller prey, possibly because they had less time to forage. So while the overall proportion being consumed stayed the same, they were foraging on a smaller portion of the population. We thought that was pretty cool!
Stay tuned for more posts, I promise to do better!
New species get introduced into novel habitats almost like clockwork in the modern era. These are termed introduced or exotic species. Typically, these introductions are the effect of anthropogenic activity. Sometimes, these species become nuisances – spreading in their new habitats via natural processes, and creating problems for native species. These nuisance exotics are called invasive species.
Green Crab - Invader!
In the marine realm, there are numerous invaders from all taxa – from plants and algae, to tunicates, crabs, mollusks, and all the way up to fish and birds. Some of the species have been here for centuries- such as the green crab, Carcinus menas, on the east coast, and the mute swan, Cygnus olor. Still, others are much more recent, such as the Asian shore crab, Hemigrapsus sanguineus, the Chinese mitten crab, Eriocheir sinensis, the lionfish, Pterois volitans,(which I’ve recently blogged about many times), and the tunicate Didemnum vexillum (which I blogged about here). Regardless of species, and length of time since invasion, these species have potential to be harmful to their new environments. The East Coast of the US is particularly hard hit by these species.
Map of invasive problem regions
Map of origins of marine invaders
So how do they get here? From a variety of ways, but perhaps most famously via ships’ ballast dumping. Ballast is simply material used by ships to control and maintain buoyancy and stability. Typically this is water, pumped into ballast tanks from the port the ship is sitting in at the time. This ballast water gets pumped in or out depending on the weight of the cargo on this ship, and so you can imagine how water could be transferred across whole oceans, bringing with it any species that happened to get sucked in to the ballast tank. This is a major source of marine invaders – including the now infamous zebra mussel, Dreissena polymorpha, which has become especially problematic throughout fresh waters of the Mississippi River, the Great Lakes, and the east coast. However, invasives can also come from aquaculture gear and species – especially as native species are fished out and replaced with non-natives to keep up food production. In addition to these non-native species used in aquaculture, other species hitch rides on them.
How Ship's Ballast works
However, as you can see above, it is possible that all invasive species are not created equal – that is, maybe not all invasives are so bad. Green fleece, known as Codium fragile, has been introduced to the east coast of America for decades. Originating from Japan, it has typically viewed as bad – it is a buoyant species which needs hard substrates to attach, including living shellfish. It got the nickname “oyster thief” since it would attach to oyster shells, and whenever a storm or strong current event occurred, the buoyant macroalgae would be swept away, dislodging oysters and taking them away from reefs and culture sites. It is clear why this is considered a problematic species.
And yet, some recent research has shown that maybe Codium isn’t all that bad. Research which I have participated in has demonstrated that Codium may act as a viable alternative habitat for native bay scallops. Why? Bay scallops have evolved a strong association with seagrasses, and the Codium canopy likely provides the same upright structure to scallops. We observe scallops frequently in association with Codium in Long Island bays, and a study conducted showed that survival of free-released and tethered scallops was the same in eelgrass and Codium, suggesting that the invader offers a similar predation refuge. This was published last year in Marine Biology (See Carroll et al, 2010, below).
From Carroll et al 2010
In addition, I have taken the research further. The aforementioned paper talked about survival on a relatively short time span – 1 week. In order to examine the longer term effects on growth I conducted a caged field experiment the past two summers at 2 field sites with eelgrass, Codium, and unvegetated sediments in close proximity to each other. The general findings have been that scallops in Codium grow at rates similar to scallops in eelgrass, however, there are site-specific differences. There are also no differences in mortality between the habitats – suggesting that dense stands of Codium aren’t having as detrimental impact of low dissolved oxygen as I originally thought. This work isn’t published yet, as I am working on a method to find the stoichiometry of the tissues, but some of the results are in the presentation I gave at CERF 2009 here: Thursday_SCI-045_1115_J.Carroll
Moon snail crawling over Codium
However, I am not the only one who sees “positive” impacts of Codium. In the most recent issue of Marine Ecology Progress Series, a team of Canadian researchers, led by Annick Drouin, higher abundance and diversity of the faunal community in eelgrass meadows invaded by Codium fragile. Using a variety of sampling methods and field manipulations, the team demonstrated higher abundance and diversity of invertebrate organisms on Codium, and in eelgrass meadows invaded by Codium, than those without Codium. The pattern of fish abundance and diversity was not different – likely because they are highly mobile and can move easily between structured habitats. It is likely that Codium just generates MORE habitat, as it is branching and canopy forming. The important thing here is the ecological implication – the lack of a negative effect on native species by the presence of this “invader.” Perhaps Codium might not be so bad after all, especially as eelgrass is declining in many regions.
Figure from Drouin et al 2011
It is possible, then that “invasive” vegetation species in the marine environment may not always be bad. In many cases, invasives may be beneficial. Numerous studies (including the ones above with Codium) have demonstrated a positive effect of invasive algal species on native fauna. Typically, the vegetation is habitat forming, and invades areas where native habitat forming vegetation has already been lost. In essence, it is replacing a lost habitat, and creating a new habitat which is functionally similar to the species which declined/disappeared. That being said, invasive algal species can be detrimental to native macrophytes through competition. However, the benefit is in enhancing native fauna, which has potential fisheries ramifications. This requires further investigation, but it is entirely possible that non-native macroalgal species might have a positive effect on a number of native fauna.
Mud crab in Codium canopy
Pipefish chillin' in Codium canopy
The above photos, and the one of the moon snail farther up the page, are all illustrations of native species of Long Island associating with the invasive Codium fragile. Now, again, there are certainly detrimental effects of invasive species, so I am not trying to be too much of an apologist for them here. However, in the absence of eelgrass, it is entirely likely that the upright, canopy forming structure of Codium creates a habitat suitable to many seagrass associated fauna. As eelgrass is declining, invasive macrophytes might be important replacement habitats for a variety of native species. Understanding how these species affect native species will be key for management of estuaries moving forward. Particularly, once established, invasives becoming increasingly expensive and difficult to remove. If some invaders might be of benefit, that relationship needs to be well understood. Hey, invasives could help bring back the bay scallop in NY (and likely is having an impact), providing a habitat as eelgrass has disappeared from many Long Island areas. Who knows where else they might be beneficial.
There will be those of you out there who disagree. I don’t blame you. Calling an “invader” beneficial certainly goes against conventional wisdom. When we first introduced the idea of Codium as a potential scallop habitat to a shellfish crowd, we were scoffed at. However, the data don’t lie. And more research points to cases where invasives may actually facilitate natives.
Drouin, A., McKindsey, C., & Johnson, L. (2011). Higher abundance and diversity in faunal assemblages with the invasion of Codium fragile ssp. fragile in eelgrass meadows Marine Ecology Progress Series, 424, 105-117 DOI: 10.3354/meps08961
Carroll, J., Peterson, B., Bonal, D., Weinstock, A., Smith, C., & Tettelbach, S. (2009). Comparative survival of bay scallops in eelgrass and the introduced alga, Codium fragile, in a New York estuary Marine Biology, 157 (2), 249-259 DOI: 10.1007/s00227-009-1312-0
So I frequent the UnderwaterTimes almost daily. I like to find out what’s going on in underwater related news. So imagine my excitement at this article today:
“Fake seagrass could help boost fish numbers”
The premise of the article states that scientists in New Zealand are using artificial seagrass to help boost fish stocks. Seagrass is an extremely vulnerable marine habitat, with worldwide losses. In some places where lush underwater meadows used to exist, the grass has been replaced by barren sediments. This can have an impact on fish stocks, as many marine species utilize seagrass as a habitat for at least some portion of their life cycle. So it comes as no surprise that scientists want to try to replenish fish stocks by enhancing seagrass. This is apparently what they are trying to do in a bay in New Zealand. Although, I imagine that the researchers are actually using ASUs to test affects of fragmentation on local fisheries species and not to actually be used to enhance species, it is interesting none-the-less.
I have a certain affinity for all things fake seagrass. Why? Well, a portion of my dissertation research involves using artificial seagrass units (ASUs) to investigate the impacts of patch size and shape (perimeter, area, and P:A ratios) has on scallop recruitment, survival and growth. As I mentioned already, seagrasses are important habitats, and bay scallops have long been known to associate with seagrass. Scallop populations are currently undergoing varying degrees of restoration (depending on location) but with restoration comes certain issues – namely, how are these little guys going to be affected by declining seagrass. In many areas where scallop populations crashed, seagrasses have also diminished in extent. Since seagrass is important for scallops, a decline in seagrass cover can have implications for scallops and their restoration. For my work, I use 2 sizes of seagrass mats, 8.5 and 17 square meters, and 2 shapes – a circle and 4 pointed star to maximize perimeter. Just in case those numbers don’t mean much to you, the small circles are just over 3 meters in diameter, the large circles around 5 meters across. The large star is 7 meters from tip to tip. These ASUs have 500 shoots per meter, consisting of 4 blades of polypropelene ribbon. It was quite the undertaking, and required many beer and pizza nights for fellow grad students, as well as help from local schools and scout troops.
I have generated some interesting, and unexpected, data. There is a wealth of literature out there about the impacts of fragmented seagrass habitats, patch configuration, edge effects, etc, that has been accumulating over the last 15 or so years. Going into the experiments, I had a pretty good idea about what I would expect to see – more scallop recruits along the edges of the mats, but higher predation at the edge. Growth to be slowest in the centers of the mats, etc. However, not everything happens the way I planned or anticipated. In particular, I have been working up some of my recruitment data, and I did not see a “settlement shadow” or edge effects due to predation. What is a “settlement shadow?” Essentially, bivalve larvae can be assumed to be passive particles, moving at the mercy of the currents. As a current comes into a seagrass meadow, the flow is attenuated. Particles settle out along the edge, and become diminished with distance into the meadow. Hence, recruitment is expected to be highest along the edges of seagrass meadows (and, also, the edges of my ASUs). On the opposite end of the spectrum, survival is expected to be the lowest along the edge, since predators are likely to have more access along the edges, and thus predator encounter with scallops should be higher. However, this isn’t what I saw. The reason? The dominant predator in my particular system is a small mud crab – not likely to be impeded by seagrass structure and essentially ubiquitous throughout the ASUs. This tells me the dominant process structuring post set scallop communities on my grass mats is predation, and the predator is apparently not impacted by fragmentation. This could have implications for restoration. I haven’t finished all the analysis yet, but it was pretty interesting. I just presented some of this work at a graduate symposium last week, and plan on presenting it again at the National Shellfish Association annual meeting in March.