The Horseshoe Crab Diaries Part Three: In Which Paul Learns Something New and Then Must Tell Everyone

Here we are near the end of the month, again lamenting the sad decline in posting frequency. But hey, look! Another post about horseshoe crabs. This must be your lucky day!

So last week I went to a talk by a guest PhD student at the aquarium who studies the ecological impact of horseshoe crabs. The take away was basically that, in the area of study, which was across several mud flats in Great Bay, New Hampshire the HSCs are foraging in nearly 100% of available space across two weeks. Diligent little chelicerates.

She also found that that in areas where the HSCs were foraging the invertebrate population was about a third of that in undisturbed mud. Basically they are exerting some pretty intense predation pressure on these mud flats.

And that brings me to the real revelation: The HSC really is, like the sea star, a top predator of the inter-tidal zone. I have been telling it all wrong. When asked I generally tell people that horseshoe crabs eat "dead stuff" or "soft stuff" because they don't have teeth or jaws. While it's probably true that they're not terribly picky and will scavenge, they are in fact hunters as well. The really cool thing I learned was that they are capable of cracking soft shell clams with their legs. I had no idea that they were so capable at being awesome predators. It really has changed my already very favorable opinion of them. They're like the sharks of the mud flats,

Is it Worth One Bird?

One of my newer favorite things to do is listen to the podcast "Radiolab." It's a science podcast, not specific to natural history, but I'll plug it anyway because, well, it's awesome. I keep thinking about one story I listened to recently about an endangered bird called the Kirtland's warbler. You should go listen to the story here:

http://www.radiolab.org/2010/jun/28/weighing-good-intentions/

For those of you who don't have the time or came up with some other excuse to skip the homework I'll quickly summarize. The warbler is very endangered partly because it nests in trees that need fires. Thanks primarily to Smokey the Bear the amount of forest fires has drastically been reduced in the US. Unfortunately, fires are an incredibly important part of forest ecology. So you have this endangered species which, by law, the DCR is required protect. How do you protect a species which relies on fires? Well, you have to set fires. One day a fire goes out of control, it creates a whole lot of damage and kills a DCR employee. The reporter basically asks the question: is it worth one bird?

The comments on the page I've linked to are also very interesting and take the question a little bit further, I think.

It also reminded me of a book I read (most of) called Out of Eden by a guy named Alan Burdick. It deals with management of invasive species and the author takes a somewhat controversial stance: that essentially we do too much, we worry too much, we waste effort that could be best spent elsewhere. I'm not sure. It's worth at least borrowing it from your library. The only reason I didn't get all the way through it is that I felt each case study was too similar and Burdick's musings were, likewise, too similar.

So I just wanted all you readers to think about these things. I spend a lot of time at my job talking about endangered species. I think it's important to consider what's actually happening on the ground. How are these things actually managed? Are we being overly cautious? Will there be an ecological collapse? Should we always err on the side of safe rather than sorry? And how do we make people care about species besides the charismatic whales and turtles? I clearly don't have answers to any of these questions but I think it's important for us to think about.

Shipworms

I’m going to try something a little different today on the Network. I have limited my topics to either local species, generally accompanied by a photo that I took myself or musings on natural history education in general. Today I’m going to start a series that introduces organisms that are oddities, phylogenetically speaking.

Perhaps my most nerdy passion is learning about phylogeny. Phylogeny is essentially the study of relationships in the tree of life. The organisms I want to take a look at are those that have diverged in interesting ways from their close relatives.

So today’s featured creature: the shipworm. I’ve been thinking a lot about the word worm. It’s one of those words that, speaking technically, carries almost no actual meaning. It’s a descriptor of body type (long and thin) and before biologists were able to start the project of classifying animals based on their actual relationships instead of what they looked like it got used as a label for all sorts of things. So we end up still using the word “worm” today to describe at least four major phyla (annelids, polychaetes, nemerteans and nematodes) and probably more that I’m missing as well as a host of other creatures that aren’t part of any of those groups.

Image courtesy of the US Geological Survey

Like this guy here. The “shipworm.” Folks tell me he’s not a worm at all. He fits the description more or less, though. He’s long and thin. But this animal is actually a bivalve mollusk, meaning his closest relatives are clams, mussels, oysters, scallops etc. and most closely the geoducks and soft shell clams. But this creature has diverged away from the “normal” bivalve physiology by growing into an organism with an elongated body. It retains the two shells of other bivalves but these have been altered into a cutting or boring device at one end of the animal.

Shipworms are so named because they are well known by sailors to bore into ships’ hulls, destroying them in the process. Wood can become so concentrated with the burrows of shipworms that it becomes nothing but an empty husk, brittle to the touch. The family of shipworms has also been dubbed “termites of the sea.”

This behavior of drilling into wood serves two purposes. First is protection. The animal will remain inside its burrow and secrete a calcareous compound to line it. The second is nutritional. These mollusks actually consume the wood and live symbiotically with a bacteria that takes nitrogen from sea water and synthesizes proteins. They also extend their siphons into the water through the open end of the burrow both to breath and to supplement their diet with plankton.

Even though a single piece of wood can become almost completely saturated with shipworm burrows they never make them long enough to interfere with a fellow shipworm. They simply stop growing. So apparently they are not only fascinating but also really great at sharing!

So the next time you pass some drift wood at the beach inspect it more carefully and you may find the little holes left by these bizarre bivalves.

Sources:

Chesapeake Bay Field Guide: http://www.chesapeakebay.net/shipworm.htm

Grave, B.H. Natural History of Shipworm Teredo Navalis, at Woods Hole, Massachusetts. Accessed at: http://www.biolbull.org/cgi/reprint/55/4/260.pdf

SusanScott.net: http://www.susanscott.net/OceanWatch2000/jun05-00.html

EOL: http://www.eol.org/pages/2578045