Saturday, June 18, 2016

Horseshoe Crabs

Note: This show first aired on June 18, 2016. 

Its June and the full moon is right around the corner and that makes some of us naturalist types on the coast of Maine excited to see horseshoe crabs. We should probably refer to them by their latin name Limulus polyphemus, because their common name is a misnomer, they aren’t crabs or even crustaceans at all. They are in the group Chelicerata, along with sea spiders, scorpions, land spiders, mites, and our favorite, ticks. Like all the Arthropods though (including all the insects and crustaceans), they have a chitinous exoskeleton and jointed legs. When you see them in the water, their armored head and body and long spike like tail look otherworldly. Their uneven erratic movement is jarring to watch. They look out of place, but what they really are is out of time. They are living fossils remaining essentially unchanged in the fossil record for the past 200 million years (they outlived the dinosaurs by a long shot)**, and with fossil ancestry going back nearly 400 million years. What they are still doing here it is hard to say, other than doing what they have always done, feeding on soft mud or sand bottoms, crawling around on the surface of the substrate or burrowing shallowly in, preying on small invertebrates like worms, bivalves and tiny crustaceans. As adults they don’t have many predators (logger head turtles are one, humans are another), and they are most at risk of predation when young, tasty little nibblelets.  

Traditional thinking has it that at the time of the late spring and early summer full and new moons, and the high tidal ranges typically associated with them, horseshoe crabs come up coastal estuaries to breed, laying eggs at the high water mark. And this timing does play out in the majority of the horseshoe crab’s range; on the broad sandy beaches of the inner bays on the mid Atlantic coastal plain, horseshoe crabs by the tens of thousands mount a beachhead assault on moonlit nights, laying millions upon millions of little blue green eggs in the sand. Look at the footage from Delaware Bay to see what I mean. Here in Maine though evidence shows that the crabs’ activity is not strongly associated with lunar period. They seem to be more cued to things like water temperature and salinity, and even weather (so us naturalist types can get over it and just go whenever things start getting warm). Down in Delaware, where horseshoe crabs are essentially at their ecological zenith, the timing is important not only because the beach habitats where these eggs are being laid are so uniform, and the tidal ranges lower than here, but also because other animals rely heavily on these eggs as a food source. Animals that are timing their arrival to Delaware Bay to coincide with the emergence of the horseshoe crabs from the sea.  Migrating shore birds like ruddy turnstones, sanderlings, plovers, and most famously perhaps red knots congregate by the tens of thousands as they migrate from the southern hemisphere, all descending on Delaware Bay for a horseshoe crab caviar feeding orgy that refuels them with nutritious high fat high protein eggs and enables the rest of their migration to high northern latitudes. This convergence is a wonder of nature. By comparison, what the horseshoe crabs do here in Maine pales.

Because they are at the absolute northern edge of their breeding range, populations of horseshoe crabs in Maine are found in isolated pockets, breeding not on wide open beach fronts but up in the estuaries of coastal rivers. They certainly provide a food resource for other marine organisms, and probably some migratory birds, but not in the keystone way they do further south. As an animal that time forgot, here in Maine they are doubly so, showing up more as a persistent oddity than a fundamental player in the food webs observed here. But as climate continues to change and waters warm many species are shifting their ranges to higher latitude. This may present difficulties for horseshoe crabs, as Maine’s coastline lacks their typical favored habitat, protected sandy beaches (most of our sandy beaches are apparently too high energy). And will all those birds that gorge themselves in the Chesapeake region be able to find them if they started breeding further north? These are the kinds questions we have to think about for all species as we watch climate change play out over our life times.

I would be remiss to not mention another reason people are interested in horseshoe crabs, and that is their blood. In areas where the crabs are abundant, they are collected at the shore and drained of much of their blood, which contains a protein that clots in the presence of gram negative bacteria. This blood factor is used to test medical equipment to ensure it sterility. If you’ve ever had surgery or an IV, it is likely you have benefited from this, and as of now there is no synthetic alternative. The crabs apparently can regenerate blood (much like we can) when returned to the ocean, which they are—and given human’s typical treatment of ocean resources, I think this shows amazing forethought. 

** Some say that calling them a "living fossil" is a misnomer, as the species in the fossil record are not the same as the modern Limulus polyphemus (which only dates back 20 million years or so). So be it. To me the term implies something that is strange to our eyes, because of how little it has changed, rather than something as weird as an orchid, weird because of the lengths to which is has yes, I think it is ok to call horseshoe crabs living fossils, even though, they aren't exactly perfectly unchanged from the Paleozoic era (see the last reference for more info).


Great videos on this site:

Older report on long term study in Maine:

Report on long term study in Maine:

Sample of the recent articles that refute the "living fossil" label