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 changed....so 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).
** 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 changed....so 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).
References:
Great videos on this site:
Older report on long term study in Maine:
Report on long term study in Maine:
http://dnr2.maryland.gov/fisheries/Pages/horseshoe-crab.aspx
Sample of the recent articles that refute the "living fossil" label http://phenomena.nationalgeographic.com/2011/11/22/in-evolutions-race-horseshoe-crabs-took-a-slower-pace/
Sample of the recent articles that refute the "living fossil" label http://phenomena.nationalgeographic.com/2011/11/22/in-evolutions-race-horseshoe-crabs-took-a-slower-pace/
