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 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:

http://naturedocumentaries.org/96/horseshoe-crabs-mating-maine/

https://www.fws.gov/northeast/pdf/horseshoe.fs.pdf

Older report on long term study in Maine:

http://www.cascobayestuary.org/wp-content/uploads/2014/07/2004_schaller_horseshoe_crab_surveys1.pdf

Report on long term study in Maine:

http://www.damariscottariver.org/wp-content/uploads/2015/12/Ten-Year-Report-Monitoring-of-the-Atlantic-Horseshoe-Crab-Limulus-polyphemus-During-the-Spawning-Season-in-the.pdf

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/
 

Eastern Tent Caterpillars

Note: This show first aired June 11, 2016.

Early summer in Maine is astonishing. One moment the forest canopy is open, allowing glimpses of the newly arrived migrating warblers, a day or two later and the broad leaves have exploded from their buds, and any chance you had of seeing a tiny fast moving bird is gone. These rapidly growing fresh green leaves are not just a beautiful annoyance to bird watchers, but a nutritious salad bar for many of the insects that those warblers come here to eat. The young newly emerged leaves have a high water and nitrogen content to support their rapid growth, and generally lack the defensive chemicals that develop age, protecting the mature leaves. The high nutrient content combined with a lack of protection make them easy targets for developing insect larvae.
 

One type of larva that everyone has been freaking out about this year is the Eastern Tent Caterpillar, aka Malacosoma americanum. If you have been around any cherry or apple trees this spring, you have seen these. They are a remarkably social insect, emerging as tiny caterpillars in May, from a mass of eggs that overwintered glued to a branch on the host tree. The larvae congregate together and immediately start spinning a large three dimensional web in the crotch between two branches, that’s the tent. This web serves as home base for this community of caterpillars. As a native of north America, they especially like native cherry trees, but are also commonly seen in apple trees, though will occasionally end up on other species. The caterpillars emerge just as the new leaves are emerging on the host tree in the spring, perfectly timed to hatch just as their food source does. As they grow they continue to spin more and more silk, enlarging their tent as they themselves need more room. The tent provides a space to congregate when they are not feeding, and a temperature regulation mechanism. The tent is usually oriented to have maximum surface area towards the sun in the morning, so the caterpillars can warm up, become active and digest their food more efficiently. They go through a series of six instars, or caterpillar size classes, shedding their soft exoskeleton and expanding their body into a new larger skin. Generally they spend all of their time on their host tree, but at the end of their larval period, the group disbands, and individuals wander off, in search of good places to pupate on their own. When you see individuals wandering about over the landscape, this is what they are doing—looking for a good spot like a tree trunk or fence post on which to spin their cocoon and undergo the three week transformation into an adult.

One would think that these caterpillars would be sitting ducks for any and all predators—after all they live in big juicy groups in large easy to spot nests. As a result they have evolved several defensive mechanisms that keep them off the menu for most birds and parasitic insects, the organisms that would be their most likely predators. First they have a behavioral tic that causes them to thrash about wildly if threatened. This makes it especially hard for a parasitic wasp to successfully inject an egg into the caterpillar. Secondly, as is noted, they feed on cherry trees, and cherry tree tissue contains cyanide, so these caterpillars can sometimes release cyanide containing juices if threatened. Thirdly, and probably most obviously, the backs of these caterpillar are lined with hundreds of irritating hairs. Any animal that eats the caterpillar has to contend with the build up of these hairs in their throats and stomachs, the effects of which can range from uncomfortable to debilitating. The Black billed cuckoo is one local bird that has evolved the ability to eat these irritating caterpillars hair and all, the birds eat their fill and then cough up the lining of their stomach, growing a new one ready for the next meal of hairy tent caterpillars.

Seeing a tree, like the black cherry tree in my yard, totally stripped of leaves by hungry caterpillars is alarming, when all around lush green leaves are spilling out of twigs. The cherry tree looks dead, but trees are not that easy to kill, and it takes more than a denuding by tent caterpillars to do in a cherry tree. They, and many other broad leaf trees, have the ability to grow another set of leaves later in the summer, from buds that form in the spring (as opposed to the ones that over winter). Watch a tree attacked by tent caterpillars this summer, by August it should have sprouted new leaves, and not a tent caterpillar in sight. The caterpillars pupate in June and emerge as adults at the end of June or into July depending on location. The adult’s job, as is the job of so many adult lepidopterans, is simply to mate as soon as possible, lay eggs and die. Those eggs, once laid, wait many long months, until next spring, before the next round of tent caterpillars emerges, and we start all over again. 

References:

http://www.maine.gov/dacf/php/gotpests/bugs/factsheets2/tent-cater-wisc.pdf

https://entomology.ca.uky.edu/ef423

http://www.maine.gov/dacf/php/gotpests/bugs/tent-caterpillars.htm

http://web.cortland.edu/fitzgerald/Easterntent.htm

http://northernwoodlands.org/outside_story/article/of-cuckoos-and-caterpillars

http://www.nytimes.com/2016/05/15/nyregion/when-caterpillars-move-in-the-yellow-billed-cuckoo-feasts.html?login=email&_r=0

http://biologylabs.utah.edu/coley/ColeyPubs/2006Coley_cats_Oikos.pdf

https://extension.tennessee.edu/publications/Documents/W227.pdf

Hatching Chickens at Blue Hill Consolidated School

On May 28, 2016 the show aired an interview I did with some students at Blue Hill Consolidated School, about their classroom chicken hatching project.

We talked about what they learned from the experience.

Photo by Mary Tobey

You can listen to the show here: http://archives.weru.org/world-around-us/2016/05/world-around-us-52116/