You are isotopes (Part II)

This is the second part in a series how isotopes work and how they are scientifically fascinating. Part I here

It turns out a horse is not just a horse, of course. The horse is a collection of atoms, and each of those atoms has a particular isotopic “flavor”, and the collection of isotope types in the horse tells a story.  At the end of the day, scientists are simply interested in reading and telling stories about our world. The tail….er, tale of the horse is written by myriad interacting processes in the universe which influence the horse’s stable isotope ratios.

As I mentioned last time, carbon-12 is much, much more common than carbon-13 is on our planet, due to nuclear fusion of helium-4 in the sun. there are nearly 99 carbon-12’s on earth for every carbon-13. But that’s the base ratio if you took our whole planet, put it in a blender and mixed it all up. If you measured a particular object, such as a horse, it likely does not follow that measure exactly. It has become differentiated from the global average by numerous factors which have altered the isotope ratio.

In isotopic chemistry, fractionation is our name for any process which creates a preference for a certain isotope. If chemical reactions had no bias toward any particular isotope, that 99 to 1 ratio of carbon-12 to carbon-13 would be present in literally everything including you and me. But it turns out that the biochemical dice are loaded- to make the ratio even more biased!

The enormous Rubisco enzyme. No one said photosynthesis was simple. Source: Wikipedia

Photosynthesis is the process by which plants take carbon dioxide gas in the atmosphere and “fix” it to make sugars, which they then use for food. The core enzyme responsible for this carbon fixation is called Rubisco (short for Ribulose-1,5-bisphosphate carboxylase/oxygenase). This enormous molecule is likely the most abundant enzyme on earth. And it turns out that it has a favorite flavor when it comes to the carbon it fixes into sugar.

In fact, the entire plant is discriminating against carbon-13 in several of the processes of photosynthesis. Carbon dioxide molecules diffuse more quickly into the plant’s leaves if they include the lighter carbon-12 rather than carbon-13. “Light” CO2 also dissolves more easily in the plant’s fluids. But the biggest fractionation happens when the Rubisco molecule gets hold of CO2 and breaks it. At each of these steps, the light carbon-12 is more likely to be used by the plant than its heavier siblings. There are various thermodynamic reasons for why this is the case, but the plant is essentially a sieve removing more of those heavy carbons at every step. At the end of the process, the plant is left isotopically “lighter” than the CO2 gas surrounding it that it breathes in.

Because you are what you eat, this means that you are suspiciously carbon-light, and there’s nothing you can do about it. Should have thought of that before you decided to be dependent on plants as the factory for your carbon-based molecules. Next time, we’ll talk about how we measure this, and the kinds of science that can happen once you have a nice consistent measurement to use to compare isotopic ratios between samples.

When a clam has a stowaway

My mussel contained a tiny half-eaten crab! - Imgur
Source: jeredjeya on Reddit

Bivalves put a lot of energy into their shells. These hardened, hinged sheaths of carbonate are an effective defense against many predators looking to get at the squishy clam’s body encased inside. Parasitic pea crabs have evolved to free-ride on the bivalves’ hard work.

 

(video courtesy Dana Shultz)

Pea crabs are small (pea-sized), very specialized parasites which live in the mantle cavity of many bivalve groups including oysters, mussels, clams and more. The mantle is the wall encasing the soft body of the bivalve, and the cavity is the space between this soft gooey tissue and the shell itself.

For a pea crab, there is no better place to be than this tiny, claustrophobic space. In fact, they can’t live anywhere else, though some species have been found in other unusual places such as inside the anuses and respiratory tracts of sea cucumbers (link SFW, fortunately, unless you’re a sea cucumber). In a bivalve host, the crab is protected from predation by the shell, and the bivalve provides a constant buffet of food as it sucks in suspended particles with its gills. The crab steals some of this food from itself before the bivalve can digest it.

As you might imagine, having a crab living in you taking your food and pinching at your gills is not an ideal arrangement for the bivalve. Pea crabs damage their hosts’ gills with their constant picking, and bivalves infected with crabs suffer slower growth than uninfected individuals, particularly for those unlucky enough to play host to the larger female pea crabs. At a certain point, the males will sneak out of their hosts and find a bivalve with a female crab inside. At this point, they mate inside the host’s shell, adding great insult to injury. The female releases her larvae, which swim out to infect new hapless bivalves and start the cycle over again.

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Aww, she’s expecting! The papers refer to this as being “with berry” which I find amusing for some reason. (photo from Dana Shultz)

 

You might think that commercial oysters with crab parasites would be thrown out, but to the contrary, finding a pea crab or its close relative the oyster crab with your meal is a cause for celebration in some areas, such as the Cheasapeake Bay. The crabs are eaten whole and often raw, and are said to have a texture akin to shrimp, with notes of sweetness and umami. Personally I prefer surprises in my Kinder eggs rather than in my shellfish, but to each their own.

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A pea crab serves as a nice side dish for this lunching sea otter. Source: Brocken Inaglory on Wikipedia

 

The Many Homes of Hermit Crabs

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My boy Harry, a purple pincher (Coenobita clypeatus) inhabiting a tapestry turban snail (Turbo petholatus) shell. These seem to be his favorite kind, even though they do not come from his native Caribbean.

Hermit crabs (superfamily Paguroidea) are most famous for using snail shells as their home, having evolved a soft, spiral abdomen to be able to use them for protection. But they are more flexible about their choice of abode than you might expect.

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This crab was likely preserved buried alive in sediment. Note how it uses its claw as a protective trap door sealing the opening of the ammonite shell. Source: Jagt et al, 2006

Different groups of shelled organisms have risen and fallen in abundance through geological time. During the time of the dinosaurs, ammonites (relatives of modern squid and octopus) were among the most common marine organisms, and hermit crabs were there to recycle their shells when they died.

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Each tiny pore (zooid) in this bryozoan contained a tiny tentacled organism. Together they grew in a shape that made for a nice hermit crab house (image 5 shows a cross section where the crab’s abdomen would fit). Source: Taylor and Schindler 2004

Mollusks aren’t the only contractors for hermit crabs. Some hermits utilize the skeletons of colonial organisms like bryozoans as a home. Bryozoans are filter-feeding colonial animals made up of thousands of tiny tentacled organisms living in the pores of a shared skeleton. The extinct bryozoan Hippoporidra lived in symbiotic partnership with hermit crabs, growing around a gastropod shell to attract a hermit crab partner. This was an example of mutualism: by providing a home for a crab, the bryozoan would be transported to new environments with plentiful food particles to eat, and also would be protected from their arch-enemy, nudibranchs (sea slugs). Some modern day hermits, such as Manucomplanus varians of the Gulf of California, have evolved very similar partnerships with live staghorn corals.

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Manucomplanus varians at Monterey Bay Aquarium

Not all hermit crabs live in hard houses. Some deep sea forms partner with anemones, with the stinging tentacles serving as an effective defense.

Source: Okeanos Explorer

The recently discovered green-eyed hermit crab, which also lives in deep water, lives in a glued-together mass of sand created by tiny anemones, which continue to grow the structure to fit the crab as it increases in size.

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The green-eyed hermit crab was found over 200 m deep off of South Africa. Source: Lannes Landschoff via Eurekalert 

Unfortunately, hermits adapted for gastropod shells are unable to find adequate homes in some areas, due to overharvesting of shells for the tourist trade as well as an excess of plastic trash. These crabs make do with whatever items that they can find. Plastic is not an ideal home material for hermits. Bottlecaps and narrow tubes do not allow the crab to fully retract for protection and leach chemicals which may harm the crab. The crabs also nibble on their shells as a source of calcium, which is obviously not possible with plastic.

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Coenobita purpureus, a land hermit crab on Okinawa. Source: Shawn Miller

But hermits continue to impress me with their flexibility and ingenuity in their search for homes. For a hermit crab, home is where the abdomen is.

 

 

Why do worms surface after a rain?

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Source: Eva the Weaver on Flickr

Biologists have long wondered at the capabilities of earthworms. When he wasn’t crafting foundational theories of modern biology, Charles Darwin discovered that earthworms assist plant growth by aerating and turning over soil. Previously derided as pests, his work helped to shed light on the dark, dirty, essential role of these silent tillers of the earth. Earthworms spend most of their lives converting organic matter into compost. They play an important role in nutrient cycling, as the castings that they leave behind are rich in important nutrients like phosphorus and calcium.

Our sunny, dry surface world is not safe for these subterranean beasts. Earthworms are not at home in daylight and are usually only there by necessity. One of the most common times to observe them is during and after a rainstorm, particularly at night. As a child, I often wondered why earthworms seemed so determined to drown themselves in puddles. Researchers have still not reached consensus on the causes for this behavior. Darwin supposed that the sicker worms were driven to the surface by flooded soil. Some propose that they are driven from the saturated soil by low oxygen content, where many drown in puddles as they attempt to find non-waterlogged soil. Other researchers are skeptical of this angle, as worms require moisture to breathe through their skin, and many species can survive immersion in water for extended periods. They suggest that the worms are using the wet conditions brought by rains to aid migration to new patches of soil, or even to mate. The reality, as often is the case, is likely something of a combination of all of the above.

One team found that different earthworm species have different tolerance levels for low oxygen conditions. The worms that can survive extended periods of immersion in water are the same species that often remains under the ground following a rainstorm. These worms have lower oxygen consumption. The worms with higher rates of oxygen consumption leave the ground when dissolved oxygen levels are too low to sustain respiration. They do so preferentially at night, partially because this is a period when they are more active, and thus respiring more oxygen, and also because at night they are less exposed to predation by birds. The early bird (at least before sunrise) really does tend to get the worm.

Some have suggested that the rhythm of raindrops stimulates the emergence of earthworms, and that the same mechanism allows fishermen (and animals) to use “worm fiddling” or charming to gather thousands of worms at a time for bait. Worm fiddling can take many forms, but one of the most common techniques observed involves rubbing a long piece of metal against a post driven into the ground. The vibrations induce thousands of worms to rise up out of their burrows.

Ken Catania, a biologist at Vanderbilt University, rejected the rain rhythm idea and proposed that worm fiddling actually works by imitating the noise of a burrowing mole, one of the most ravenous predators of worms. Rather than fleeing from the simulated rain, the worms are fleeing the jaws of their greatest enemy, only to be trapped at the surface by a different adversary. Like a whale driven to beach itself by noise pollution or the threat of a predator, surfaced earthworms deserve our sympathy, particularly if they’re destined to become our fishing bait.