You are isotopes (Part I)

As you may well know, every element is defined by its number of protons contained the nuclei of its atoms. Hydrogen has one. Carbon has six. This is non-negotiable. But every element can be found in multiple “flavors” known as isotopes. This flavor depends on the isotope’s atomic mass, which is determined by the number of neutrons present in the nucleus of that atom. Neutrons are kind of like atomic ballast. Unlike protons, which have a positive charge, they are neutral, but they do have a mass. Different isotopes have different numbers of neutrons, determining their atomic mass but preserving its particular elemental identity (which would only change if you changed the number of protons present).

Let’s focus on carbon, an element which I think about daily, though every element has isotopes and I could pick many other examples. Hope you’re OK with that, but if not it’s my blog so deal with it. So carbon has been found or created in up to 15 flavors. A whopping 98.9% of all the carbon on Earth occurs as carbon-12 (written as 12C), which has six protons and six neutrons, adding up to an atomic mass of about 12 atomic mass units (amu). It’s the most common because it’s the product of three helium-4 isotopes fusing together, each weighing 4 amu + 4 amu + 4 amu adding to make a single carbon-12. This is a very common reaction in stars, and because you are stardust, it is also the most common flavor of carbon in you.

But we make other flavors by adding neutrons. You can make carbon-13 with six protons and seven neutrons. This is a rare flavor, accounting for almost all of the remaining 1.1% of carbon found on earth. It is also the only other stable form of carbon. I note that it’s stable because all the other 13 known flavors of carbon are unstable, and many are only known from the laboratory because they are too short-lived to be found in the environment.

It turns out that if an element’s atomic nucleus is too light, or too heavy, that element will become radioactive and decay with time, continuously firing off pieces of itself out of frustration. Carbon-14 is the most famous and common of these radioactive isotopes of carbon, and it still only makes up 1 in every million million atoms of carbon on earth. Carbon-14 fires off particles and decays into nitrogen-14 because it is more stable orientation for the protons and neutrons to be in, for physics reasons I won’t get into here.

Carbon-14 does this in a very predictable, methodical pattern. It’s difficult to predict when an individual carbon-14 atom will do this, but if you take any object you have just created, like a piece of pottery, for example, you can be pretty much certain that in 5,730 years, only 1/2 of the carbon-14’s will still be present. The rest decided they’d rather be nitrogen-14. This is non-negotiable and you’d best learn to accept it. But it means that we can sniff out the age of a lot of interesting mysterious objects if we know the amount of carbon-14 present in the environment (which we often do) and measure the amount present in the object today. You have some restrictions. For example, for objects that are too old, too little of the carbon-14 would be left for you to measure accurately.

Carbon-14 dating, often just called radiocarbon dating, is very useful in figuring out the ages of stuff, but I’m mostly interested in the stable isotopes of carbon. Next week I’ll talk about why that is, and what kind of questions I can answer by looking at amounts of different stable carbon isotopes in a sample. See you then!

In grief for the America that was

I used to have pride in being American. I’m grateful for all the opportunities and privileges I’ve had being a citizen of this storied democracy. I held onto hope when my countrymen doubted the citizenship of my president. I held on when we elected a fraudulent, undignified xenophobe to succeed him. I held on when we left a climate agreement out of spite and began rolling back environmental protections left and right. But today, I don’t know if I can do it. I don’t really feel like part of this place anymore.

I’m still trying to figure out my career. If I stay here after grad school, I’ll be a more inward looking person; thinking about my own life, friends and family. But I’m looking into leaving. I don’t really feel like I can be part of this shared enterprise anymore. It’s really out of exhaustion more than anger. I’m tired of this feeling of grief. It’s analogous to the feeling I’d have after losing a loved one, when I just want to disconnect. The country I grew up in is no more, and it may be time to move on.

Fossil-Fueled Life

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Thick aggregates of Bathymodiolus mussels on a cold seep site off of Nantucket. Source: NOAA

Humans aren’t the only users of fossil fuels. In many parts of the ocean, natural gas (methane) is constantly bubbling out of the sediment. These areas are known as cold seeps and are often a marker of productive fossil fuel reservoirs in the crust underneath. The name cold seep is somewhat of a misnomer (they are often slightly warmer than surrounding waters), but they are indeed much cooler than the more famous hydrothermal vents, which form due to geothermal activity. They are often found in shallower waters than hydrothermal vents, which generally occur in the deepest regions of the ocean where the earth’s crust is rifting. As with hydrothermal vents, cold seeps provide a unique opportunity for ecosystems to arise which are based on chemical energy, rather than solar-powered photosynthesis like the rest of the biosphere. Unusual life forms harness the chemical energy of the methane and sulfide emitted at these seeps, and many can also be found at the more famous hydrothermal vents.

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Crabs feeding on a bed of Bathymodiolus. Source: NOAA

Bathymodiolus mussels, vesicomyid clams, and other bivalves thrive at these seep environments, and have evolved to partner with bacteria in their gills and stomachs which can directly consume and produce energy from the reaction of methane and sulfate continuously being sourced from the seep underneath, creating bicarbonate and sulfide as products. This form of metabolism, chemosynthesis, is distinct from the familiar photosynthesis/respiration that most life-forms at the surface use to create energy. The partnership between a multicellular animal and chemosynthetic bacteria is called chemosymbiosis. These reactions are of course being utilized by all sorts of microbes not partnering with bivalves, but the bacteria that take on metazoan hosts have the advantage of a stable environment and a constant flow of fresh seep gas brought in by the bivalves’ gills. The bivalves feed on the products of the microbe’s hard work. As they are some of the only inhabitants that can tolerate the oxygen-free, toxic environment of seeps, they form dense thick shell beds wherever a seep appears.

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A bed of tubeworms and a deep-living octopus in the Arctic. Source: MBARI via BBC

Other chemosymbiotic groups use the hydrogen sulfide byproducts from seeps, which they oxidize into elemental sulfur. The most prominent of these are the vestimentiferan tubeworms, which lack mouth or anus and are totally dependent on the activity of the bacteria that they house in a modified digestive tract. Some of these worms have been found in seeps in extreme environments, such as the truly cold cold seeps of the Arctic. They are less spectacular in appearance than their bright red counterparts from hydrothermal vents, but are believed to live for an extremely long time, depending on the consistency of the seep that they inhabit.

 

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Hesiocaeca methanicola, one species of the poorly understood “ice worms.” Source: NOAA via Wikipedia

At the extreme pressures and low temperatures of the deep ocean, methane can freeze in combination with water molecules, forming structures called clathrates. Much of the deep ocean floor is dusted with deposits of clathrates. Microbes which feed on clathrates are believed to be a food source for grazing polychaete “ice worms.” These unusual organisms can survive up to 96 hours without a whiff of oxygen, an unbelievable feat for a moving, multicellular organism.

Imaginative artist representation of a probe visiting a vent under the ice of Europa. Source

Cold seep environments are perhaps merely one specific manifestation of a vast, poorly understood collection of biota which do not depend on the sun for their energy. We do not yet understand many of the deep ecosystems which may be present within the earth’s crust, in the deep ocean and trapped under polar ice. NASA studies cold seep and hydrothermal environments as the best analog for the conditions life could experience in the frozen oceans of Europa and Enceladus. It is eerie to think that such alien ecosystems  exist merely a few kilometers off of our familiar shores, and were using fossil fuel energy far before humans figured out how to combust it.

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Methane bubbling out of the seafloor off of the Virginia coast. Source: NOAA

 

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.