icthyology – excalculus

[First published at http://excalculus.tumblr.com/post/89487231607/the-annual-fish on June 21 2014. Edited December 2018 – note that many of the image links are broken at this time.]

For the next in my series of odd animals, I think I’ll be sticking with the ludicrously hardcore fish theme. (Previous contender: the icefish. Hemoglobin is for wimps.) Say hello to genus Nothobranchius, the annual killifishes of Africa.

*Nothobranchius rachovii.* All images link to their sources.

Amazingly beautiful. Tiny and fragile. Thrives in… temporary puddles in the African desert?

First, a bit of background. Nothobranchius are tropical and subtropical fish native to parts of east and southeast Africa known for seasonal rainfall. This sort of area tends to be difficult for any animal due to the extreme dryness for much of the year, and aquatic life in particular has a very hard time. Fish from these kinds of environments tend to be very hardy or have interesting survival mechanisms – the lungfish, for instance, is famously capable of breathing air and sealing itself in a moisture-preserving mucous cocoon. The barramundi of Australia and southeast Asia is so adept at surviving in stagnant, drought-stricken riverbed pools that it is now in high demand for aquaculture. The little killifishes, though, don’t have any of that. When confronted with the question of how to survive the complete drying of the environment, their answer is simple: don’t.

When the unreliable rains stop and the pools they inhabit dry up, all the adult fish die. In fact, they may already be dead of old age – due to the pressure to grow and reproduce quickly these fish have some of the shortest lifespans of any vertebrate. What’s left behind are eggs, buried in mud in watertight casings and suspended in the state known as diapause.

So now we have eggs that can survive prolonged drying and temperatures from -8 to 40 degrees Celsius, but that’s not the only thing they have to do. Waiting for water to trigger development would set them too far behind, so the embryos begin to grow when laid but enter diapause at several discrete stages. Most eggs are timed to reach their final stage of development around when the annual rains would begin, but not all – if there is a drought or a freak early rainstorm any embryos that hatched or developed too far would die. Eggs can wait for months or years at any of these diapause points before continuing to the next one, so that at any given time there will be viable eggs ready to hatch.

The staggered development is amazing enough on its own as an adaptation to a harsh environment, but the closer you look the more incredible it gets. One would expect the embryos to develop most quickly under optimal conditions for fish survival, and for the most part this is true – but in Nothobranchius guentheri (and likely other species as well), the presence of adult fish in the same body of water prevents eggs from developing past the first diapause stage. (Inglima et al. 1981) This effect can be reduced by removing adults or aerating the water, and probably serves to conserve the pool of dormant eggs for future seasons. In the same species, temperature and photoperiod during mating and incubation changes at what stage and for how long eggs enter diapause (Markofsky and Matias, 1977) – meaning the process is tailored depending on the season in which the eggs were produced to maximize their chance of completing development at a good time. And if that weren’t amazing enough, fish from different localities are so adapted to the specific area they live in that they show dramatic differences in lifespan and aging patterns (Terzibasi et al. 2008) even when bred for several generations in a laboratory. The development and hatching of the eggs isn’t a simple staggered timer, it’s an intricate process that takes into account the locality, conditions when the eggs were laid and conditions in the environment during development. And that’s only the mechanisms we know about: it’s entirely possible there are more.

Speaking of N. furzeri, the species from that last study, it holds the dubious distinction of being the shortest-lived vertebrate on the planet that can be bred in captivity with an average lifespan of 9 weeks (maximum 12). The overall crown was stolen from it by a coral goby with a reproductive cycle of 25 days and a lifespan of 59 days, but that’s a story for another time. The rest of the genus ranges from similarly mayfly-esque on the short end to the longest-lived species managing to survive up to a year or so with proper aquarium care. And there is a huge amount of interest in providing proper aquarium care, because although they hail from muddy puddles and spend their short adult lives in a rush of eating and breeding these are some of the most beautiful fish in the hobby, with colors and finnage that would not look out of place on a coral reef.

Inglima, Kenneth, Alfred Perlmutter, and Jules Markofsky. “Reversible stage‐specific embryonic inhibition mediated by the presence of adults in the annual fish Nothobranchius guentheri.” Journal of Experimental Zoology 215.1 (1981): 23-33.
Markofsky, Jules, and Jonathan R. Matias. “The effects of temperature and season of collection on the onset and duration of diapause in embryos of the annual fish Nothobranchius guentheri.” Journal of Experimental Zoology202.1 (1977): 49-56.
Terzibasi, Eva, et al. “Large differences in aging phenotype between strains of the short-lived annual fish Nothobranchius furzeri.” PloS one 3.12 (2008): e3866.

[Originally published at http://excalculus.tumblr.com/post/71595885559/meet-the-icefish on December 29, 2013. Edited December 15, 2018. All images link to their original sources.]

I have free time, and that means more biology! Today’s creature is one I’ve been meaning to write about for a while: the crocodile icefish, Pagetopsis macropterus.

Photographed at McMurdo Station, Antarctica.

The name seems pretty self-explanatory – crocodile for the long, flat, toothy snout, and icefish because it lives exclusively in the frigid waters just off the coast of Antarctica. But there’s something very strange going on with this fish, one of the symptoms of which can be seen just by looking closely: it’s ghostly pale, almost transparent. Its internal organs are visible through its body wall, and some of its skeletal structure is faintly visible.

Note that the backs of the eyeballs and the shape of the skull are visible through the top of the head.

Now, transparency in fish is unusual but not unheard of. Generally this is due to a combination of small size and lack of skin pigment, as in some small aquarium fishes. But despite being scaleless the icefish has markings, so it’s not a pigmentation issue. For a clue as to what’s going on, look back at the first picture. Specifically, look at the gill covers. On a fish this transparent you should be able to see the gills clearly, right? Anyone who has ever bought whole fish at a market now knows there is something odd afoot, for the gills of a healthy live or recently dead fish are bright red with oxygenated blood. Stale fish have a brownish purple color. Icefish?

Completely and utterly white. That’s bizarre enough, but things get really strange when you cut one open.

Original link (defunct): http://images.aad.gov.au/img.py/20b2.jpg?width=640&height=429

For anyone who has never seen the inside of a normal fish, a comparison:

Innards of *Notothenia coriiceps*, another Antarctic fish closely related to the icefish.

Internal organs should not be white, but the icefish seems to be doing just fine. Something has changed, something that normally gives flesh, viscera and fish gills color and opacity. The gills are actually the best clue here, as they are essentially made of blood vessels in order to extract oxygen from the water. Let’s compare icefish blood to that of a close relative:

Icefish blood on left, *Notothenia coriiceps* blood on the right.

And there’s your answer. Icefish blood looks more like water than blood, because the icefish is unique among vertebrates for lacking red blood cells. That’s incredible enough for a closer look, so genes were sequenced and it was found that the key change here was a deletion in the genes that code for the oxygen-carrying protein hemoglobin.

Hemoglobin genes of icefishes and friends.

Hemoglobin has an alpha chain and a beta chain. Sequencing the DNA of many icefish species and several relatives reveals that icefishes retain part of the alpha chain sequence, but the beta chain is missing entirely or remains as scrambled fragments. (Near et al. 2006) This genetic fossil tells us that the ancestors of the icefish had normal hemoglobin genes, but managed to lose them.

So now we know why icefish have clear blood, but how on Earth did this one make it out of quality control? Hemoglobin is… somewhat important. It’s how oxygen gets from lungs or gills to the rest of the body. Every other vertebrate relies on hemoglobin to survive. And if you thought the malarkey stopped there, you thought wrong: On top of this, there have been not one, but several distinct mutation events in which icefishes lost myoglobin as well. (Myoglobin is an oxygen-binding protein found in vertebrate muscle. This is what makes muscle red.)

Figure borrowed from Sidell and O’Brien, 2006. Red bars represent mutations resulting in loss of myoglobin. Note that our friend Pagetopsis macropterus has gone all in.

The fact that the icefish still exists suggests that flinging your oxygen-binding proteins out the window either does not affect survival or confers an advantage, which seems blatantly untrue.

One theory is that as counterproductive as it looks, this may actually be an adaptation to the icefish’s habitat. Antarctica is, to use a highly technical term, balls cold. The water it lives in is about at the freezing point of even seawater. This is so cold that the entire larger family that icefish belong to have evolved blood proteins that function as antifreeze in order to function in an environment that is simply too cold for most fish to survive. (The antifreeze proteins are another interesting case – analysis reveals that they developed from pancreatic digestive enzymes.) Liquids become more viscous as they cool, and normal blood is already thick, sticky stuff. At sub-zero temperatures it becomes very difficult to pump through an animal’s body. By removing red blood cells the icefish makes its blood much thinner and easier to pump, meaning there might be a selective advantage in freezing but nutrient-rich waters.

There’s one glaring problem, though: by every scientific measure, losing hemoglobin and myoglobin has no benefit. (Sidell and O’Brien, 2006) Let’s go through the data.

Icefish lacking hemoglobin have massive heart sizes and blood volumes compared to their red-blooded relatives from the same environment. Calculating cardiac energy consumption based on heart size and blood pressure shows that they expend about twice as much energy pumping blood as a fish with red blood cells.
Icefish have much denser capillary beds than their relatives. This reduces the distance oxygen has to diffuse to get to a given area of tissue, suggesting that there isn’t exactly an abundance of oxygen to work with.
When icefish hearts are isolated and their function measured, hearts from species lacking myoglobin perform significantly worse than those from species that have myoglobin. When myoglobin function is blocked the myoglobin-negative hearts are unaffected, but the myoglobin-positive hearts lose so much function that they are actually worse off. The fact that the myoglobin-negative hearts come out ahead in this situation means they have performance adaptations specifically to compensate for lack of myoglobin.

That last item is positively damning. If you have to put so much effort into adapting to your “adaptation,” it’s likely not an adaptation at all. All this points to the fact that the hemoglobin and myoglobin deletions were in fact bugs that the species managed to survive rather than features that made them more fit. (I beg you to read the paper – it’s got some very neat pictures of hearts and capillaries, a lot of great information, and it’s titled “When bad things happen to good fish.”)

The fact that they live in Antarctica in the first place is likely the only reason they got away with it: Cold water has several interesting properties, one of which is an increased ability to dissolve gases. As a matter of fact, the waters of the Antarctic are effectively saturated with oxygen. Combined with scaleless skin for better absorption, an overpowered heart, and a spectacularly dense capillary network, it’s possible for a vertebrate to get enough oxygen using what is essentially an insect-style open circulatory system.

So there you have it: a fish so hardcore that it bleeds antifreeze and survived deleting all its oxygen-carrying proteins.

Thomas J. Near, Sandra K. Parker, H. William Detrich; A Genomic Fossil Reveals Key Steps in Hemoglobin Loss by the Antarctic Icefishes, Molecular Biology and Evolution, Volume 23, Issue 11, 1 November 2006, Pages 2008–2016, https://doi.org/10.1093/molbev/msl071
Sidell, Bruce D., and Kristin M. O’Brien. “When bad things happen to good fish: the loss of hemoglobin and myoglobin expression in Antarctic icefishes.” Journal of Experimental Biology 209.10 (2006): 1791-1802.

Tag: icthyology

The Annual Fish

The Icefish