Slimy, tentacle-faced hagfish look like aliens and, accordingly, possess several PhD theses’ worth of unusual traits. Their body fluids have a similar number of dissolved salts to seawater, although because hagfish control the levels of some ions like calcium in their blood, the mix of salts is slightly different. Hagfish are also famously tolerant of low oxygen conditions (hypoxia), which comes in handy when they burrow inside rotting whales to dine. In some fishes, hypoxia disrupts how they manage the delicate balance between their internal salts and the salts in their surroundings as they cut back on the energy-intensive process of ion movement to save energy. But what about hagfish with their unusual balance of salts? And what happens when oxygen is scarce? Chris Glover of Athabasca University, Canada, and Greg Goss of the University of Alberta, Canada, decided to find out.
Glover and Goss hypothesized that their slimy beasts would do the same as other fish when oxygen was in short supply: decreasing calcium uptake and accumulation. They tested this idea by placing hagfish in air-tight jars and tracking the movement of mildly radioactive calcium into their tissues as they used up the available oxygen.
Unlike the hagfish in open jars, the hagfish in sealed jars moved calcium quickly into most of their tissues, including skin, liver, muscle, heart, plasma and brain. Low oxygen levels also bumped up the rates of calcium build-up in their gut and especially their gills; the guts of the fish in the low oxygen conditions had nearly 4 times more calcium than the guts of the fish in oxygen-rich conditions after adjusting for calcium contained in the blood flowing through the tissue. Since hagfish have no bones to leech calcium from, these results meant that, contrary to expectations, hagfish responded to hypoxia by importing more calcium into their bodies.
When an animal does something you don't expect, the next questions are usually: how and why? Tackling the first question, Glover and Goss focused on how calcium moved across hagfish skin and gut tissue when it was isolated from the rest of the body with the goal of understanding the role each tissue played in the hypoxic fish's calcium binge.
They discovered that low oxygen in the environment reduced calcium uptake by the skin, but only when the experiment was performed in unnaturally low calcium conditions, so this was unlikely to be a major mechanism of calcium accumulation in living animals. However, the results in the gut tissue were more puzzling. Despite surging calcium levels in the gut under hypoxia, there were no differences in the calcium uptake rates between the guts of the hagfish that had been kept in normal levels of oxygen and those that were oxygen starved. The authors mused that perhaps the sealed-in hagfish had gulped down seawater as a reaction to the stress, accidently flushing the tissue with calcium in the process. With the skin and gut ruled out as major importers of calcium into the body, the researchers determined that the gills were the source of the calcium detected in the closed-jar hagfish, which squares with the impressive rate of calcium accumulation observed in this tissue.
Hagfish are peculiar creatures, so it's perhaps unsurprising that they respond so differently from most other earthbound fishes. As for why hypoxic hagfish pull in so much calcium through their gills, that remains a mystery. Calcium could possibly aid in coping with low oxygen conditions by constricting blood vessels or protecting brains cells. Alternatively, the calcium accumulation might not be a strategy at all and could simply be a result of hagfish moving more water – and consequently calcium – through their gills; but that's a study for another time.