Few fish can deal with such extremes as a small killifish from the Middle East. Alex Bell introduces the bulletproof Arabian toothcarp.
ENVIRONMENTAL CONDITIONS PLAY a huge role in restricting where fish can survive, with water temperatures a major determining factor for the survival of a species. This is because the majority of them are ectothermic, meaning the temperatures of the surrounding environment heavily influence their metabolism, growth, activity and, as fishkeepers are well aware, it can also be a trigger for spawning events. All ectotherms have adapted to the speed of these processes under the temperatures they evolved in, and when moved outside of this range it can drastically impact their ability to function effectively.
Similarly, fish are largely constrained to an environment by the forces of diffusion and osmosis. Having evolved to live in equilibrium with certain parameters of water hardness, pH or salinity, a shift away from those parameters will lead to osmotic stress, or other physical stresses. Fish species only have a certain, often somewhat narrow, degree of tolerance.
Life at the extremes
One species fares a little better than most at the extremes: the Arabian toothcarp, Aphaniops dispar (formerly Aphanius dispar). Not only does this stocky little fish thrive in coastal saline habitats around the Arabian Peninsula, but also inland, in pools filled with hypersaline or fresh water—at the extremes, it has been found in water with a colossal 175ppt salt (by contrast, seawater typically holds between 33-38ppt). There are plenty of euryhaline fish species (those that can tolerate a wide range of salt concentrations) out there, but few can tolerate the extent that this remarkable fish can, and especially not for the duration that they can.
As well as its ability to withstand such varied concentrations of salt and still maintain osmotic balance within its body, its temperature tolerance is as wide ranging as the peninsula it inhabits. Reportedly, populations can survive temperatures as low as 4°C, and as high as 40°C, although they may not always thrive at these extremes—especially if the population does not experience these naturally. It’s an ongoing debate whether A. dispar is actually just one species or is instead a species group—several species have already been split out. This could go some way to explain why some individuals don’t fare so well in some conditions as compared to others.
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