Deserts aren't the most obvious aquatic environments, but rare water holes and fissures crop up in most deserts; even Death Valley in the USA. Even more remarkably, some of these waterholes have been colonised by fish, and one, the 3 m×7 m Devil's Hole, is home to the planet's entire population of Cyprinodon diabolis, better known as the Devils Hole pupfish. Sadly the hole's vulnerable residents became a `causecélèbre' in the late 1960s, when water-pumping activity threatening the pupfish's survival. Activists mobilised, and a 1976 landmark US Supreme Court ruling forced the pumping to stop. But by then the Devils Hole pupfish was in serious danger, so conservationists set up 3 refuges to establish backup populations. However, within 5 years, ecologists noticed that the refuge populations were losing some of the Devil's Hole distinctive characteristics; the pupfish's heads and eyes were smaller than those of the original population and their bodies became deeper. Curious to know whether environmental factors could be driving the fish's morphological modifications, Gabrielle Nevitt and Sean Lema decided to see if diet restriction could alter morphological characteristics in a close, but unthreatened, relative; the Amargosa River pupfish (p. 3499).
Hiking to the river's Tecopa Canyon, Lema collected Amargosa River pupfish before returning to Nevitt's lab in the University of California at Davis. Collecting freshly laid eggs, Lema waited for the larvae to hatch before dividing them into three groups; one was fed an unrestricted diet, another a moderate diet and the third on a severely restricted diet. Having set each of the tanks to the Devil's Hole temperature, 33°C, Lema noticed that the tanks' temperatures varied a little, so he factored temperature into his analysis of the fish's development too. Monitoring the fish over 126 days, Lema recorded the size of their heads and body depth relative to their body length. Sure enough, fish on the restricted diet had restricted growth and relatively large heads and shallow bodies, just like the Devils Hole pupfish. But most surprisingly, the fish that had been exposed to water that was just 1°C warmer than the experimental temperature failed to develop pelvic fins, just like the true Devils Hole pupfish. Both temperature and diet restriction had a big effect on the developing fish's morphology. `We hadn't expected such a strong impact on fin development' admits Lema.
So what could be driving these drastic physical changes? Lema explains that Don Brown's 1997 Proc. Nat. Acad. Sci. publication suggested that low thyroid hormone levels could prevent zebrafish larvae from developing pelvic fins. Were the underfed pupfish's thyroid hormone levels low? Measuring the fish's whole body hormone levels, Lema found that they were considerably lower than the well-fed fish.
Next Lema and Nevitt decided to see what happened to developing fish larvae when their thyroid levels had been lowered pharmacologically, and were amazed to see that the fish's growth on a good diet was unrestricted, but that they failed to develop pelvic fins. The team suspect that the Devils Hole pupfish's metabolism is significantly affected by their poor diet in hot water, naturally reducing their thyroid hormone levels, resulting in their unique appearance.
But what hope does this offer for the Devils Hole pupfish's survival? Lema suspects that the fish's best hope is a carefully controlled captive breeding program, and with their current numbers at record low levels, the fish have never before needed help more than they do now.
- © The Company of Biologists Limited 2006