Blind Mexican tetras that live in caves face a number of challenges compared with other members of the species that live in above-ground rivers, the greatest being the lack of reliably available food. Since plants and algae need light to photosynthesise, they don’t exist in subterranean depths and thus the cave dwelling populations of Astyanax mexicanus have had to find ways to deal with long periods of nutrient deprivation without starving to death.
Some of those strategies have already been observed by other studies, and include changes to circadian rhythm, decreased metabolic rate, and elevated body fat. However, a team of researchers led by Nicolas Rohner of the Stowers Institute for Medical Research in Missouri, US, looked at three tetra populations named for the caves they inhabit – Tinaja, Pachón and Molino – and monitored their blood glucose to see how it differed to laboratory-raised populations of their surface-dwelling cousins.
The results show that compared to the lab-raised cohort the cave fish have elevated blood glucose levels for longer after both feeding and fasting, followed by a far larger drop after 21 days.
The cavefish populations all lacked the ability to maintain stable blood glucose levels – they exhibited dysregulated glucose homeostasis, in the jargon – which is a key mechanism for controlling insulin production. Low levels of insulin are the cause of Type 1 diabetes in humans, but Rohner and his colleagues found that in cave-dwelling tetras the condition is a help, not a hindrance.
“Our findings suggest that diminished insulin signalling is beneficial in a nutrient-limited environment and that cavefish may have acquired compensatory mechanisms that enable them to circumvent the typical negative effects associated with failure to regulate blood glucose levels,” the scientists write in a paper published in the journal Nature.
Part of the story appears to be a mutation on an insulin receptor gene which may affect the efficiency of insulin binding in the Tinaja and Pachón populations. A similar end is being achieved by the Molina population – which split from the surface-dwelling mainstream later than the other ntwo – via a different and as-yet-unknown mechanism.
What’s particularly surprising about this finding is that the usual life-shortening effects of insulin resistance and hyperglycemia in other species – humans, for example – don’t appear to manifest in the Tinaja and Pachón fish populations.
It’s speculated that there are some other mutations or adaptations at work which might help mitigate the effects of elevated blood glucose, to the point where they actually appear healthier and remain fertile compared for longer than their surface cousins.