Smell-Shocked

The suspected risks to fish of climbing ocean acidity keep escalating. First, the suggestion was just that anemonefish and others might not get home, although many broader implications of that study (published in the "Proceedings of the National Academy of Sciences") were left for others to pursue. Now, wiredscience points to more dramatic conclusions from a new, related study in the same journal that suggests, only slightly hyperbolically, that the actual effect is tantamount to a death wish:

Researchers discovered the potentially deadly problem through a series of experiments on common reef-dwelling fish that were raised in seawater with acidity levels resembling what’s expected by the century’s middle and end.

“Instead of avoiding the odor of a predator, they’re attracted to it,” said biologist Douglas Chivers of the University of Saskatchewan. “When you take them into the wild, their behavior has changed. We ended up with huge mortality.”

Five to nine times the mortality, actually! Here's the "Proceedings of the National Academy of Sciences" executive-style summary:

As atmospheric carbon dioxide increases, dissolved CO2 in seawater will increase, and lower the surface ocean’s pH. Previous studies have examined how ocean acidification may impact species that depend on seawater pH to create skeletons and shells, but the consequences for noncalcifying species such as fishes are less well understood. Philip Munday et al. examined the effect of acidification on clownfish and damselfish larvae, at values that could occur this century if atmospheric CO2 continues to increase at the current rate above the present-day concentration of nearly 390 ppm. The authors placed larvae in seawater at 700 and 850 ppm, and observed the fishes’ response to the odor of a predator. According to the authors, half of the larvae that initially avoided the odor were less able to detect it after 4 days in seawater at 700 ppm CO2, and at 850 ppm all of the larvae appeared to become attracted to the scent. When transplanted to a natural coral reef habitat, larvae that had been exposed to the elevated CO2 levels spent more time near predators, and had five to nine times the mortality rate of controls. The authors suggest that by the end of the century, ocean acidification could threaten the sustainability of some fish populations.

And here's the money shot from that abstract:

Our results show that additional CO2 absorbed into the ocean will reduce recruitment success and have far-reaching consequences for the sustainability of fish populations.

On a slightly related note, I stumbled across a factoid not long ago that didn't exactly surprise me but rather fazed me because it was one of those things I simply hadn't pondered. It was this: In all mammals, marine and terrestrial, the sense of hearing was evolved in order to hear sounds in an airborne environment. But it's tempting to flip that point on its head for olfactory abilities: Isn't it the case that olfaction was evolved long before our ancestors wriggled out of the water and thus was tailored to sensing water-borne smells? (I'd put a sperm's ability to smell its way to an egg in its liquid environment pretty darned high on my list of Darwinian "do's.")

I don't know the answer, but I resolve to find out more about what adaptations the move to land precipitated (pun intended!).