Acidifying oceans are bad news for larval fish

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The settlement behaviour of young fish could be significantly altered in more acidic oceans.

Studies have shown that increasing ocean acidity can affect the settlement behaviour of young clownfishes, but now it seems that other reef fishes could be affected too.

In a study to be published in a forthcoming issue of the journal Coral Reefs, Brynn Devine and colleagues show that while there was little significant change in the settling behaviour of three damselfishes in more acidic oceans, the timing of the settlement could be significantly altered.

The authors studied three species of Pomacentrus damselfishes: the Ambon damselfish (Pomacentrus amboinensis), Whitetail damselfish (P. chrysurus), and Lemon damselfish (P. moluccensis). The Ambon damselfish is a habitat generalist and shows no preference for specific habitats, while the Whitetail damselfish prefers to live amongst coral rubble and the lemon damselfish among live coral.

Conducting their studies at Lizard Island in the Great Barrier Reef, the authors collected settlement-stage damselfish larvae. They then conducted a series of experiments at both present-day carbon dioxide levels and levels elevated to almost twice those of current levels (which is projected to happen within this century).

In their first set of experiments, they tested the olfactory preferences of the settlement-stage damselfishes when presented with habitat odour from hard coral, soft coral and coral rubble using a two-channel flow chamber. The subject fish were presented with pair-wise choice tests of (1) hard coral versus soft coral, (2) hard coral versus coral rubble, (3) soft coral versus coral rubble and a blank seawater versus seawater as a control.  

In the second set of experiments, the authors released settlement-stage larvae overnight in outdoor aquaria that contained three different microhabitats (hard coral, soft coral and coral rubble) in order to test whether habitat selection was affected by increased ocean acidification when all sensory cues are available.

Lastly, the authors assessed the settlement rates of whitetail damselfish larvae across two lunar cycles.

The authors found that although exposure to elevated carbon dioxide disrupted the ability of larvae to discriminate between habitat odours in olfactory trials, it had no effect on the habitats selected at settlement when all sensory (especially visual) cues were available.

However, elevated carbon dioxide levels had a significant effect on the settlement timing of the Whitetail damselfish, with peak settlement rates occurring around the new moon in current conditions and around the full moon at elevated carbon dioxide levels.

This is worrying, as rising carbon dioxide levels could cause larvae to settle at unfavourable times, with potential consequences for larval survival and population replenishment.

For more information, see the paper: Devine, BM, PL Munday and GP Jones (2011) Rising CO2 concentrations affect settlement behaviour of larval damselfishes. Coral Reefs doi:10.1007/s00338-011-0837-0

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