Oceans acidification endangers cuttlefish

Falling pH inhibits squid growth and damages their balance organs

The squid species Loligo pealeii is still widespread in the Atlantic © NOAA / NMFS / SEFSC
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Cuttlefish could have a hard time in the future because the increasing acidification of the oceans makes their survival difficult, as an experiment by US researchers now shows. In acidic water, their eggs grow slower and the juveniles stay smaller. At the same time they develop deformed organs of equilibrium so that they lose their orientation and swim in a circle. The endangerment of squid by the acidification will have far-reaching consequences, the scientists warn in the journal "PLoS ONE". Because the animals are both ecologically and economically an important part of the marine fauna.

The increasing carbon dioxide content of the air not only heats the climate, it also affects the oceans. Because sea and atmosphere are in constant exchange. Part of the greenhouse gas dissolves in the seawater, where it forms carbonic acid and other chemical compounds that make the water more acidic. But this has consequences especially for marine life such as corals, shells, sea urchins or calcareous algae. Because they rely on their calcium shells and skeletons to be able to absorb enough calcium carbonate from the water. In acidic water, however, this biomineralization is made more difficult, instead lime also increasingly dissolves from already existing shells.

Recently, there is increasing evidence that other marine animals are affected by a sinking pH. For example, the larvae of clown fish already lose their sense of smell below a pH of 7.8. For comparison, the normal acidity of seawater is currently between 7.9 and 8.25. The consequences of the increasing acidification of squid has been unclear. Max Kaplan, Aran Mooney and their colleagues from the Woods Hole Oceanographic Institution (WHOI) have now investigated this in an experiment.

Laboratory basin as the acid ocean of the future

For this purpose, the researchers caught several male and female squid species of the species Loligo pealeii in the Vineyard Sound in Massachusetts. These ten-armed, reddish-colored cephalopods are common along the east coast of North America, their body about as long as a human forearm. The captured animals were kept in the lab until they mated and the females began storing their egg capsules.

Kaplan and his colleagues collected the egg capsules and placed them in two different seawater basins. One was ventilated with normal outside air, the acidity of the water therefore corresponded to that in today's North Atlantic. The second basin received air with an increased carbon dioxide content, making the water three times more acidic than it is today. It roughly matched the value predicted for many ocean regions by the end of this century. The researchers observed how long the larvae needed and measured for hatching and examined the juveniles at regular intervals. display

Small and pretty disoriented

The result: In almost all measured parameters, the acidic water had significant effects: the squid embryos in the acidic water developed more slowly and slipped later. "That's not exactly good if you're just a solid mass of eggs on the bottom of the sea that can easily be eaten by fish, " says Mooney. After hatching, the size of the juveniles remained on average less than five percent behind those of their conspecifics in the less acidic water.

But not only that: the squids, which are exposed to the higher pH value, also developed malformed balance organs. These usually consist of tiny crystals of calcium carbonate, called statoliths. Based on the movements of these crystals in special sensory cells, the cuttlefish recognize their position in the water and can thus control their swimming movements. Instead of regular, fully developed statoliths, the squids grown in acidic water have much smaller crystals with a misfolded structure, as the researchers report.

Easy prey for predators

"This suggests that animals at high CO2 levels have problems forming these crystals, " explains Mooney. The acidic water causes the calcium carbonate to constantly re-open. As a result, the squid get problems in orientation and therefore easy prey for predators. The result of the experiment shows that the consequences of marine acidification could be far more extensive than previously assumed.

"Squid are at the center of the marine ecosystem - almost all marine animals feed on them or eat them, " explains Mooney. If anything happens to them, it will affect many levels of the marine food chain. So squid are an important food for tuna and other commercially important fish species. In many regions, however, the cuttlefish are caught and sold as coveted seafood.

(Public Library of Science, 03.06.2013 - NPO)