To escape from predatory sea stars, black turban snails grazing on algae will quickly crawl up and out of tide pools. But as seawaters become more acidic, snails are losing their flight response. The finding, published in Proceedings of the Royal Society B this week, is another stark example of how ocean acidification is altering marine animal behavior – and predator-prey systems as a result.
As carbon dioxide from human activities continues to enter the ocean, seawater pH levels have been dropping. And while many studies have focused on the effects of acidification on different species, the effects on interactions between species isn’t understood as well.
To study how ocean chemistry changes impact tide pool ecology, a team led by Brittany Jellison of Bodega Marine Laboratory collected ochre sea stars (Pisaster ochraceus) and black turban snails (Tegula funebralis) from mid-intertidal pools near California’s Bodega Bay in 2014. These snails live in the mid-intertidal zone of the rocky shores from Vancouver Island in Canada south to Baja California in Mexico. Their densities are the highest in wave-protected areas like rock pools, where they consume microalgae and seaweeds. The sea stars inhabit similar areas – low- and mid-intertidal zones from Alaska to Baja California – though they tend to be more common in wave-exposed sites. These carnivorous sea stars rarely leave the water to forage, so crawling out of the tide pool is an effective escape response for the herbivorous snails.
Back in the lab, the snails were exposed to waterborne cues emitted by the sea stars in special aquaria with seawater of varying pH levels – from present levels to expected levels for the year 2100. Higher acidity, the team found, impaired the snails’ flight response, and they spent most of their time submerged below the waterline. But it wasn’t because high acidity slowed the snails down or dampened their ability to detect predator cues. Exactly what prevents the snails’ escape response remains unclear for now.
Just one change in a predator-prey interaction could alter the structure of the whole shoreline community. Reduced predator avoidance means fewer snails and higher densities of the algae they eat – and that’s just one part of the larger, complex food web they inhabit.