Are Animals Getting Smaller and Smaller with Rising Temperatures?

Nassarius corniculus (left column) and Cyclope neritea (right column) living at CO2 seeps (top row) showing shell dissolution and apex truncation compared to shells collected from ambient pH (bottom row) / V. Garilli et al., Nature Climate Change 2015
Janet Fang 22 Apr 2015, 18:47

Prehistoric organisms who survived mass extinctions in the past were often smaller than the ones who came before them. This phenomenon is called the “Lilliput Effect,” named after the land of tiny people in Gulliver’s Travels. According to a new study published in Nature Climate Change this week, the stunted growth of sea creatures might be a genetic response to ocean acidification, one of the consequences of excessive carbon dioxide levels. Shrinking enabled them to survive extinction, and it seems to be happening again right now.

The rate of ocean acidification these days is especially worrisome because many previous mass extinction events were linked to climate warming and elevated CO2. For example, during "the Great Dying" at the end of the Permian 252 million years ago, atmospheric CO2 levels increased by a factor of six. In the immediate aftermath of events like that one, many of the shelled survivors (bivalves, snails, and friends) were smaller than before—and some stayed small for millions of years. Is dwarfing an adaptation for dealing with the higher energetic costs of maintaining a shell in such acidic, shell-dissolving waters? 

To investigate, an international team led by Vittorio Garilli of APEMA-Paleosofia and Riccardo Rodolfo-Metalpa of Institut de Recherche pour le Développement compared sea snails living in naturally acidified conditions near CO2 seeps with sea snails living in places with ambient seawater pH. Specifically, they collected Nassarius corniculus and Cyclope neritea from shallow-water CO2 seeps off Vulcano Island of northern Sicily and from three “normal” reference sites in the Mediterranean. To see how these gastropods cope with (and potentially adapt to) acidification, the team examined their shell morphology, dissolution, and repair using scanning electron microscopy.

The two species adapted to acidified seawater at CO2 seeps were smaller (and more corroded) than those found in normal pH conditions—by about a third. These Lilliputian snails also had higher energy consumption, but much lower metabolic demand. That means that, over several generations, they adapted their metabolic rates to cope with the CO2 that's absorbed by the water. 

“Not only do they demonstrate a similar magnitude and direction of body size change as fossil organisms, but they also reveal the physiological advantages of dwarfing,” study co-author Marco Milazzo from University of Palermo says in a news release. “These physiological changes allowed the animals to maintain calcification and to partially repair shell dissolution,” Garilli adds.

And we used to think these pH values were too low for calcification to happen! The ability to adapt through dwarfing, it seems, may have (and still do) offer advantages as CO2 emissions rise. "The smaller organisms can survive high carbon dioxide concentrations because they don't need as much oxygen,” study co-author Jason Hall-Spencer of Plymouth University tells New Scientist. "That's the nub of the analysis.”

Images: V. Garilli et al., Nature Climate Change 2015 (top), Plymouth University via Phys.org (middle)

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