Purple sea urchins residing in small hollows excavated in a reef. Photo copyright Anne M. Rosenthal.
What happens to ocean animals as increasing carbon dioxide acidifies the ocean? Can populations evolve quickly enough to survive?
Along the Pacific Coast, that just might happen: Intermittent upwelling from the deep already brings marine organisms into contact with acidic waters. And to survive, populations must harbor gene versions – alleles – that allow them to cope with an acidic pH. But how rapidly can these adaptive alleles become dominant in a population? Experiments by Melissa H. Pespeni and colleagues, in the Palumbi laboratory at Hopkins Marine Station, Stanford University, showed surprisingly fast adaptation in larval populations of the purple sea urchin, Strongolocentrotus purpuratus.
Along the Pacific Coast, that just might happen: Intermittent upwelling from the deep already brings marine organisms into contact with acidic waters. And to survive, populations must harbor gene versions – alleles – that allow them to cope with an acidic pH. But how rapidly can these adaptive alleles become dominant in a population? Experiments by Melissa H. Pespeni and colleagues, in the Palumbi laboratory at Hopkins Marine Station, Stanford University, showed surprisingly fast adaptation in larval populations of the purple sea urchin, Strongolocentrotus purpuratus.
A fragment of the test or skeleton of the purple sea urchin. The tiny holes are pores where tube feet protrude through the test. Photo copyright Anne M. Rosenthal.
One of the most strikingly beautiful invertebrates along West Coast seashores, S. purpuratus sports the calcareous skeleton typical of sea urchins, a "test" complete with moveable spines, as well as pores where tube feet – sucker-tipped extensions of its hydraulically operated water vascular system – protrude. Like sea stars, brittle stars, and sea cucumbers, sea urchins are members of the phylum Echinodermata and bear both bilateral and radial symmetry.
As vegetarians, purple sea urchins consume a variety of algae, gnawing away with five renewable teeth, part of a mouth complex known as Aristotle's lantern. The purple sea urchin's teeth, and most likely its spines, also come in handy for excavating urchin apartment complexes – clusters of urchin-sized hollows located in reefs where conditions are optimal for urchin survival. Urchins serve as prey for their close cousins, the sea stars, and are favored morsels for sea otters, apex predators that keep urchin populations under control; urchin pigment even collects in the bones of otters, staining them purple.
The magnificent purple sea urchin bears rows of spines in a radial symmetry. Photo copyright Anne M. Rosenthal.
Skeletal rods composed of calcite (CaCO3) are instrumental to the larva's ability to swim and feed, and thus, to its survival over four or so months of drifting and feeding within the plankton before settling down to become a sessile adult.
Micrograph of the pluteus larva of a sea biscuit, a type of sand dollar, showing the skeletal rods. Photographed under polarized light microscopy by Bruno C. Vellutini.
But here's the bottom line for an urchin larva: It has to extract calcium carbonate from ocean water to build its skeletal rods more quickly than they dissolve back into the ocean. And the more acidic the water, the faster calcium carbonate dissolves.
Carbon dioxide comes into play because, when it is absorbed by water, it forms carbonic acid. Therefore, as carbon dioxide concentrations increase in the air, ocean waters gradually acidify. This process is spelled out in a viewer-friendly video by Stephen Palumbi, Stanford University (Hopkins Marine Station) on the Microdocs YouTube channel: http://tinyurl.com/17fvbs3
Compared to other organisms, purple sea urchins have a head start in terms of genetic diversity. One study showed that purple sea urchin populations bear more versions of their genes than any other organism yet studied – approximately a thousand times more than humans, for example. But would this genetic diversity confer survival of some individuals under acidic conditions predicted for the future? And if so, would an actual shift in the frequency of adaptive alleles be observed under conditions of lower pH?
Sea urchin larva. Photo courtesy of Stephen Palumbi.
Carbon dioxide comes into play because, when it is absorbed by water, it forms carbonic acid. Therefore, as carbon dioxide concentrations increase in the air, ocean waters gradually acidify. This process is spelled out in a viewer-friendly video by Stephen Palumbi, Stanford University (Hopkins Marine Station) on the Microdocs YouTube channel: http://tinyurl.com/17fvbs3
Compared to other organisms, purple sea urchins have a head start in terms of genetic diversity. One study showed that purple sea urchin populations bear more versions of their genes than any other organism yet studied – approximately a thousand times more than humans, for example. But would this genetic diversity confer survival of some individuals under acidic conditions predicted for the future? And if so, would an actual shift in the frequency of adaptive alleles be observed under conditions of lower pH?
Prior experiments with S. purpuratus grown in the laboratory had shown poor larval survival in water acidified to the future levels anticipated. However, judging that crowded conditions and still waters might have stymied survival in previous experiments, Pespeni and colleagues designed a new experiment featuring more spacious quarters and continuous water movement akin to the constantly shifting ocean.
Melissa Perpeni injects urchin to prompt spawning. Photo by Dan Griffin, courtesy of Stephen Palumbi.
Spawning sea urchins in laboratory. Photo by Dan Griffin, courtesy of Stephen Palumbi.
Male and female sea urchins were then collected from seven populations along 1200 kilometers of the Pacific Coast. For each population, scientists mixed eggs from ten females with a mixture of sperm from ten males. Resulting planktonic larvae were raised under today's ambient CO2 conditions or under experimentally elevated CO2 representative of likely future conditions.
Melissa Pespeni uses a microscope to check on sea urchin larvae. Photo by Dan Griffin, courtesy of Stephen Palumbi.
Then, using advanced molecular biology techniques, Pespeni monitored the collective genome of both populations of sea urchin larvae – those under present-day pH and those under anticipated acidic conditions – over time as the larvae matured. Despite few telltale differences in size or appearance, rate of development, or competency to settle, there were pronounced genetic changes in the population of surviving experimental larvae as time went on. These genetic changes were, in fact, related to cellular processes that regulate pH and build skeletons.
The findings indicated the beginning of an evolutionary response to increased acidity and implied that purple sea urchin populations not only have the genetic components to survive lower pH levels, but that the level of these alleles can quickly increase in purple sea urchin populations.
Yet this might or might not benefit the population in the long run, notes Pespeni on her web site, since concurrent loss of genetic diversity could cost the sea urchin population in other ways. Specifically, as a result of evolving to survive acidic conditions, the population might no longer have sufficient plasticity to respond successfully to other environmental pressures, such changes in temperature, fluctuations in food availability, or presence of disease-causing organisms. Further research may lend insight to whether sea urchin populations can adapt to multiple stressors simultaneously.
But an even bigger question looms on the horizon, indicates Pespini. While genetic resilience to ocean acidification in purple sea urchin populations looks promising, what about organisms with tiny population sizes, low genetic variability, and little or no exposure to highly acidic conditions? It is these organisms that may prove most vulnerable.–Anne M. Rosenthal
A video by Stephen Palumbi and Eric Sanford, UC Davis (Bodega Marine Laboratory) on the Microdocs YouTube channel, describes the experiment in lay terms and shows the set-up:
https://www.youtube.com/watch?v=wDcDqULXDOE
https://www.youtube.com/watch?v=wDcDqULXDOE
References:
Melissa H. Pespeni (now a post-doctoral researcher at Indiana University) web site:
Pespeni, M.H. et al., 2013, Evolutionary change during experimental ocean acidification. PNAS 110(17): 6937-6942. PDF available through Pespeni website.
Pespini, M.H. as quoted in Karoi, L., April 17, 2013 blog post, Evolve and Adapt – Sea Urchins' Climate Change Strategy, take part.com
Buchsbaum, R., et al. 1987. Animals without backbones, 3rd edition. University of Chicago Press, Chicago.
Durham, J.W., et al. 1980. Echinoidea: The Sea Urchins in Intertidal invertebrates of California by Morris, R.H., Abbott, D.P., and E.C. Haderlie, Stanford University Press, Stanford, CA
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