Threat of Ocean Acidification to Echinoderms

Did you know the exhaust that billows from vehicles causes significant problems for sea stars? While most understand that the industrial smoke pouring into the air has a negative effect on our planet, who knew that it directly affects the chemistry of the ocean and the well-being of echinoderms like sea stars, sea urchins, and brittle stars?

Image Courtesy of: NOAA (National Estuarine Research Reserve Collection)

Image Courtesy of: NOAA (National Estuarine Research Reserve Collection)

The truth is that carbon wastes from industrial pollution can be absorbed by sea waters, an issue that leads to increasing acidity of the marine environment. This chemical phenomenon makes it extremely difficult for echinoderms to create their hard skeletons. Without strong skeletal structures, these organisms lack sturdy foundations for muscle attachment required for movement and feeding. It also makes them more vulnerable to fatal damages by predators. Thus, heightened levels of carbon output have resulted in drastic declines in echinoderm populations. In due course, if carbon emissions are not reduced, our oceans will continue to become more acidic and these organisms may face possible extinction in the future.

Changing the Chemistry of Sea Water

The ocean absorbs half the estimated amount of carbon dioxide that humans pump into the atmosphere. Rising concentrations of carbon dioxide makes sea water more acidic because, in aqueous environments, carbon dioxide is converted to carbonic acid. The hydrogen ions produced by this acid then react with carbonate anions, forming a compound called bicarbonate. This reaction reduces the availability of carbonate ions in forming other compounds, including calcium carbonate, a key component to the echinoderm skeleton.

Even though carbon dioxide has increased the acidity of our oceans by only a small amount, such small changes can still have drastic effects on marine life. In order to measure the concentration of hydrogen ions in the ocean, the pH log scale is used. The scale ranges from 1, most acidic, to 14, least acidic. Currently, the pH of the ocean surface already measures 0.1 pH unit lower than that of the pre-industrial time period. By the end of the century, the ocean's pH is projected to drop 0.3 more units. While such small changes in pH may appear insignificant, they can drastically increase the concentration of hydrogen ions in the oceans. The hydrogen ions then compete with calcium ions for the negatively charged carbonates. If pH levels continue to drop at the estimated rate, lower and lower concentrations of carbonate ions will be available for biologically essential reactions.

A Battle for Carbonate

After carbon dioxide from the polluted atmosphere dissolves into sea water (thereby creating carbonic acid), a battle between the two positively-charged particles ensues. The growing concentration of hydrogen ions reduces the availability of carbonate, a crucial component of skeletal formation in echinoderms. In order to successfully create structures from calcium, echinoderms require a continuous supply of ions to supersaturate the site of deposition, known as a locus. Positively-charged calcium ions (Ca2+) reacts with negatively-charged carbonate (CO32-) to form calcium carbonate. Clusters of this compound then aggregate into crystals that eventually form the echinoderm structures. Increasing levels of carbonic acid not only slow construction, but may also inhibit calcification completely. Calcification is a process that typically occurs in areas with slightly basic sea water. Without the full calcification of vital skeletal structures, echinoderms struggle to survive and are more vulnerable to lethal damages.

In echinoderms, calcium carbonate forms a unique structure called the stereom, a skeletal meshwork that makes an echinoderm skeleton light to carry but still resistant to breakage. Simple or compound calcium carbonate plates called ossicles provide echinoderms with a vital framework necessary for movement. In sea stars, each arm's articulating ossicles extend in two rows to protect nerves and vessels while also providing locations for muscle attachment. Ossicles also form protective knobs called tubercles and moveable or fixed spines. In addition, the calcium carbonate plates form unique pincer-like structures called pedicellariae that are used for defense and food capture. Brittle stars create ossicles known as vertebra in their segment-like appendages. Some sea urchins build modified podia (feet-like structures) that attach to the rigid calcareous plates of a hard shell known as the test. Certain species of echinoderms, such as sea urchins, also utilize their calcareous plates for feeding. Comprised of five calcareous plates with muscles, for example, an arrangement called Aristotle's Lantern controls the movements of five teeth that are used for grinding food.

In summary, changes in ocean chemistry inhibit the growth of calcium carbonate structures that echinoderms employ for locomotion, support, and defense, thereby threatening their survival. A continual effort must be made to reduce the carbon emissions that contribute to the acidification of our oceans. Only by reducing the amount of carbon dioxide humans pump into the atmosphere can creatures like sea stars and sea urchins survive.

Further Reading:

Beedlow, et al., 2004. Rising Atmospheric Carbon Dioxide and carbon sequestration in forests. Frontiers in Ecology Volume 2 (6): 315-322.

Burns, W., 2008. Saving Biological Diversity: Balancing Protection of Endangered Species and Ecosystems. Springer US, New London.

Brusca and Brusca, 2003. Invertebrates. Sinauer Associates, Sunderland.

Crenshaw, M.A., 1990. Biomineralization Mechanisms, 1-3 in Joseph Carter. Skeletal Biomineralization: Patterns, Processes and Evoloutionary Trends Volume One. Van Nostrand Reinhold, New York.

Currey, 1990. Biomineralization Mechanisms, 14 in Joseph Carter. Skeletal Biomineralization: Patterns, Processes and Evoloutionary Trends Volume One. Van Nostrand Reinhold, New York.

Doney, 2006. The Dangers of Ocean Acidification. Scientific American Volume 294(3):58-65.

Garrison, Tom, 2002. Oceanography- An Invitation to Marine Science. Brooks/Cole Thomas Learning, United States.

Lowenstam, H., 1989. On Biomineralization. Oxford University Press, New York.

Mann, K.H., 2006. Dynamics of Marine Ecosystems. Blackwell Publishing, Malden.

Pearse, 1987. Living Invertebrates. Blackwell Publishing, Palo Alto.

Willmer, P., 1990. Invertebrate Relationships-Patterns in Animal Evolution. Cambridge University Press, Cambridge.

Author: Nina Pelletier, Biology, Saint Anselm College

Reviewed by Falishia Sloan and Yangguang Ou

Published by Maria Huang

JYI staff members in the Research Department help student authors every step of the writing, editing, and peer-review process.
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