Sea urchins are invertebrates that occupy a globally important position in marine food webs, often acting as the primary grazers in coastal ecosystems. Like many ocean organisms, they are highly sensitive to changes in seawater chemistry. Rising atmospheric carbon dioxide, primarily from burning fossil fuels, is fundamentally altering ocean conditions. This change, known as ocean acidification, threatens the survival and function of sea urchins.
The Chemistry of Ocean Acidification
Ocean acidification begins when the ocean absorbs carbon dioxide (\(\text{CO}_2\)) released into the atmosphere. Approximately one-third of this \(\text{CO}_2\) is taken up by the surface ocean, triggering a cascade of chemical reactions. When \(\text{CO}_2\) dissolves in seawater, it reacts with water (\(\text{H}_2\text{O}\)) to form carbonic acid (\(\text{H}_2\text{CO}_3\)). Carbonic acid quickly dissociates into a bicarbonate ion (\(\text{HCO}_3^-\)) and a free hydrogen ion (\(\text{H}^+\)).
The increase in the concentration of these hydrogen ions is what causes the seawater’s \(\text{pH}\) to drop, making the water more acidic. Since the start of the Industrial Revolution, the average \(\text{pH}\) of surface ocean waters has fallen by approximately 0.1 units. This drop represents an increase in acidity of about 30 percent due to the logarithmic nature of the \(\text{pH}\) scale.
A primary consequence of this chemical shift is the reduction of available carbonate ions (\(\text{CO}_3^{2-}\)), which are essential building blocks for calcifying organisms. The excess hydrogen ions from the carbonic acid reaction bond with the existing carbonate ions in the water. This process effectively makes the carbonate less available for sea urchins and other marine life to use in building and maintaining their calcium carbonate structures.
Effects on Sea Urchin Development and Structure
The primary biological consequence of ocean acidification is the stress placed on the sea urchins’ ability to create and maintain their skeletons, known as calcification. They rely on calcium carbonate to build their hard, spherical tests and their feeding apparatus, Aristotle’s lantern. Reduced availability of carbonate ions makes it energetically challenging to secrete and sustain these structures.
Larval stages are exceptionally vulnerable to these chemical changes. Exposure to elevated \(\text{CO}_2\) concentrations leads to impaired development and significantly reduced growth, resulting in smaller skeletal lengths. If the \(\text{pH}\) drops too low, the tiny calcium carbonate spicules forming the larval skeleton can be corroded. Low \(\text{pH}\) levels also decrease the swimming speed and motility of sperm, drastically reducing fertilization rates and compromising reproductive success.
Ocean acidification also imposes a substantial metabolic cost on both juvenile and adult sea urchins as they attempt to maintain internal balance. They must expend extra energy to regulate the \(\text{pH}\) of their internal coelomic fluid and cells, a process known as acid-base regulation. This compensation requires up-regulating ion transport mechanisms, which demands a significant increase in energy expenditure. Studies show that this energy is diverted away from growth and tissue repair, resulting in reduced overall somatic growth and decreased reproductive output for adult urchins.
Broader Ecological Role of Sea Urchins
Sea urchins function as keystone herbivores in many shallow-water marine environments, and the stress from ocean acidification on their populations has ecological implications. In kelp forest ecosystems, their intense grazing activity controls the abundance of macroalgae. A decline in sea urchin populations due to stress could lead to an overgrowth of kelp and other algae, altering the entire habitat structure.
Conversely, high densities of urchins can clear-cut a seabed, creating a low-diversity habitat known as an “urchin barren.” The impact of acidification on grazing rates is complex. Some research indicates that higher \(\text{CO}_2\) conditions can reduce the feeding rate of urchins, which would decrease grazing pressure and benefit kelp. Regardless of the specific outcome, any change to the urchin’s health and population size fundamentally shifts the balance of the ecosystem.
On coral reefs, sea urchins are crucial for resilience because they graze on fast-growing macroalgae that compete with corals for space. Where fishing has reduced the population of herbivorous fish, the role of sea urchins in controlling algal growth becomes more pronounced. The physiological compromise of sea urchins from ocean acidification poses a double threat. It directly reduces the population of a calcifying organism while simultaneously jeopardizing the health of coral reefs that rely on them for algal control.