The sea urchin, a member of the echinoderm phylum, serves as a significant grazer in marine habitats, particularly in kelp forests and on coral reefs. As a keystone species, its feeding behavior helps maintain the balance of these diverse ecosystems. However, the world’s oceans are rapidly absorbing excess atmospheric carbon dioxide, a process known as ocean acidification (OA), causing a measurable decrease in ocean pH. This drop fundamentally changes seawater chemistry and poses a direct threat to the sea urchin’s physiological and developmental processes. Impacts range from the molecular level, affecting the ability to build and maintain body structure, to broad ecological consequences that could destabilize entire ocean food webs.
The Chemistry of Decreasing pH
Ocean acidification begins with the absorption of atmospheric carbon dioxide into the surface seawater. This absorbed gas reacts with water to form unstable carbonic acid. The carbonic acid quickly dissociates, releasing hydrogen ions, which directly causes the pH decrease. Since the pH scale is logarithmic, a change of only 0.1 pH unit represents a roughly 26% increase in hydrogen ion concentration.
This chemical reaction also reduces the concentration of carbonate ions, necessary building blocks for calcifying organisms. Their availability is measured by the carbonate saturation state. As the ocean pH drops, the saturation state decreases, making it chemically more difficult for marine life to form and maintain their calcium carbonate structures. Calcification becomes an energetically expensive process.
Impact on Skeletal Structure and Growth
The primary physical effect of decreased pH is the impairment of the sea urchin’s ability to build and maintain its calcium carbonate skeleton (test) and protective spines. The urchin’s structure is made of high-magnesium calcite, a highly soluble form of calcium carbonate, making them particularly vulnerable to corrosive seawater. Studies show that lower pH levels directly reduce the calcium carbonate content in the shell, leading to structural modifications.
To counteract the surrounding water’s lower pH, sea urchins must expend significant energy regulating the acid-base balance within their internal fluids. This reallocation of energy away from normal functions results in reduced growth rates in both juvenile and adult urchins. Increased porosity in the skeletal plates under acidic conditions suggests a loss of structural integrity, making them more susceptible to physical damage and predation.
The corrosive environment can also lead to decalcification, where existing skeletal material dissolves, especially in extremely acidic conditions. Adult urchins exposed to pH levels around 7.6 or 7.8 show signs of physiological stress and reduced growth of gonads and body size. The adult energy budget shifts, prioritizing survival functions over growth and reproductive development.
Impact on Reproduction and Early Development
The earliest stages of the sea urchin life cycle are the most sensitive to drops in ocean pH. Reduced fertilization success is a documented effect, as gametes show impaired function in lower pH water.
The planktonic larval stage is acutely affected, exhibiting higher rates of mortality and abnormality when exposed to acidified seawater. Larvae in lower pH conditions, such as 7.2, often fail to develop to the pluteus stage or show severe morphological abnormalities, including smaller skeletal rods. The formation of the first internal skeletal elements is compromised, leading to malformed or stunted structures.
Larval development is often delayed as the organisms struggle to cope with chemical stress. This delay, combined with the need to expend more energy on metabolic regulation, increases their time in the water column. An extended larval period increases exposure to predators, reducing the number of new juveniles that settle on the seafloor. Metamorphosis, the transition from planktonic larva to benthic juvenile, is also significantly delayed or completely failed under low pH conditions.
Broader Ecological Consequences
The negative impacts on sea urchin survival, growth, and reproduction have substantial ripple effects across the marine ecosystem. Sea urchins are important grazers of macroalgae, and their decline can fundamentally alter habitat structure. A population decrease can lead to the unchecked growth of kelp or other algae, potentially transforming kelp forests into barren areas dominated by a few species, known as a phase shift.
The compromised health and reduced abundance of urchins also affect species that rely on them for food. Sea urchins are a major food source for predators, including sea otters, lobsters, and some fish species. A decline in sea urchin populations due to ocean acidification can lead to a reduction in the fitness or population size of these higher trophic level consumers.
The overall result is a disruption of the food web, creating trophic imbalances that challenge the resilience of the entire marine community. Compromised sea urchin populations, due to reduced survival and reproductive output, may not be able to fulfill their ecological role as regulators of primary producers.