Sea urchins are spiny, globe-shaped marine invertebrates that play a significant ecological role as grazers. As calcifying organisms, they have a unique relationship with oxygen (O2) and carbon dioxide (CO2). They require O2 for metabolism and generate CO2 as a waste product, but CO2 is also a fundamental building block for their rigid internal skeleton, the test, which is composed of calcium carbonate. This dual requirement creates a delicate physiological balance challenged by changes in seawater chemistry.
Internal Gas Exchange and Metabolic Needs
Sea urchins acquire oxygen to power cellular respiration primarily through their extensive network of tube feet and, in some species, five pairs of small, external gills near the mouth. Oxygen uptake and carbon dioxide expulsion rely entirely on diffusion across these thin-walled structures. This process brings the internal body fluid into close proximity with the seawater.
The internal body fluid, known as the perivisceral coelomic fluid (PCF), transports gases throughout the body. O2 is consumed in the cells to generate energy, constantly producing CO2 as a byproduct. This CO2 must be efficiently removed to prevent an acidic buildup.
Maintaining acid-base homeostasis is a physiological challenge for the sea urchin. If metabolic CO2 cannot be efficiently diffused into the surrounding water, its concentration rises inside the PCF, dropping the internal pH. This internal acidification, known as hypercapnia, can disrupt cellular functions. Thus, the flow of oxygen for energy is linked to the continuous expulsion of waste CO2 to maintain a stable internal environment.
The Dual Role of Carbon Dioxide in Calcification
While metabolic CO2 must be removed as waste, the molecule is also an indispensable resource for constructing the urchin’s test and spines. This process, called biomineralization, involves precipitating calcium carbonate (CaCO3) from the internal body fluids. Sea urchins must actively regulate the fluid chemistry at the site of calcification to facilitate this structural growth.
The key step involves converting internal CO2 into bicarbonate ions (HCO3-), which combine with calcium ions (Ca2+) to form the solid skeletal material. Specialized cells create a microenvironment saturated with these ions, allowing crystal formation. Calcification is an intracellular process, meaning the urchin has biological control over the fluid where the skeleton is formed. This control shields the process to an extent from the external environment.
This internal chemical manipulation demonstrates the dual role of CO2 within the organism. The sea urchin uses a portion of its metabolic CO2, and potentially CO2 absorbed from seawater, as a source for the bicarbonate needed for skeletal growth. The ability to precisely manage this internal fluid chemistry allows the urchin to grow its armor while managing the acidifying effects of its own metabolism.
External Environmental Stress and Physiological Response
The balance between O2 requirement and CO2 management is threatened by rising atmospheric CO2 dissolving into the ocean, known as ocean acidification (OA). As seawater CO2 concentration increases, the water’s pH decreases, making the environment more acidic. This change directly impairs the urchin’s ability to expel metabolic waste.
Increased external CO2 reduces the necessary diffusion gradient between the urchin’s internal body fluid and the surrounding seawater. This makes it harder for the animal to offload metabolic CO2, leading to chronic internal hypercapnia. To counteract this, the urchin must expend energy on active ion transport mechanisms to maintain a stable internal pH.
This physiological stress creates a metabolic trade-off, redirecting energy from growth, reproduction, or immune function toward acid-base homeostasis. In adult sea urchins, exposure to elevated CO2 reduces calcification rates, making it harder to grow or repair the test. Larval stages are particularly vulnerable, often exhibiting impaired development and increased mortality under moderately acidified conditions. External CO2 stress impedes the passive expulsion of metabolic waste CO2 and makes using CO2 for calcification more energetically expensive. The overall result is a physiological burden that compromises the urchin’s structural integrity and its ability to thrive.