Sea urchins are spiny marine invertebrates and significant grazers in ocean ecosystems worldwide. They are herbivores that can influence coastal habitats, sometimes leading to “urchin barrens” when their populations are unchecked. As ectotherms, their internal temperature is regulated by the environment, making water temperature the primary factor determining where they successfully live and thrive.
Global Thermal Diversity
The distribution of the over 950 species of sea urchins depends entirely on the temperature of the surrounding seawater. Sea urchins inhabit all oceans, ranging from the tropics to the polar regions and down to depths of over 5,000 meters. While the temperature limits for the entire class Echinoidea are extremely broad, the specific range for any single species is much narrower.
Tropical species, such as the Caribbean Diadema antillarum and Echinometra lucunter, have a high tolerance for heat, with upper lethal limits often exceeding 35°C. For example, the upper lethal limit for several Caribbean species occurs between 35.1°C and 37.1°C. In contrast, polar species, like the Antarctic sea urchin Sterechinus neumayeri, are accustomed to near-freezing conditions.
Temperate species, such as the purple sea urchin (Strongylocentrotus purpuratus), inhabit a more moderate range that shifts seasonally. The optimal performance window for many Caribbean species falls between 26°C and 32°C. The sea urchin Heliocidaris crassispina demonstrates a wide tolerance, with high and low temperature limits of approximately 33.3°C and 3.9°C respectively.
Temperature’s Role in Sea Urchin Biology
Temperature exerts direct control over the internal functions of sea urchins, influencing their metabolic rate, feeding efficiency, and growth. As water temperature increases within a species’ tolerable range, its metabolic rate generally rises, requiring more energy and increasing feeding activity. For Heliocidaris crassispina, the optimal growth temperature for its larvae is around 28°C, where it exhibits the fastest growth.
Temperature is also critical for reproduction, acting as a cue for gonadal development and spawning. Small temperature shifts can determine if a population successfully reproduces. While a small increase in temperature can boost a sea urchin’s gonadal index, excessive warming impairs juvenile development. The optimal temperature for fertilization in red sea urchins (Mesocentrotus franciscanus) is between 12°C and 22°C, with fertilization rates sharply decreasing outside this range.
Larval development is extremely sensitive to temperature, often representing a bottleneck for population success. For the tropical sea urchin Lytechinus variegatus, larvae showed reduced survival and growth when chronically exposed to temperatures above 30.5°C. The developmental velocity of embryos is directly linked to temperature, with a species’ optimal range corresponding closely to its breeding season’s water temperature.
Survival Limits and Environmental Stress
Every sea urchin species possesses defined thermal limits that exceed its ability to survive. The upper thermal maximum is the temperature at which the animal experiences physiological failure, such as the inability to right itself, leading to cell damage and mass mortality. A lower lethal limit also exists, below which the animal suffers from chilling injury and metabolic collapse.
When these limits are surpassed, basic functions fail; for example, the righting reflex and the lantern reflex (used for feeding) are suppressed at elevated temperatures. For various Caribbean species, the upper limit for performing the righting reflex occurs between 34.0°C and 36.9°C, very close to the lethal temperature. Exposure to temperatures just above the norm causes severe stress, leading to pathological changes in the gonads and impaired reproductive output.
Ocean warming is pushing many sea urchin populations to the brink of their thermal tolerance, especially those in tropical areas already living close to their upper limits. Rising temperatures reduce the thermal safety margin—the buffer between a species’ average habitat temperature and its lethal temperature—making them highly vulnerable to marine heatwaves. This stress can lead to shifts in population distribution or localized collapses.