Euechinoidea and Cidaroidea respond differently to ocean acidification

Area of Science:

• Marine Biology
• Oceanography
• Physiology

Background:

• Increasing atmospheric carbon dioxide (CO₂) causes ocean acidification, impacting marine organisms.
• Extracellular pH regulation is crucial for marine life survival.
• Sea urchin groups, Euechinoidea and Cidaroidea, exhibit different evolutionary histories regarding ocean acidification events.

Purpose of the Study:

• To investigate the differential responses of Euechinoidea and Cidaroidea sea urchins to reduced seawater pH.
• To compare the extracellular acid-base balance regulation mechanisms between these two sea urchin groups.

Main Methods:

• Studied the tropical cidaroid Eucidaris tribuloides and temperate/tropical euechinoids Tripneustes ventricosus and Paracentrotus lividus.
• Measured coelomic fluid pH and bicarbonate concentrations under reduced pH conditions.
• Utilized carbon stable isotope signatures (δ¹³C) to trace carbon sources.

Main Results:

• Euechinoids (T. ventricosus, P. lividus) compensated for reduced seawater pH by increasing coelomic fluid buffer capacity and bicarbonate concentration.
• Cidaroids (E. tribuloides) showed no significant changes in coelomic fluid pH or bicarbonate levels.
• Carbon isotope signatures indicated bicarbonate in euechinoids likely originated from seawater, unlike in cidaroids.
• Feeding rates remained unaffected in all studied species.

Conclusions:

• Euechinoids possess the capacity to regulate extracellular acid-base balance in response to ocean acidification.
• Cidaroids do not exhibit apparent regulation of extracellular pH under reduced seawater pH conditions.
• Further research is needed to explore the energetic costs and distribution of pH regulation mechanisms across more sea urchin species.