Ocean Acidification

Highlights

• First study on pteropod response to ocean acidification in the eastern Arabian Sea.
• High pteropod abundance during fall inter monsoon season due to food availability.
• pH in the Arabian Sea was low during south west monsoon with pH_T upto 7.75
• Pteropod shell dissolution was observed under acidified conditions
• Protrusions through the pteropod shell were observed under acidified conditions

Abstract

The rapid rise in atmospheric CO₂ and its subsequent uptake by the oceans has led to ocean acidification and other associated changes in the marine ecosystem. The recent reports of the shoaling of the aragonite saturation horizon in the northern Indian Ocean are particularly alarming, as they pose a serious threat to the survival of calcareous organisms. Pteropods, also known as sea-butterflies, are believed to be highly susceptible to ocean acidification due to their thin aragonite shell. In our study in the eastern Arabian Sea, we found low pH conditions with surface pH_T as low as 7.751 during late South-west monsoon (SWM). The pteropod abundance is high during the fall inter-monsoon (FIM), suggesting that the system continues to sustain productivity even after the cessation of peak monsoon activity. This also implies that the food availability regulates pteropod abundance in the eastern Arabian Sea. As pteropods are key components of food sources for many marine species, such as fish, any changes in their abundance can have cascading effects on the marine food web. To show how pteropods will be affected in futuristic elevated CO₂ conditions, a CO₂ manipulation experiment was conducted in the eastern Arabian Sea during December 2024. Pteropods belonging to Creseis acicula from the eastern Arabian Sea were subjected to pH_T = 7.470, and pCO₂ = 1734 μatm under controlled conditions. Our findings suggest that acidification led to the dissolution of pteropod shells. Acidification also led to protrusion through the shells, and these protrusions varied in length up to 88 μm. These structural alterations represent an acute response of pteropod shells to reduced pH, highlighting their rapid vulnerability to acidification stress. These observed protrusions need to be assessed further to determine if they provide any competitive advantage in combating or minimizing the impact of ocean acidification.

Mayuri R., Suhas S. S., Biraja K. S., Ajay K. J., Anoop B. & Sarvesh V., 2026. Pteropod vulnerability to ocean acidification in the eastern Arabian Sea. Estuarine, Coastal and Shelf Science 340: 110046. doi: 10.1016/j.ecss.2026.110046. Article (subscription required).