Which organisms are affected the most?

High-sensitivity groups

This page focuses on organisms where acidification directly interferes with calcification (shell/skeleton building) or early-life development. Sensitivity still depends on local conditions like temperature, oxygen, food supply, and water mixing.

Pteropods (sea butterflies)

Thin shells • important in cold-water food webs

Pteropods are tiny swimming snails with very thin calcium-carbonate shells. When seawater becomes more corrosive, their shells can dissolve or become harder to build—especially in high-latitude oceans where acidification can show up sooner.

• Shell dissolution risk increases in low-carbonate waters.
• They’re a key prey item for fish and other animals in some regions.

Oysters

Larvae are especially vulnerable • reef builders

Oyster larvae need stable carbonate chemistry to form their first shells. Acidification can reduce survival and slow growth, which matters both for ecosystems (reef habitat) and for aquaculture.

• Early life stages often show the strongest sensitivity.
• Healthy reefs also support water filtration and coastal biodiversity.

Mussels

Shell formation • growth and strength can change

Mussels build shells from calcium carbonate. Under acidified conditions, they may spend more energy on maintaining shell material, leaving less energy for growth and reproduction—especially when combined with other stressors like warming and low oxygen.

• Shell thickness and mechanical strength can be affected.
• Impacts often depend on food availability and local water chemistry.

Stony corals

Calcium-carbonate skeletons • reef structure

Many reef-building corals deposit calcium-carbonate skeletons. Acidification can reduce calcification rates, which can weaken reef growth and resilience—especially when heat stress is already causing bleaching.

• Lower carbonate availability can slow skeleton building.
• Reefs protect coastlines and provide habitat for diverse marine life.

Sea urchins

Calcified structures • development can be sensitive

Sea urchins have calcified parts and early life stages that can be sensitive to carbonate chemistry. Acidification can alter larval development and may affect growth and survival depending on the species and conditions.

• Larval stages can be a bottleneck for population success.
• Effects vary widely across species and habitats.

Coccolithophores (calcifying phytoplankton)

Microscopic calcifiers • important in carbon cycling

Coccolithophores are phytoplankton that produce tiny calcium-carbonate plates (coccoliths). Their response to changing chemistry is complex and species-specific, but shifts can matter because these organisms influence ocean carbon cycling.

• Responses differ by species, nutrients, and light conditions.
• They play a role in marine food webs and biogeochemistry.