Corals Spin Tiny Vortices to Get Oxygen, but Heat Breaks the System

Corals rely on symbiotic algae for oxygen during the day, but at night they must pull oxygen from the surrounding water. Since diffusion through the thin boundary layer at the coral surface is too slow to meet their needs, corals use hairlike cilia to generate vortices that actively circulate oxygen-rich water to their tissues.

A 2014 study from MIT and the Weizmann Institute of Science first revealed this active ventilation, overturning the assumption that corals were passive recipients of diffused oxygen. The cilia were previously thought to serve mainly as brooms for clearing mucus and debris.

In a study published in Science in May 2026, an international team exposed the stony coral Porites lutea to incrementally warmer water up to 39 degrees Celsius in dark tanks to isolate the cilia's nighttime function. Using high-speed cameras and fluorescent oxygen-reactive nanoparticles, they tracked cilia beating frequency and oxygen flow in real time.

As water temperature rose, the corals' metabolic demand for oxygen increased, and the cilia beat faster to compensate. However, warmer water holds less dissolved oxygen, so the accelerated vortices began delivering oxygen-depleted water. The cilia's own energy consumption further outpaced the oxygen supply.

At approximately 37 degrees Celsius the cilia started to slow, and at 39 degrees they stopped entirely, leading to coral death. Researchers caution that thermal limits vary by species and local adaptation, but note that ocean temperatures nearing 38 degrees Celsius have already been recorded in places such as Florida.

The mechanism triggering faster ciliary beating in heat remains unknown. Possibilities include changes in seawater viscosity, which decreases with temperature, or sensory processing through the coral's nerve net. Corals lack a central brain but possess distributed neurons that may detect heat or oxygen shortage.

A companion study published the same month found that cilia are arranged in hexagonal units on each polyp, creating corkscrew-like vortices that simultaneously push away sediment and direct nutrients toward polyp mouths. This structural integration suggests the coral skeleton and living tissue are functionally coordinated.

Marine biologists not involved in the work say the findings shift attention from bleaching alone to deoxygenation as a direct, fatal threat from warming. Patchy bleaching patterns on reefs may correlate with areas of reduced water flow and oxygen supply, and researchers plan to test cilia under normal light-dark cycles and non-thermal bleaching triggers to untangle the relationship.