Microplastic chemicals are disrupting how coral babies find each other

On certain nights when the moon is new, millions of coral eggs and sperm rise from reefs across Hawaiʻi and swirl through the water column, guided by lunar cycles and chemical signals that tell them when to meet and where to settle. It's one of nature's most precisely timed events — except now those chemical signals are being scrambled by something invisible: microplastic leachate, the toxic residue that leaches from plastic fragments breaking down in the ocean.

For decades, researchers studying coral and plastic pollution focused on one thing — how adult corals handle ingesting or absorbing plastic particles. But a new study from the University of Hawaiʻi at Mānoa suggests the real problem might be happening earlier, in the moment when coral reproduction begins. Coral biologist Keiko Wilkins and her team published their findings in Frontiers in Marine Science, revealing that microplastic leachate interferes with coral reproduction in ways that vary depending on which stage of development you're looking at.

"The effects that we're seeing on the fertilization and the effects that we're seeing on the larvae settling are very different," Wilkins explained. This distinction matters because it means the threat isn't a single problem to solve — it's multiple pressure points where reproduction can go wrong.

Coral reproduction happens one of two ways. Spawning corals release eggs and sperm into the water for external fertilization, creating those visible clouds of reproductive material. Brooding corals release larvae that have already been fertilized internally. Both methods depend on chemical communication in the water — signals that tell sperm where eggs are, and signals that tell larvae whether a reef is a safe place to settle and grow.

The chemical interference

When microplastics break down in seawater, they release chemicals that mimic or mask these natural signals. For spawning corals, leachate can reduce fertilization success rates. For larvae, the contaminated water makes it harder for them to recognize suitable settlement sites. The Hawaiian archipelago sits dangerously close to the Pacific Garbage Patch, meaning coral reproduction here happens in waters already loaded with microplastic pollution.

What makes this particularly concerning is the timing. Coral spawning events are synchronized and brief — often just a few nights per year when conditions align. If leachate disrupts fertilization during those narrow windows, an entire year's reproductive output can be lost. Larvae have a similarly tight window: they can only settle for a few days before they either find a reef or die in open water.

The research adds another layer to an already complex picture of coral stress. These reefs are already contending with warming oceans, acidification, and disease. Microplastic leachate isn't replacing those threats — it's compounding them, making it harder for coral populations to recover even when other conditions improve.

Wilkins' work is among the first to examine this specific mechanism, which means there's still much to learn about how widespread the problem is and whether certain coral species are more vulnerable than others. What's clear is that the chemical pollution from plastics operates on a different timescale than the visible debris — it's working in the background, interfering with the invisible conversations that keep reefs alive.