Microplastics impact coral reproduction at multiple stages: Report

Scientists have found chemicals leaching from microplastics have significant negative impacts on coral reproduction and settling of their larvae — two most crucial processes for reef survival.

The first of its kind research presented evidence of impacts at multiple levels depending on the species, polymer type to which the coral is exposed and time, indicating delayed cumulative effects at various life stages of corals. 

The study published in Frontiers in Marine Science explored the effects of microplastic leachate on coral planula larvae settlement and survival. 

Planula larvae of the broadcast spawning species Montipora capitata and the brooding species 'Harbor Porites' were exposed to microplastic leachate from four plastic types [nylon, polypropylene (PP), high-density polyethylene (HDPE) or low-density polyethylene (LDPE)] at three concentrations — 50, 100 or 200 particles per litre — for a period of seven days.

Corals have two main modes of sexual reproduction: brooding and spawning, the study explained. “Brooding coral species internally produce eggs and sperm, which fertilise and develop into planula larvae within the coral. Alternatively, spawning coral species release egg and sperm into the water column where external fertilisation and development occur. Planula larvae undergo settlement and metamorphosis where they change from a planktonic to benthic stage,” it said. 

The process of site selection and development by corals is heavily influenced by chemical cues providing attractive substrates. Since corals are sessile after settlement, they likely encounter sinking particles or less dense, biofouled particles.

Conversely, coral planula larvae move throughout the water column and likely encounter a variety of particles with different densities (sinking and floating plastics), the researchers wrote.

Earlier studies from 2019 and 2024 have found effects of microplastics on coral reproduction but noted limited effects of weathered polypropylene on fertilisation of the broadcast spawning coral Acropora tenuis. However, it did not find significant effects on embryo development and larval settlement.

“There was a significant effect of particle size on abnormalities of the fertilised eggs exposed to large-weathered plastics resulting in significantly lower fertilisation success. The mechanism for this decline in fertilisation was assumed to be due to physical contact with the particles. More recently, we found negative effects of microplastic leachate on fertilisation and fatty acid quantity and composition of Montipora capitata eggs likely due to the endocrine-disruptive nature of many plastic additives,” the authors noted.

Observing that the previous studies focussed on impacts of microplastic pollution on the gametes — a reproductive cell of broadcast spawning coral species, the scientists decided to find the chemical effects on planula larvae development that remain understudied to identify full effects on coral reproduction.

The report came as coral reefs across the world are under tremendous heat stress from January 1, 2023 to September 2025, with the bleaching-level heat stress impacting 84.4 per cent of the world’s coral reef area. Mass coral bleaching has been documented in at least 83 countries and territories, according to United States-based National Oceanic and Atmospheric Administration Satellite and Information Services. 

“Microplastic leachate had negative effects on survival and settlement of Montipora capitata and Harbor Porites planula larvae with species-specific and time-dependent effects seen. Harbor Porites planula larvae had higher survival and settlement compared to those of M capitata in many circumstances,” the study noted.

Planula larvae exposed to LDPE-200 demonstrated lower survival than in several treatments in June for Harbor Porites species. Between the species, negative effects were seen in planula larvae survival of both species in the HDPE-100 and LDPE-200 treatments with more significant effects seen in Harbor Porites, the authors said.

Both the polymer type and concentration influenced survival, with significant effects seen on the final days of the experiment (days five and seven), suggesting a delayed or cumulative negative effect, according to the report.  

The researchers concluded that considering the vulnerability of the early life stages of corals and the many other stressors that they currently face, a greater understanding of microplastic leachate effects on different life stages of corals is needed to understand potential effects on recruitment and replenishment of reefs.