# Genomic analyses support locally derived crown-of-thorns seastar outbreaks in the Pacific

## TL;DR

Genomic analysis shows crown-of-thorns seastar outbreaks in the Pacific are locally derived, not spread across oceans, and reveals new insights into their population structure and possible species classification.

## Contribution

The study uses low-coverage whole-genome sequencing to demonstrate local origins of COTS outbreaks and challenges existing species classifications.

## Key findings

- Pacific COTS populations are highly structured and outbreaks are locally derived, not spread through open ocean.
- Three main COTS lineages are geographically restricted to Hawai‘i, French Polynesia, and the West Pacific.
- Current COTS populations have the highest effective sizes in the last million years, suggesting human or climate influence.

## Abstract

Crown-of-thorns seastars (COTS, Acanthaster spp.) are the most notorious coral predators, whose devastating outbreaks cause recurrent and extensive coral depletion across Indo-Pacific reefs. However, the spread potential of COTS outbreaks and the anthropogenic role in their initiation have remained a subject of intense debate for over five decades.

Here, using low-coverage whole-genome sequences of 247 COTS, we show that Pacific COTS populations are highly structured, indicating that outbreaks do not spread through open ocean, but instead are locally derived. Pacific COTS populations are grouped in three main lineages geographically restricted to Hawai‘i, French Polynesia, and the West Pacific, with the latter showing further significant genetic substructure. Phylogenomic analyses indicated that the Hawai‘i COTS lineage likely represents a different undescribed species and challenged the species status of both A. cf. solaris and the Eastern Pacific COTS species (A. ellisii), as the latter appeared as the sister group of the French Polynesia COTS lineage. Additionally, we show that current COTS populations present the highest effective sizes of the last million years, suggesting that human and/or climate change may influence COTS population sizes.

Overall, our study highlights the improvements brought by low-coverage whole-genome sequencing approaches in resolving the phylogeny and connectivity patterns of a keystone species in understudied regions of the Pacific Ocean.

The online version contains supplementary material available at 10.1186/s12915-025-02350-4.