Impacts of Ocean Acidification on Early Life-History Stages and Settlement of the Coral-Eating Sea Star Acanthaster planci

Sven Uthicke,Symon Dworjanyn,Miles Lamare,Andrew Peter Negri,Neal Cantin,Michelle Liddy,Rebecca Albright,Pamela Kamya,Maria Byrne,Danilo Pecorino,Tilmann Harder

doi:10.1371/journal.pone.0082938

Abstract

Coral reefs are marine biodiversity hotspots, but their existence is threatened by global change and local pressures such as land-runoff and overfishing. Population explosions of coral-eating crown of thorns sea stars (COTS) are a major contributor to recent decline in coral cover on the Great Barrier Reef. Here, we investigate how projected near-future ocean acidification (OA) conditions can affect early life history stages of COTS, by investigating important milestones including sperm motility, fertilisation rates, and larval development and settlement. OA (increased pCO2 to 900–1200 µatm pCO2) significantly reduced sperm motility and, to a lesser extent, velocity, which strongly reduced fertilization rates at environmentally relevant sperm concentrations. Normal development of 10 d old larvae was significantly lower under elevated pCO2 but larval size was not significantly different between treatments. Settlement of COTS larvae was significantly reduced on crustose coralline algae (known settlement inducers of COTS) that had been exposed to OA conditions for 85 d prior to settlement assays. Effect size analyses illustrated that reduced settlement may be the largest bottleneck for overall juvenile production. Results indicate that reductions in fertilisation and settlement success alone would reduce COTS population replenishment by over 50%. However, it is unlikely that this effect is sufficient to provide respite for corals from other negative anthropogenic impacts and direct stress from OA and warming on corals.

Highlights

Carbon dioxide (CO2) concentrations in the atmosphere have increased by 40% over the past 250 years and are likely to double by the end of this century [1]
Increased atmospheric CO2 leads to increased sea surface temperatures and a reduction in ocean pH, decreased carbonate and increased dissolved inorganic carbon (DIC) concentrations [2]
While some marine primary producers such as seagrasses, phytoplankton and macroalgae may benefit from increased DIC [3,4], many calcifying organisms exhibit reduced calcification due to lower saturation states of carbonate (e.g. [5,6,7])

Summary

Introduction

Carbon dioxide (CO2) concentrations in the atmosphere have increased by 40% over the past 250 years and are likely to double by the end of this century [1]. Increased atmospheric CO2 leads to increased sea surface temperatures and a reduction in ocean pH, decreased carbonate and increased dissolved inorganic carbon (DIC) concentrations [2]. While some marine primary producers such as seagrasses, phytoplankton and macroalgae may benefit from increased DIC [3,4], many calcifying organisms exhibit reduced calcification due to lower saturation states of carbonate Susceptibility of corals to increased pCO2 varies with species [9], it appears that many structurally complex corals will be lost, leading to a decline in habitat available to a variety of other species and changes to ecosystem structure and function [10]. The underlying causes of these outbreaks have long been debated [13]; higher larval survivorship caused by greater food (phytoplankton) availability, driven by agricultural land runoff, is currently the most widely accepted hypothesis to explain primary COTS outbreaks [14]

Methods

Results

Conclusion