Detection of a High-Density Brachiolaria-Stage Larval Population of Crown-of-Thorns Sea Star (Acanthaster planci) in Sekisei Lagoon (Okinawa, Japan)

Momoka Kojima,Takashi Nakamura,Satoshi Nagai,Sayaka Kai,Lawrence Bernardo,Kazuo Nadaoka,Takahiko Kameda,Kouki Fukuoka,Tatsuya Natori,Kohta Ikehara,Atushi Watanabe,Go Suzuki,Nina Yasuda,Shunsuke Kitazawa

doi:10.3390/d8020009

Momoka Kojima, Takashi Nakamura + Show 12 more

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https://doi.org/10.3390/d8020009

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Abstract

Outbreaks of the crown-of-thorns sea star (Acanthaster planci) are likely to be strongly associated with drastic changes in larval survival influenced by food availability. However, no quantitative or qualitative data are available on the distribution of A. planci larvae in the field nor on the environmental factors that influence their survivorship. Here we use a DNA barcoding approach to describe the distribution of A. planci larvae in Sekisei Lagoon, Ryukyu Archipelago, Japan after conducting three days of high-intensity sampling. High densities (53.3 individuals/m3) of A. planci larvae were found outside of Yonara Channel, which is the largest reef channel in this lagoon. Surprisingly, most (94%) of the aggregated larvae were advanced-stage brachiolaria. Considering that it takes several days to develop to this stage, this result demonstrates that A. planci larvae were floating for some time and maintaining a high-density population. However, this dense larval cloud disappeared immediately after a typhoon. No spatial correlation was found between larval density and either nutrient or chlorophyll a concentrations, suggesting that A. planci larvae do not necessarily aggregate in nutrient-rich water. These data suggest that some high-density populations of late developmental stage A. planci larvae were produced under a low phytoplankton concentration and could potentially trigger an adult outbreak. Consequently, our data suggest that adult outbreaks may not necessarily be triggered by food availability alone.

Highlights

Outbreaks of the crown-of-thorns sea star (COTS; Acanthaster planci), which feeds voraciously on corals, have threatened Indo-Pacific coral reefs for the last few decades [1]
A feeding experiment revealed that A. planci larvae starve at chlorophyll a (Chl-a) concentrations 2 μg/L [8]
The aim of this study was to understand basic A. planci larval ecology, including how A. planci larvae are distributed around a coral reef, whether A. planci larvae are found in nutrient-rich water, which may increase their survival, and whether a natural disturbance, such as a typhoon, affects larval population dynamics

Summary

Introduction

Outbreaks of the crown-of-thorns sea star (COTS; Acanthaster planci), which feeds voraciously on corals, have threatened Indo-Pacific coral reefs for the last few decades [1]. The A. planci pelagic larval stage is 2–6 weeks in duration [9,10,11], during which the larvae feed on phytoplankton [12,13]. The major cause for the dramatic fluctuations in larval survival is thought to be nutrients from terrestrial runoff [12,13,14]. This inflow of nutrients increases during heavy rainfall years with flood events and triggers the increased phytoplankton that forms a major component of the A. planci larval diet. Another study showed phenotypic plasticity of A. planci larvae depending on phytoplankton availability, which supports the larval survival hypothesis [16].

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