Mucous Secretion and Cilia Beating Defend Developing Coral Larvae from Suspended Sediments.

Gerard F Ricardo,Ross J Jones,Peta L Clode,Andrew P Negri

doi:10.1371/journal.pone.0162743

Abstract

Suspended sediments produced from dredging activities, or added to the sediment budget via river runoff, are a concern for marine resource managers. Understanding the impact of suspended sediments on critical life history stages of keystone species like corals is fundamental to effective management of coastlines and reefs. Coral embryos (Acropora tenuis and A. millepora) and larvae (A. tenuis, A. millepora and Pocillopora acuta) were subjected to a range of suspended sediment concentrations of different sediment types (siliciclastic and carbonate) to assess concentration-response relationships on ecologically relevant endpoints, including survivorship and ability to metamorphose. Embryos were subjected to short (12 h) suspended sediment exposures from ages of 3–12 hours old or a long (30 h) exposure at 6 hours old. Neither the survivorship nor metamorphosis function of embryos were significantly affected by realistic sediment exposures to ~1000 mg L-1. However, some embryos exhibited a previously undescribed response to dynamically suspended sediments, which saw 10% of the embryos form negatively buoyant cocoons at siliciclastic suspended sediment concentrations ≥35 mg L-1. Scanning electron and optical microscopy confirmed the presence of a coating on these embryos, possibly mucus with incorporated sediment particles. Cocoon formation was common in embryos but not in larvae, and occurred more often after exposure to siliciclastic rather than carbonate sediments. Once transferred into sediment-free seawater, functional ~36-h-old embryos began emerging from the cocoons, coinciding with cilia development. Ciliated (> 36-h-old) larvae exposed to suspended sediments for 60 h were also observed to secrete mucus and were similarly unaffected by suspended sediment concentrations to ~800 mg L-1. This study provides evidence that mucous secretion and cilia beating effectively protect coral embryos and larvae from suspended sediment and that these mechanisms may enhance their chances of successful recruitment.

Highlights

Coral reefs provide a range of benefits to coastal communities through tourism, fishing and coastal protection, and have been collectively valued at US $9.9 trillion/yr. globally [1]
As part of an experimental sequence to investigate the effects of sediments on the early lifehistory stages of corals and understand the risk associated with turbidity-generating events during coral spawning periods [9, 11, 17], we examined the survivorship and metamorphosis response of embryo and larvae of several tropical coral species after exposure to different sediment types and concentrations
The 3-h old embryos exposed for 12-h to ~800 mg L-1 suspended sediment concentrations (SSCs) fragmented upon agitation and so it was not possible to quantify survivorship

Summary

Introduction

Coral reefs provide a range of benefits to coastal communities through tourism, fishing and coastal protection, and have been collectively valued at US $9.9 trillion/yr. globally [1]. Inshore reefs are frequently exposed to suspended sediment concentrations (SSCs) < 5 mg L-1, but subject to spikes of >100 mg L-1 usually associated with cyclonic activity [15, 16]. These particles have the potential to affect both existing populations of key reef-building taxa, as well as reproduction processes and recruitment of new individuals to these populations[17, 18]. The impact of sediment on the planktonic stage remains poorly explored compared with the fertilisation [9,10,11, 20, 21] and settlement stages, both which often show susceptibility to low sediment levels [22, 23]

Results

Discussion

Conclusion