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

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.

Microplastic Contamination Has Limited Effects on Coral Fertilisation and Larvae

“…The limited effect of microplastics on embryo development and larval settlement are consistent with other studies that have shown some tolerance of these early life stages of corals to other types of physical stressors (e.g., sediment and coal [52,73,74]). The effects on coral fertilisation by natural particulates is due to interactions with sperm that results in sperm sinking, sperm limitation at the surface near the buoyant eggs and ultimately reduced fertilisation rates [54].…”

“…However, since there was a significant difference in abnormalities between control and blank treatments, we cannot decipher whether the observed abnormalities were caused by the presence of plastic particles or mechanical agitation (i.e., rolling). The coral larvae exposed to microplastics were ciliated and mobile and these characteristics, along with an ability to produce mucous, has been shown to protect larvae from exposures to very high concentrations of sediments [73]. Similar mechanisms are likely to have protected the swimming larvae in these tests to all microplastic types.…”

Immobilisation of living coral embryos and larvae

“…This protective mechanism is like that described for Acropora spp. embryos, which form protective cocoons in response to suspended sediments 11 . We suggest that the optimal timing of coral encapsulation is, therefore, after embryos are ciliated, but prior to decline in buoyancy, for ease of collection (Fig.…”

Cumulative effects of suspended sediments, organic nutrients and temperature stress on early life history stages of the coral Acropora tenuis

“…However, larval settlement success was significantly reduced when embryos were exposed to suspended sediments, revealing the latent effects that treatments can have on later developmental processes. A possible mechanism by which suspended sediments act on coral embryo development has recently been described35, whereby sediments cause the production of mucus by early embryos (~6-24-h old) of A. tenuis and A. millepora ; however, that study did not detect subsequent effects on larval survivorship or settlement35. A difference between the two studies was that carbonate sediments were used in ref.…”