Metal pollution as a potential threat to shell strength and survival in marine bivalves

Bryce D Stewart,Stuart R Jenkins,Charlotte Boig,Catherine Sinfield,Kevin Kennington,Andrew R Brand,William Lart,Roland Kröger

doi:10.1016/j.scitotenv.2020.143019

Abstract

Marine bivalve molluscs, such as scallops, mussels and oysters, are crucial components of coastal ecosystems, providing a range of ecosystem services, including a quarter of the world's seafood. Unfortunately, coastal marine areas often suffer from high levels of metals due to dumping and disturbance of contaminated material. We established that increased levels of metal pollution (zinc, copper and lead) in sediments near the Isle of Man, resulting from historical mining, strongly correlated with significant weakening of shell strength in king scallops, Pecten maximus. This weakness increased mortality during fishing and left individuals more exposed to predation. Comparative structural analysis revealed that shells from the contaminated area were thinner and exhibited a pronounced mineralisation disruption parallel to the shell surface within the foliated region of both the top and bottom valves. Our data suggest that these disruptions caused reduced fracture strength and hence increased mortality, even at subcritical contamination levels with respect to current international standards. This hitherto unreported effect is important since such non-apical responses rarely feed into environmental quality assessments, despite potentially significant implications for the survival of organisms exposed to contaminants. Hence our findings highlight the impact of metal pollution on shell mineralisation in bivalves and urge a reappraisal of currently accepted critical contamination levels.

Highlights

Marine bivalve molluscs deliver a range of ecosystem services such as water filtration and sediment consolidation and provide a quarter of the world's seafood (Daily, 2003; FAO, 2018; Grabowski et al, 2012)
For this purpose we conducted a systematic study of the lethal damage of scallops caught at two opposite sites near the Isle of Man, namely Bradda and Laxey, measured the level of metal contamination in the sediments at these sites, determined the mechanical fracture strength of the shells for both sites, as well as a number of additional control sites, and analysed the shell structure for scallop shells from Bradda and Laxey
While scallops caught at Bradda Inshore and Offshore experienced lethal damage levels of 2–7%, those caught at Laxey revealed consistently higher mortality rates between 8.5 and 11%

Summary

Introduction

Marine bivalve molluscs deliver a range of ecosystem services such as water filtration and sediment consolidation and provide a quarter of the world's seafood (Daily, 2003; FAO, 2018; Grabowski et al, 2012). The thickness and microstructure of shells is highly variable between both individuals and areas, which appears to be predominately driven by environmental conditions, as it is not explained by genetic differences (De Noia et al, 2020; Telesca et al, 2019; Vendrami et al, 2017). This variability can affect the mechanical strength of shells, and an organism's survival (Grefsrud et al, 2008; Grefsrud and Strand, 2006; Vendrami et al, 2017). Increased storm events due to climate change will increase runoff and further elevate introduction of metals into marine environments (EEA, 2015), even from long disused mining areas

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