Is an Apicomplexan Parasite Responsible for the Collapse of the Iceland Scallop (Chlamys islandica) Stock?

Árni Kristmundsson,Ásthildur Erlingsdóttir,Mark A Freeman,Linsheng Song

doi:10.1371/journal.pone.0144685

Árni Kristmundsson, Ásthildur Erlingsdóttir + Show 2 more

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https://doi.org/10.1371/journal.pone.0144685

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Abstract

Due to the total and unexpected collapse of the Iceland scallop, Chlamys islandica, stocks around Iceland during the 2000s, a commercial fishing ban has been imposed on this valuable resource since 2003. Following the initial identification of an apicomplexan parasite in the scallops, a long-term surveillance program was established to evaluate the effect of the parasite on the population. The infections were highly prevalent in all shell sizes throughout the study. However, the parasite only impacts mature scallops where they cause severe macroscopic changes, characterized by an extensively diminished and abnormally coloured adductor muscle. A highly significant relationship was observed between infection intensity and gonad and adductor muscle indices. The first four years of the study, were characterized by high infection intensity and very poor condition of the adductor muscle and gonads, whilst during subsequent years, infections gradually decreased and the condition of the scallops improved. Histopathological changes were restricted to the presence of apicomplexan zoites which were widely distributed, causing varying degrees of pathology in all organs. In heavy infections, muscular and connective tissues were totally necrotized, destroying significant parts of numerous organs, especially the adductor muscle, digestive gland and gonads. The progression of the disease was in good synchrony with the mortality rates and the subsequent decline observed in the scallop stock and recruitment indices. Our findings strongly suggest that the apicomplexan parasite played a major role in the collapse of the Iceland scallop stock in Breidafjordur. In addition to causing mortality, the infections significantly impact gonad development which contributes further to the collapse of the stock in the form of lower larval recruitment. Furthermore, compelling evidence exists that this apicomplexan pathogen is causing serious disease outbreaks in other scallop populations. Similar abnormal adductor muscles and the parasite itself have been identified or observed in association with other mass mortality events in several different scallop species and commercial stocks in the northern hemisphere.

Highlights

Iceland scallop, Chlamys islandica (Muller, 1776) (Mollusca: Pectinidae) is a cold water bivalve species inhabiting the boreal-subarctic transition zone
The almost complete collapse in the population of the Iceland scallop, Chlamys islandica, experienced in Breidafjordur in West Iceland became apparent in the years 1999–2000 [3,20,21] and in 2005–2006 the stock index was only at 16% of its average size from previous years 1993–2000 (MRI, 2007)
The observed progression of the apicomplexan infection is in good synchrony with the mortality rates and the subsequent decline observed in the scallop stock and recruitment indices

Summary

Introduction

Chlamys islandica (Muller, 1776) (Mollusca: Pectinidae) is a cold water bivalve species inhabiting the boreal-subarctic transition zone. In the NE Atlantic it is found around Iceland, in the Barents Sea and from the northern part of Norway, south to Bergen and the Stavanger Fjord. In the NW Atlantic, it is found in West Greenland, from Thule in the north to Cape Farewell in the south and along the eastern coast of Canada, from Cumberland Peninsula, Hudson Bay and south to Massachusetts [1]. Commercial fisheries of Iceland scallop have mainly been in Icelandic waters, and on a smaller scale off West Greenland, in the Barents Sea and off the Atlantic coast of North America [1]. The fisheries in Iceland date back to 1969 with the main fishing grounds in Breidafjordur West Iceland, constituting 60–100% of the total catch, with the scallop industry forming a vital part of the economy in that region [2]. From 1980 to 2000, the annual catch ranged from 8,500–17,000 tn, reaching a peak in the years 1983–1986 followed by a stable 8,000–9,000 tn catch from 1990–2000 [3]

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