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

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.

Harmless sea snail parasite causes mass mortalities in numerous commercial scallop populations in the northern hemisphere

“…SAP causes severe histopathological changes in the Iceland scallop which was comprehensively described by Kristmundsson et al . 5 . The histopathology of M. kathae in the whelks is minor, even in those with extensive infections.…”

“…All Iceland scallops and whelks tested positive using a diagnostic PCR, initially developed for the SAP 5 . Furthermore, DNA sequencing showed that the SSU rDNA of Merocystis kathae and SAP was 100% identical.…”

“…Highly pathogenic mollusc-infecting apicomplexans are known, for example the one infecting scallops (herein referred to as scallop apicomplexan or SAP). SAP has been shown to be largely responsible for the total collapse of a population of Iceland scallop Chlamys islandica in Iceland 5 , and an unidentified apicomplexan negatively impacting oyster populations in New Zealand 6 . Furthermore, strong indications exist that SAP causes regular mass mortalities in a number of commercial scallop species inhabiting different geographic areas 5 , 7 , 8 .…”

Partial 18S rRNA sequences of apicomplexan parasite ‘X’ (APX), associated with flat oysters Ostrea chilensis in New Zealand

“…In addition, many lineages of apicomplexans are closely associated with marine invertebrates, including a number from commercially significant bivalve species from around the world (Morado et al 1984, Whyte et al 1994, Tuntiwaranuruk et al 2004, Aranda et al 2011, Uddin et al 2011, Kristmundsson et al 2015. This includes the unnamed and poorly understood APX found in NZ flat oysters that has been associated with mass mortality events in the largest commercial flat oyster fishery in NZ (Cranfield et al 2005).…”

“…For example, a mass mortality event of the Iceland scallop Chlamys islandica stock in Breidafjordur, west Iceland, is believed to have been caused by an apicomplexan, Aggregata sp. (Kristmundsson et al 2015). It was estimated that during 2000−2006, the main scallop population in Iceland decreased by 84%, despite a halt in commercial fishing from 2003.…”