Marteilia refringens and Marteilia pararefringens sp. nov. are distinct parasites of bivalves and have different European distributions

Abstract

Marteilia refringens causes marteiliosis in oysters, mussels and other bivalve molluscs. This parasite previously comprised two species, M. refringens and Marteilia maurini, which were synonymized in 2007 and subsequently referred to as M. refringens ‘O-type’ and ‘M-type’. O-type has caused mass mortalities of the flat oyster Ostrea edulis. We used high throughput sequencing and histology to intensively screen flat oysters and mussels (Mytilus edulis) from the UK, Sweden and Norway for infection by both types and to generate multi-gene datasets to clarify their genetic distinctiveness. Mussels from the UK, Norway and Sweden were more frequently polymerase chain reaction (PCR)-positive for M-type (75/849) than oysters (11/542). We did not detect O-type in any northern European samples, and no histology-confirmed Marteilia-infected oysters were found in the UK, Norway and Sweden, even where co-habiting mussels were infected by the M-type. The two genetic lineages within ‘M. refringens’ are robustly distinguishable at species level. We therefore formally define them as separate species: M. refringens (previously O-type) and Marteilia pararefringens sp. nov. (M-type). We designed and tested new Marteilia-specific PCR primers amplifying from the 3’ end of the 18S rRNA gene through to the 5.8S gene, which specifically amplified the target region from both tissue and environmental samples.

Declining Populations of Mytilus spp. in North Atlantic Coastal Waters—A Swedish Perspective

“…Sporadic detection of Bonamia in flat oysters has been reported from Norway (OIE 2009) and from the Limfjord in Denmark (Madsen 2017, ICES 2018, but so far it has not been found in mussels; however, Martelia refringes known as an oyster parasite in Southern Europe has also caused decline and even extinction of local Mytilus beds in France. This species has thus far not been detected north of France, but a survey in the UK, Norway, and the Swedish west coast identified a closely related genotype, distinguished as Marteilia pararefringes (Kerr et al 2018), thus far only detected in mussels. Infection levels and prevalence of both M. refringes and M. pararefringes peak during summer.…”

High mortality of mussels in northern Brittany – Evaluation of the involvement of pathogens, pathological conditions and pollutants

“…The drastic decrease in prevalence observed in Brest between May and September could be explained by death of infected mussels. Histological examination does not allow discriminating between the congeneric species Marteilia refringens and M. pararefringens; although both species have been detected in mussels, M. pararefringens is more frequently reported (Kerr et al 2018).…”

The Importance of Marine Bivalves in Invasive Host–Parasite Introductions

“…It has been suggested that the parasite spread with hull-fouling mussels from the Mediterranean, but ultimately the north-westwards spread is not fully understood (CABI, 2020a datasheet 73758). (Carrasco et al, 2015;Kerr et al, 2018).…”