Contents lists available at ScienceDirect Journal of Invertebrate Pathology journal homepage: www.elsevier.com/locate/jip Shell deformity as a marker for retrospective detection of a pathogenic unicellular alga, Coccomyxa sp., in mytilid mussels: A first case study and research agenda Michael Zuykov a, ⁎ , Galina Kolyuchkina b , Philippe Archambault c , Michel Gosselin d , Julia Anderson a , Christopher W. McKindsey e , Graeme Spiers a , Michael Schindler f a Harquail School of Earth Sciences, Laurentian University, Sudbury, ON, Canada b Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia c Département de biologie, Université Laval, Québec, QC, Canada d Institut des sciences de la mer de Rimouski, Université du Québec à Rimouski, Rimouski, QC, Canada e Fisheries and Oceans Canada, Maurice-Lamontagne Institute, Mont-Joli, QC, Canada f Department of Geological Sciences, University of Manitoba, Winnipeg, MB, Canada ARTICLE INFO Keywords: Shell deformity Biomarker Blue mussels Mytilus edulis Coccomyxa sp. Pathogen ABSTRACT An L-shaped shell deformity (LSSD) on the posterior shell edge is known exclusively in wild mytilid mussels infected with photosynthetic Coccomyxa-like algae. LSSD forms due to the appearance of extra shell material; it only occurs if the mussel is heavily infected with the alga. Traditionally, observation of high amount of the green spots (algal colonies) on a large area of host soft tissues (most of the mantle and in adductor muscle) has been used to indicate a high infection rate. We examined 300 Mytilus spp. (100 small, 20–30 mm; 200 large, 40–60 mm) with a high degree of LSSD (parameter “d” > 5 mm) from the Lower St. Lawrence Estuary (Québec, Canada). Green spots were absent in two large mussels, and were only present along the mantle posterior edge in 14 large mussels; other individuals had high infection levels. Our observations suggest that some individuals could be in a state of remission, or, even more optimistically - mussels may be able to resist the pathogen. LSSD is the stable through-time marker for detection of mytilid mussels that are or were infected with Coccomyxa algae, and, thus, may provide information for the study of mussel immunity and control of alga distribution/migration in coastal waters worldwide. Unicellular green algae Coccomyxa parasitica Stevenson & South, and related taxa described in open nomenclature as Coccomyxa spp., are pathogenic microorganisms that invade seven species (and a hybrid) of wild marine bivalves. The hosts are mostly mytilid mussels and infec- tion has been reported in shallow coastal waters of several sites around the world (Zuykov et al., 2018a), including the Estuary and the Gulf of St. Lawrence (Fig. 1). Two diagnostic signs of infection with Coccomyxa are the focus of the research reported here. First, the infection causes major changes in mussel soft body parts (Sokolnikova et al., 2016) in- cluding the presence of green spots (GS), a dense distribution of algal colonies, on the surface and within soft tissues. The most colonies occur on the inner and outer mantle surface (Fig. 2H,J,L); although the outer surface is always characterized by slightly more GS than the inner surface, here we consider mantle as an entire tissue. The infection level classification has been based on the spatial distribution and abundance of GS within host soft tissues (Gray et al., 1999; Mortensen et al., 2005). Second, the posterior shell edge (PSE) (built with a layer of calcite prisms) exhibits L-shaped shell deformity (LSSD) (Fig. 2E,F,I,K) that appears on different sizes of animals due to the deposition of extra shell material (Fig. 2G). Thickened posterior shell margin, in comparison with a sharp margin in uninfected mussels (Fig. 2C), initiated the proposal of a new shell measurement parameter, called “d” Zuykov et al. (2014) (Fig. 2E,I,K). Zuykov et al. (2014) classified LSSD degree in 4 groups on the basis of the value of the parameter “d” (from gr. I – 0 mm to gr. IV – > 6 mm); the LSSD degree differs between individuals of the same ontogenetic stage. As we proposed recently, extra shell material precipitation occurs due to photosynthesis of a great number of Coccomyxa cells via light-enhanced calcification (Zuykov et al., 2018a; Zhao et al., 2019). It is important to note, however, that the correlation between LSSD appearance (start/end of its formation) and infection rate remains problematic. In the present study we compared the infection level in mytilid https://doi.org/10.1016/j.jip.2019.107311 Received 21 August 2019; Received in revised form 11 December 2019; Accepted 14 December 2019 ⁎ Corresponding author. E-mail address: michael.zuykov@yahoo.com (M. Zuykov). Journal of Invertebrate Pathology 169 (2020) 107311 Available online 16 December 2019 0022-2011/ © 2019 Elsevier Inc. All rights reserved. T