Does photosynthesis provoke formation of shell deformity in wild mytilid mussels infested with green microalgae Coccomyxa? – A conceptual model and research agenda

Endobiotic microalgae in molluscan life

“…Since the first description of a microalgae in C. cardissa (Kawaguti, 1941;Kawaguti, 1950), photosymbiosis has been identified in more 30 living species. At least most of these cases (approximately 27) are concentrated within a single family, Cardiidae (Vermeij, 2013), and 5 host species belong to Mytilidae (Zuykov et al, 2018b), indicating strong patterns of phylogenetic clumping. All tested representatives of three fragine genera (Lunulicardia, Fragum and Corculum) (Kirkendale et al, 2021), Tridacna and Hippopus are entirely photosymbiotic while sampled members of the other seven fragine genera and subgenera (Trigoniocardia, Americardia, Apiocardia, Ctenocardia, Papillicardium, Parvicardium and Microfragum) are not identified as photosymbiont hosts (Vermeij, 2013).…”

“…The shell is triangular in shape, with rounded edges and thin concentric growth lines without divergent ribs. Whereas the shell of C. parasitica-infected mussels is often highly eroded and the growth lines on the posterior margin are irregular (Zuykov et al, 2014;Zuykov et al, 2018b). A large collection of pearls is occasionally found in the mantle tissues.…”

First report of signs of infection by Coccomyxa‐like algae in wild blue mussels, Mytilus spp., in the Gulf of Maine (USA, Maine)

“…Among microbes that can be found associated with bivalve mollusks only pathogenic unicellular algae Coccomyxa spp. cause the formation of L‐shaped shell deformity (LSSD) on the posterior shell edge (Figures 1 and 2) in several species of mytilid mussels (Zuykov, Anderson, & Pelletier, 2018). Previous investigations of blue mussels, Mytilus edulis L., M. trossulus Gould and their hybrids, infected with Coccomyxa sp.…”

“…Previous investigations of blue mussels, Mytilus edulis L., M. trossulus Gould and their hybrids, infected with Coccomyxa sp. (GenBank accession number KJ372210) from the Estuary and the Gulf of St. Lawrence, Canada (Zuykov et al, 2014; Zuykov et al, 2018; Zhao et al, 2019; Zuykov et al, 2020) have demonstrated the following facts which are important to note for further discussion: (1) LSSD only occurs if the mussel is heavily infected with Coccomyxa algae. (2) LSSD degree (value of parameter “d” as defined on Figures 1c and 2b,c) does not depend on mussel age. (3) The core of LSSD is built with extra shell material. (4) Significant proportion of mussels with LSSD will show green spots (GS) in the mantle and adductor muscle. (5) In studied mussel beds, the number of individuals with LSSD usually exceeded several dozen animals per square meter. …”

“…To date, there are no monitoring programs targeting pathogenic Coccomyxa alga in bivalves. Circa fifteen papers reported that infected mussels are known from a few sites in coastal waters of Spain, Norway, Denmark, Russia, Falkland Islands and Canada (Mortensen, Harkestad, Stene, & Renault, 2005; Zuykov et al, 2018). Nevertheless, LSSD, green spots and the presence of pearls in infected tissues make infected mussels non‐attractive for harvesting.…”