Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aquaculture Mytilus edulis and Styela clava assimilate picophytoplankton carbon through feces and pseudofeces ingestion R. Sonier a, ⁎ , L.A. Comeau a , R. Tremblay b , F. Olivier c,e , T. Meziane c , B. Genard b,d a Fisheries and Oceans Canada, Gulf Fisheries Centre, P.O. Box 5030, Science Branch, Moncton, NB E1C 9B6, Canada b Institut des Sciences de la Mer, Université du Québec à Rimouski, 310 allée des Ursulines, C.P. 3300, Rimouski, QC G5M 1L7, Canada c Unité Mixte de Recherche ‘Biologie des organismes et écosystèmes aquatiques’ (BOREA, UMR 7208), Sorbonne Universités, Muséum national d'Histoire naturelle, Université Pierre et Marie Curie, Université de Caen Basse-Normandie, Université des Antilles, Centre National de la Recherche Scientifique, Institut de Recherche pour le Développement-207, CP53, 61 rue Buffon, 75005 Paris, France d Les laboratories Iso-BioKem Inc., 367, rue Gratien-Gélinas, Rimouski, QC G5M 0V1, Canada e MNHN, Station Marine de Concarneau, place de la croix Place de la Croix, BP 225, 29182 Concarneau cedex, France ARTICLE INFO Keywords: Picophytoplankton Isotopic labelling Aquaculture Feces Carbon cycling Blue mussel Tunicate ABSTRACT Picophytoplankton (PPP) may contribute to the bulk of primary production in nutrient-rich marine ecosystems. In this study, we quantifed the capacity of blue mussels (Mytilus edulis) and clubbed tunicates (Styela clava) to exploit PPP. In the water, we introduced PPP cells that were isotopically labelled with 13 C and using the same enrichment approach, we measured the secondary transfer (recycling) of carbon via the assimilation of pseu- dofeces and feces. Results show that both M. edulis and S. clava assimilate PPP carbon by ingesting PPP cells directly from water. In addition, PPP assimilation rates were similar in both species. Most interesting was our observation that both species assimilated PPP carbon contained in pseudofeces and feces, including large fecal pellets produced by S. clava. We conclude that within cultured and invasive flter feeders, PPP carbon is recycled via ingestion of feces and pseudofeces among individuals growing in close proximity. In aquaculture settings, cultured bivalves and fouling tunicates may provide an ecological service by removing excess PPP in nutrient- rich systems via direct and secondary ingestion. 1. Introduction Solitary ascidians, such as the clubbed tunicate Styela clava, are a nuisance species and may cause considerable economic losses to many blue mussel (Mytilus edulis) aquaculture operations in Atlantic Canada (Carver et al., 2003; Ramsay et al., 2014). Tunicates reproduce sea- sonally (Bourque et al., 2007), producing free-swimming larvae that settle and grow rapidly on mussel grow-out structures, including mussel socks, lines, and buoys (Boothroyd et al., 2002). Solitary ascidians, such as S. clava, are efcient flter feeders that can deplete suspended par- ticles or even control phytoplankton communities in the water column (Thompson and MacNair, 2004). Numerical modelling suggests that solitary ascidians in longline mussel farms can increase particle deple- tion by 30%–47% relative to non-infested farms (Comeau et al., 2015). Unlike mussels, which capture food particles with a ciliated gill system, ascidians employ a mucus net to capture food particles. This mucus net is produced in the endostyle (Goodbody, 1974; Riisgård and Larsen, 2000) and is elongated and rectangular (mesh like), consisting of transverse and longitudinal flaments (Flood and Fiala-Medioni, 1981). Laboratory-based retention experiments using artifcial particles sug- gest that the mucus net of S. clava is 70% efcient in retaining 1 μm particles (Randløv and Riisgård, 1979). However, for natural particles ranging between 0.2 and 3.0 μm, the feeding efciency of solitary as- cidians has been reported as being lower (59% ± 2%) (Comeau et al., 2015). In bivalves, retention efciency (RE) of small particles is much lower than RE reported for ascidians. For instance, Sonier et al. (2016) reported an RE of 20% ± 2% for cultivated M. edulis feeding on 0.2–2.0 μm picophytoplankton (PPP). After PPP are captured on gills of mussels or mucus nets of ascidians, the proportion of PPP assimilated by ingestion and digestion processes are unknown. In mussels, any captured PPP can be rejected in pseudofeces following sorting with labial palps. In addition, assuming that the energy (or carbon) from PPP ingestion is not totally assimilated by M. edulis, it is conceivable that PPP carbon in feces can be transferred and further utilized. Excreted organic matter represents a potential food source, but as- similation rates by flter feeders has been poorly quantifed and https://doi.org/10.1016/j.aquaculture.2020.735868 Received 21 February 2020; Received in revised form 18 August 2020; Accepted 20 August 2020 ⁎ Corresponding author. E-mail addresses: Remi.Sonier@dfo-mpo.gc.ca (R. Sonier), Luc.Comeau@dfo-mpo.gc.ca (L.A. Comeau), Rejean_Tremblay@uqar.ca (R. Tremblay), Frederic.Olivier@mnhn.fr (F. Olivier), meziane@mnhn.fr (T. Meziane), Bertrand_Genard@uqar.ca (B. Genard). Aquaculture 531 (2021) 735868 Available online 25 August 2020 0044-8486/ © 2020 Elsevier B.V. All rights reserved. T