Microalgal Community in Fecal Pellets of the Freshwater Pearl Mussel Margaritifera laevis (Bivalvia: Margaritiferidae) in a Drainage Ditch Connected to the Hida River, Gifu Prefecture, Central Honshu, Japan

Abstract

Microalgae in fecal pellets of the freshwater pearl mussel Margaritifera laevis were identified to the lowest possible taxon and measured to identify the characteristics of microalgae ingested by the mussels. As a case study, fecal pellets were obtained from adult and juvenile mussels collected from a drainage ditch connected to the Hida River, Gifu Prefecture, central Honshu, Japan in June and October 2015. Ditch water was also collected in October, and microalgae in these samples were analyzed. We identified five microalgal phyla in mussel fecal pellets: Cyanobacteria, Euglenozoa, Ochrophyta, Chlorophyta, and Charophyta. Cells and colonies of Ochrophyta (Bacillariophyceae) accounted for 98–99% of microalgae in the fecal pellets. Only 5–6% of Ochrophyta cells and colonies in fecal pellets had chloroplasts compared to 14–16% in ditch water. These results indicate that Ochrophyta is the major phylum of microalgae ingested by adult and juvenile mussels in the sampling site and that a portion of those cells and colonies were likely assimilated by the mussels. In contrast, 75% or more of the Chlorophyta cells and colonies in fecal pellets had chloroplasts, and thus the Chlorophyta were considered to not be actively assimilated by mussels. Microalgae in mussel fecal pellets ranged from 5 to 690 μm, the suggestion being that organic matter within this size range is potential food for the mussel. The characteristics of microalgae in fecal pellets of both adult and juvenile mussels were the same. The knowledge newly obtained in this study will contribute to clarifying food items of M. laevis in the field.