Investigation of vitamin B12 concentrations and tissue distributions in larval and adult Pacific oysters and related bivalves

Vitamin B12 (B12) is an essential micronutrient for all animals, but is not present in plants and is produced de novo only by bacteria or archaea. Accordingly, humans must derive required B12 from eating animal products or vitamin supplements, as deficiencies can lead to severe health issues including neuropathy. An often overlooked source in the human diet of B12 is shellfish, in particular bivalves, which have significantly higher levels of B12 than other animal sources, including all vertebrate meats. Origins and key metabolic processes involving B12 in bivalves remain largely unknown, despite the exceptionally high levels. In this study, we examined in several Australian bivalve species, hypotheses concerning B12 utilisation and uptake through diet or microorganism symbiosis. Vitamin B12 is not distributed evenly across different tissues types of the Pacific oyster, the commercial scallop and Goolwa cockle (pipi), with higher accumulation in the oyster adductor muscle and gill, and mantle and syphons of the Goolwa cockle. Oyster larvae before first feeding already contained high amount of B12; however, a significant decrease in B12 concentration post metamorphosis indicates a higher utilisation of B12 during this life event. We demonstrated that microalgal feed can be supplemented with B12, resulting in an enriched feed, but this did not result in an increase in larval B12 concentrations when oyster larvae were fed with this diet relative to controls, thus supporting the theory that a B12 producing microbiome within bivalves was the potential source of B12 rather than feed. However, B12 concentrations in the digestive tract of adult oysters were low compared to other tissue types, which might challenge this theory, at least in adults. Our findings provide insight into B12 uptake and function in bivalve species, which will aid the promotion of bivalves as suitable B12 source for humans as well as provide crucial information to the aquaculture industry in relation to optimisation of vitamin supplementation in bivalve hatchery production.