Searching for external sources of the riboflavin stored in earthworm eleocytes

Riboflavin (vitamin B2) is essential to maintain immune potency in animals and plants. So far is accepted that animals cannot synthesise riboflavin; they rely on plant-sourced diets and intestinal bacteria for their supplies. A unique feature of earthworm ‘hepatocyte-like’ chloragocytes and chloragocyte-derived eleocytes floating in celomic cavity is the storage of riboflavin within intracellular granules. The hypothesis was that vegetarian food-deprivation or antibiotic/antifungal treatment inhibits riboflavin accumulation in eleocytes of Eisenia andrei. The 7-week starvation inhibited worm body weight gain and worm reproduction but had insignificant effects on celomocytes, both amoebocytes and eleocytes, and eleocyte riboflavin accumulation. The 1 week or 3 week antibiotic exposure had insignificant effects on worm coelomocytes and riboflavin content. Thus, a vegetarian diet and intestinal bacteria are not the exclusive or perhaps even the main sources of eleocyte riboflavin. The role of endosymbionts in earthworm flavonoid economy warrants targeted investigation. Moreover, the possibility of horizontal transfer of riboflavin biosynthesis genes from bacteria/fungi to earthworm genomes cannot be neglected.