Developmental ecomorphology of the epibranchial organ of the silver carp, Hypophthalmichthys molitrix

Silver carp regularly consume and digest particles of food as small as 5 μm. This ability drives their efficient consumption of phytoplankton and because they feed low on the food chain they have an important place in aquaculture worldwide. In North America, where they are considered invasive, silver carp deplete food resources for native species and in so doing occupy increased niche space. Here, we determine the ontogenetic stage and size at which silver carp are morphologically capable of primarily feeding on particles <10 μm. Ecological studies on this species have shown that there is an ontogenetic shift in diet as predominantly zooplanktivorous juveniles later switch to eating much smaller phytoplankton. The occupation of this new trophic niche presents both a metabolic and a mechanical challenge to these fish, since it is unclear how they can efficiently feed on such small particles. We hypothesize that the epibranchial organ (EBO) in silver carp is essential in aggregating these small particles of food, allowing the species to consume mass quantities of tiny particles, thus mitigating metabolic constraints. In this study, we investigate early ontogeny of the EBO in silver carp to determine when this structure achieves the requisite morphology to become functional. We find that at around 80 mm standard length (SL) the EBOs are consistently filled with food, demonstrating that this accumulating organ has become functional. This size corresponds with previous ecological data documenting important shifts in the type of food consumed. While the basic bauplan of the EBO is established very early in ontogeny (by 15 mm SL), multiple waves of histological maturation of muscle, cartilage, gill rakers and epithelium ultimately form the functional structure.