Effects of prey density and flow speed on plankton feeding by garden eels: A flume study

Feeding by zooplanktivorous fish depends on their foraging movements and the flux of prey to which they are exposed. While prey flux is a linear function of zooplankton density and flow speed, those two factors are expected to contribute differently to fish movements. Our objective was to determine the effects of these factors for garden eels, stationary fish that feed while anchored to the sandy bottom by keeping the posterior parts of their bodies inside a burrow. Using a custom-made flume with a sandy bottom, we quantified the effects of prey density and flow speed on feeding rates by spotted garden eels (Heteroconger hassi). Feeding rates increased linearly with prey density. However, feeding rates did not show a linear relationship with flow speed and decreased at 0.25 m s⁻¹. Using label-free tracking of body points and 3D movement analysis, we found that the reduction in feeding rates was related to modulation of the eel's movements, whereby the expected increase in energy expenditure was avoided by reducing exposure and drag. No effects of flow speed on strike speed, reactive distance or vectorial dynamic body acceleration (VeDBA) were found. A foraging model based on the body length extended from the burrow showed correspondence with observations. These findings suggest that as a result of their unique foraging mode, garden eels can occupy self-made burrows in exposed shelter-free sandy bottoms where they can effectively feed on drifting zooplankton.

Summary: Analysis of feeding by anchored garden eels in a flume shows their unique strategies, modulating dependence on burrows and foraging movements, to enable effective feeding at a wide range of flow speeds.

Life in the flow: unique adaptations for feeding on drifting zooplankton in garden eels

A major challenge faced by sessile animals that feed in the flow is to maintain effective feeding postures while enduring hydrodynamic forces. Garden eels exhibit an exceptional lifestyle: feeding on drifting zooplankton while being 'anchored' in a burrow they dig in the sand. Using underwater observations, sampling and three-dimensional video recording, we measured the feeding rates and characterized feeding postures of garden eels under a wide range of current speeds. We show that the eels behaviorally resolve the trade-off between adverse biomechanical forces and beneficial fluxes of food by modulating their body postures according to current speeds. In doing so, the eels substantially reduce drag forces when currents are strong, yet keep their head well above bottom in order to effectively feed under conditions of high prey fluxes. These abilities have allowed garden eels to become one of the rare oceanic fishes that live in sandy, predation-rich habitats and feed on zooplankton while being attached to the bottom.