The cost of avoiding predators: A bioenergetic analysis of diel vertical migration by the opossum shrimp Mysis diluviana

The freshwater opossum shrimp Mysis diluviana can undergo extensive diel vertical migration (DVM) to feed in shallow, prey rich strata at night. Bright moonlight limits their night-time migration presumably due to predator avoidance. Using a linked, foraging-bioenergetics model, we evaluated the cost of avoiding predators by simulating the effects of prey density, water temperature, and light intensity on daily feeding and growth of M. diluviana in Lake Pend Oreille, Idaho, USA. We found that when mysid distribution was not limited by moonlight intensity, simulated food consumption (10.3 J day⁻¹) increased 1.6-fold compared to estimated consumption (6.1 J day⁻¹) based on their observed, vertical distribution. Moreover, simulated growth of mysids (0.61 mg day⁻¹) increased 74% compared to that estimated from observed distribution patterns (0.35 mg day⁻¹), when they were located in deeper, darker strata. Given recent insights into partial DVM by M. diluviana, we note that proximate factors associated with predator avoidance in pelagic (light availability) and benthic (hunger level, body size and reproductive status) habitats may convey complimentary benefits to M. diluviana fitness by reducing predation mortality and increasing metabolic efficiency.