Unveiling predator-specific mobbing in marine invertebrates

Mobbing is a complex and well-known antipredator strategy where prey attack their predators, but it is almost exclusively described in birds and mammals. This may be a result of taxonomic bias, limiting the investigation into this type of ‘complex’ antipredator behavior in many taxonomic groups. Invertebrates are generally considered to employ simpler predator avoidance strategies; the only examples of mobbing or predator approach behavior among invertebrates are from stinging or biting insects (often eusocial, so benefit from colony-level defense strategies). Here we describe novel mobbing behavior in the common marine shrimp Palaemon pugio , which repeatedly approached and attacked crab and fish predators. The shrimp not only effectively drove away fish predators and avoided predation from fish and crabs with mobbing behavior, but they did so based on predator identity and threat; shrimp were more likely to mob crabs but more likely to aggressively mob fish, which showed a greater avoidant response to mobbing than crabs. Our results suggest that some invertebrates exhibit complex behavioral strategies like mobbing that we mistakenly assumed were limited to vertebrates. • Grass shrimp ( Palaemon pugio ) show mobbing, a novel aquatic invertebrate behavior. • Two mobbing styles observed: cautious saltatory and fast ‘buzzing’ approaches. • Shrimp mob blue crabs more than fish or non-predatory mud crabs. • Mobbing has low predation risk (∼0.5%) for shrimp. • Findings suggest complex antipredator behavior occurs in invertebrates.