Why do Eurasian otters eat so few invasive blue crabs?

Abstract Invasive species can become key trophic resources for native predators, and thus understanding predator-invasive prey interactions is relevant to assess invasion impacts. The Atlantic blue crab ( Callinectes sapidus ) has recently invaded coasts and lower stretches of rivers across the Western Palearctic, often establishing very abundant populations. The blue crab is an important prey for a large number of predators within its native range, but predation upon invasive populations is still virtually unknown. The Eurasian otter ( Lutra lutra ) is a flexible predator, known to incorporate invasive prey into its diet when they become available. We studied otter diet in a small estuary of southern Spain hosting a very abundant blue crab population. The role of the blue crab as otter prey was only anecdotal since, out of 223 spraints (otter faeces) collected between 2021 and 2023, its remains were found in a single one. The avoidance of the blue crab by otters, and also by other predators, seems to be a generalised phenomenon in the invaded range of this crustacean. Predation on invasive blue crabs may require a long-lasting learning process to overcome the crab’s anti-predatory structures and behaviours. Trophic interactions between invasive blue crabs and native predators deserves attention for three main aspects: (i) its implications for the functioning and structure of invaded food webs, (ii) its potential role in limiting the invasion, and (iii) the interest in the generation of new predatory traits among animal populations.