Heads and tails of individual differences: the role of environmental complexity in cognitive development of juvenile lobsters.

Experiencing environmental complexity during early life is fundamental for vertebrate neural development, with profound and often irreversible effects on later behavioural and cognitive outcomes. However, studies on the early stages of invertebrate development are scanty. European lobsters (Homarus gammarus) are routinely reared in captivity for subsequent release in the context of restocking and stock-enhancement. The impoverished captive conditions they experience may exert long-term effects on their development, ultimately impairing their success after release. Here we tested the effects of environmental complexity during early life on learning (N = 38) and personality (N = 132) of juvenile lobsters. Lobsters experienced a full enrichment (with both substrate and shelter), a partial enrichment (either substrate or shelter), or bare standard conditions as a control. We assessed lobsters' exploration and activity patterns via repeated open field tests, quantified their learning in a double T-maze, and investigated the possible presence of a speed-accuracy/flexibility trade-off in decision-making. We found that juvenile lobsters raised in enriched environments were more active compared to lobsters raised in control conditions. They were also faster in taking decisions and in correcting erroneous ones compared to controls, while decision speed did not predict the accuracy of its outcome. Finally, while all lobsters reached the learning criterion, the effects of environmental treatment on learning speed were only detectable in individuals that developed a wild-type morphological phenotype (i.e. asymmetrical claws), who showed a higher information-seeking tendency and required more trials. Overall, these results indicate a key role of early-life experience of environmental complexity in the development of behavioural and cognitive traits that can drive success in the wild.