Leaning left: how turn bias and habitat complexity shape spatial cognition in a coral-reef fish

Abstract In coral reef ecosystems, anthropogenic change is rapidly altering habitat structure, but the consequences for fish cognition remain poorly understood. We tested whether structural habitat complexity influences spatial learning and strategy use in the humbug damselfish ( Dascyllus aruanus ). Juveniles were reared in aquaria containing either high- or low-complexity coral structures and trained in a T-maze task, requiring the use of egocentric (turn-based) and/or allocentric (landmark-based) strategies to locate a reward. Performance was assessed using the number of trials to reach a learning criterion and latency to make correct decisions. Probe trials with conflicting cues assessed navigation strategies. Turn direction was the strongest predictor of performance. Fish trained to turn left reached criterion in fewer trials and consistently relied on egocentric strategies, indicative of a lateralisation bias. In contrast, right-turning fish increasingly relied on landmark cues during probe trials and made faster decisions when doing so, suggesting that allocentric strategies reduced the cost of acting against intrinsic turn biases. Contrary to our expectation, environmental complexity did not affect the number of trials required to reach criterion. However, high-complexity fish located the reward faster during early training and showed longer decision latencies during probe trials, suggesting greater sensitivity to landmark information when cues conflicted. Our results suggest that spatial task performance may be shaped more by turn biases and strategy use than by environmental complexity alone, highlighting the importance of considering intrinsic behavioural constraints when interpreting spatial cognition.