Conditioning cultured <scp> <i>Diadema antillarum</i> </scp> through uncaged mid‐water grow‐out shifts behavior and morphology

Abstract Introduction The long‐spined black sea urchin Diadema antillarum is a keystone herbivore on Caribbean reefs, yet population recovery following mass mortality events has been slow and spatially variable. Restocking programs increasingly rely on aquaculture, but rearing conditions may generate behavioral and morphological shifts that affect post‐release survival. Objectives This study evaluated whether a short‐term uncaged mid‐water grow‐out treatment could condition aquaculture‐reared Diadema to express traits more similar to wild conspecifics. Methods Juveniles reared in an uncaged mid‐water treatment were compared with individuals maintained in standard aquaculture nurseries and with wild individuals. We assessed retention, relative growth rate, diurnal sheltering behavior, and spine characteristics. Results The proportion of Diadema remaining in the uncaged mid‐water treatment was low, with 19 of 64 Diadema present after 6 weeks. Remaining Diadema individuals were larger than aquaculture counterparts, although relative growth rates did not differ significantly among treatments. Mid‐water conditioned individuals exhibited diurnal sheltering behavior and spine thickness comparable to wild Diadema , whereas aquaculture reared individuals sheltered less during the day and had thinner spines. Conclusion Exposure to natural environmental conditions for 6 weeks during grow‐out partially restored wild‐like behavioral and morphological traits in aquaculture reared Diadema , indicating that conditioning can improve survival chances of restocked individuals.