In cooperatively breeding species, non-breeding individuals provide alloparental care and help in territory maintenance and defence. Antipredator behaviours of subordinates can enhance offspring survival, which may provide direct and indirect fitness benefits to all group members. Helping abilities and involved costs and benefits, risks and outside options (e.g. breeding independently) usually diverge between group members, which calls for status-specific differentiated behavioural responses. Such role differentiation within groups may generate task-specific division of labour, as exemplified by eusocial animals. In vertebrates, little is known about such task differentiation among group members. We show how breeders and helpers of the cooperatively breeding cichlid Neolamprologus savoryi partition predator defence depending on intruder type and the presence of dependent young. In the field, we experimentally simulated intrusions by different fish species posing a risk either specifically to eggs, young, or adults. We used intrusions by harmless algae-eaters as a control. Breeders defended most when dependent young were present, while helper investment hinged mainly on their body size and on the potential threat posed by the respective intruders. Breeders and helpers partitioned defence tasks primarily when dependent young were exposed to immediate risk, with breeders investing most in antipredator defence, while helpers increased guarding and care in the breeding chamber. Breeders’ defence likely benefits helpers as well, as it was especially enhanced in the treatment where helpers were also at risk. These findings illustrate that in a highly social fish different group members exhibit fine-tuned behavioural responses in dependence of ecological and reproductive parameter variation.