Synergistic effects of conspecifics and food on growth and energy allocation of a damselfish

Grouping organisms may suffer reduced growth due to food deprivation in the presence of larger conspecifics, but this cost can be outweighed by advantages of reduced predation risk in larger groups. Here, tagged new juveniles of a small, grouping damselfish Dascyllus aruanus, were added to small coral heads that were either empty or supported small groups of larger conspecifics. Half of the coral heads with conspecifics and half of those without were subjected daily to additions of brine shrimp, similar to natural zoo-plankton food, over a three-week period. Growth was measured as increase in body length, change in body condition (lipid content), and overall energy gain. Body lipid reserves were higher (21%) in recruits subjected to added food and in the presence of conspecifics (13%). Total energy was higher (53%) in recruits with added food but was not affected by conspecific presence. There was an interaction between conspecific presence and food supplementation treatment on change in body length, a commonly used growth estimator. These differences suggest that the benefits of extra food and effects of conspecific presence differ depending on the metric used to measure growth, but that presence of larger conspecifics enhanced the increased growth that resulted from higher food intake.