Evaluating the species-specific effects of multiple predators on shared prey helps to identify the mechanisms regulating predator impacts. Here, we investigate the trophic impact of two predators of the Chilean rocky subtidal, the fish Graus nigra [F] and the octopus Robsonella fontaniana [O] on their shared prey, the porcelain crab, Petrolisthes violaceus. Manipulative laboratory experiments were performed to evaluate mortality and behavior of porcelain crab in different treatments, including prey alone; prey with single predator ([O] or [F]); and prey with combined predators ([F + F], [O + O], and [O + F]). Different prey densities (N = 3, 9, and 12 individuals) were used to explore density-dependent predation mortality for single and combined predators using an orthogonal (crossed) design that allows to tease apart the effects of predator taxonomic identity from prey density. Behavioral responses of interacting species were registered every 4 h for 2 consecutive days, and were used to identify predator foraging strategies and prey refuge use. Mortality values (proportional prey mortality and predator per capita consumption) were compared among treatments. Results indicated differences in mortality among treatments. The cause of increase of prey proportional mortality was due to an increase in prey availability outside the refuge in fish treatments at high initial prey densities, while it was density independent in octopus’ treatments. Changes in predator per capita consumption were registered depending on the predator combination and the initial prey density. These patterns may reflect contrasting predator foraging strategies and changes in refuge use by porcelain crabs, suggesting effects on prey mortality and predator impacts.

评估多个捕食者对共享猎物的特定物种影响有助于确定调节捕食者影响的机制。在这里，我们调查了智利岩石下潮汐的两个捕食者鱼Graus nigra [F]和章鱼Robsonella fontaniana [O]对它们共同捕食的瓷蟹螃蟹Petrolisthes violaceus的营养影响。进行了操作性实验室实验，以评估瓷蟹在不同处理方式（包括单独捕食）中的死亡率和行为。具有单个捕食者（[O]或[F]）的猎物；和带有联合捕食者（[F + F]，[O + O]和[O + F]）的猎物。不同的猎物密度（N = 3、9和12个个体）使用正交（交叉）设计探索单个和联合捕食者的密度依赖性捕食死亡率，该设计允许从捕食者密度中区分出捕食者生物分类身份的影响。连续2天每4小时记录一次相互作用物种的行为反应，并用于识别捕食者的觅食策略和避难所的使用。比较了两种治疗方法的死亡率值（成比例的猎物死亡率和捕食者的人均消费）。结果表明治疗之间的死亡率差异。捕食者成比例死亡率增加的原因是由于在高初始猎物密度的情况下，在鱼类处理中的避难所之外的猎物可利用性增加，而在章鱼的处理中它与密度无关。根据捕食者组合和初始猎物密度记录了捕食者人均消费的变化。这些模式可能反映了不同的捕食者觅食策略和瓷蟹对避难所的使用变化，表明对猎物死亡率和捕食者影响有影响。