Niche separation, ontogeny, and heterospecific alarm responses in centrarchid sunfish

Behavioral responses to alarm cues in aquatic species are typically examined with emphasis on the potential survival benefits accrued by conspecific receivers. By contrast, heterospecific responses to alarm cues and changes in responses with ontogeny in fishes are relatively unexplored. Taking an ecological niche perspective, we hypothesized that the response patterns of fish to risky chemical cues should be closely related to their degree of niche differentiation, which increases with ontogeny. We tested this hypothesis using the responses of adults from sympatric bluegill (Lepomis macrochirus) and pumpkinseed (L. gibbosus) populations to the alarm cues of conspecific and heterospecific adults and juveniles, including water as a control treatment. Responses measured consisted of changes in body posture (time spent with the dorsal fin <30º, between 30º and 60º, or >60º) and behavior (times spent still, frozen, at the surface, or on the bottom of the tank). Both adult bluegill and pumpkinseed spent significantly more time with their fins held >60º in response to adult versus juvenile alarm cues, with these responses mediated by donor species such that adult conspecific cues resulted in greater responses than heterospecific cues. The same general pattern was observed in the behavioral measures. These results demonstrate that behavioral response patterns to chemical alarm cues in sunfishes are highly plastic, and are likely related to niche separation in adults. Our findings open new lines of research into the role of ecological niches in shaping behavioral responses of fish to risky information.

水生生物对警戒信号（alarm cues）的行为响应研究，通常聚焦于同种个体（conspecific）接收者由此获得的潜在生存收益。与之相对，鱼类对异种个体（heterospecific）警戒信号的响应，以及随个体发育（ontogeny）产生的响应变化，迄今尚未得到充分探索。本研究从生态位（ecological niche）视角出发，提出假设：鱼类对风险性化学信号的响应模式，应与其生态位分化程度紧密相关，而该分化程度会随个体发育逐步提升。为验证该假设，我们以同域分布的蓝鳃太阳鱼（Lepomis macrochirus）和南瓜籽太阳鱼（L. gibbosus）成体为研究对象，观测其对同种、异种成体及幼体的警戒信号的行为响应，实验设置以纯水作为对照处理。本研究测定的响应指标包括躯体姿态变化（记录背鳍角度<30°、30°~60°及>60°的持续时长）与行为模式变化（记录静止、僵直、上浮至水面以及沉至试验缸底部的持续时长）。实验结果显示，相较于幼体警戒信号，蓝鳃太阳鱼与南瓜籽太阳鱼成体在响应成体警戒信号时，背鳍保持>60°的时长均显著更长；且该响应受信号供体物种调控：同种成体警戒信号引发的响应强度显著高于异种成体警戒信号。行为指标的观测结果亦呈现相同的总体趋势。上述结果表明，太阳鱼类对化学警戒信号的行为响应模式具有高度可塑性，且可能与其成体的生态位分化密切相关。本研究结果为探索生态位在塑造鱼类对风险性信息的行为响应过程中所发挥的作用，开辟了全新的研究方向。