Coping with continual danger: Assessing alertness to visual disturbances in crucian carp following long-term exposure to chemical alarm signals

- Supernatants of centrifuged fish-skin homogenates contain chemical alarm signals.
- Crucians display the strongest instant alarm reaction to extract of conspecific skin.
- Alertness-effects from long-term exposure to chemical alarm signals were questioned.
- Fright reaction to visual disturbances was not found after 7 weeks of exposure.
- Instant acting alarm signals are not the causative agents for inducing alertness.

Chemical alarm signals in fish are passively released into the water from ruptured epidermal cells, and induce instant fright responses in conspecifics. Fish also display alarm responses to injured heterospecific skin, as well as to scent of piscivorous predators that have ingested prey. A conspicuous alertness to visual disturbances has also been observed in fish following long-term exposure to extracts of filtered, homogenized skin, but the chemical inducers of such vigilance are actually unknown. We tested if a continual exposure to water-soluble alarm signals, from either conspecifics or heterospecifics, affects alertness of fish. Based on previous experience, it was assumed that alertness could be detected following visual disturbances. Naïve crucian carp were initially exposed to the aqueous extracts of centrifuged skin homogenates, from either conspecifics, or from one out of four heterospecific species (tench, perch, Arctic charr, and brown trout). Darting movements, inter-individual distances, and vertical distribution were used to measure behavioral fright responses released by the test stimuli. After seven weeks of continual exposure to the same extracts, behavioral observations were repeated during visual disturbance. Compared with fish that were long-term exposed to skin extracts of tench or charr, crucian carp exposed to extracts of conspecifics, or to extracts of trout or perch, displayed lower inter-individual distances before being visually disturbed. However, no apparent fright responses were observed following such disturbances, and fish that had been continually exposed to conspecific chemical alarm signals displayed feeding behavior. Our results revealed that fish under assumed continual stress, induced by long-term presence of water-soluble alarm cues, only moderately changed their behavioral pattern. This further demonstrates that the aqueous part of extracts from homogenized skin does not contain any causative agents for inducing any conspicuous alertness.