Love thy prickly neighbor? Sea urchin density affects risk assessment in damselfish

Abstract

In coral reef communities, sea urchins and reef fish have a variety of commensal relationships. However, if fish view urchins as a potential form of predator protection, then urchin presence should influence reef fish risk assessment. We investigated whether dusky damselfish (Stegastes nigricans) perceive burrowing urchins (Echinometra mathaei) as a form of predator protection by asking whether and how urchin presence explains variation in flight initiation distance, the distance a fish fled an approaching threat. We found that when individuals were subjected to a high-risk approach (i.e., one that started relatively close to the fish), and urchin density was high, dusky damselfish tolerated a closer approach than when subjected to high risk with low urchin density. However, when there was low risk and high urchin density, the damselfish fled sooner. We conclude that under high risk, damselfish perceive burrowing urchins as providing added security from approaching threats.