Changes in the darkness of four body features of bluegill sunfish (Lepomis macrochirus Rafinesque) during aggressive encounters

The darkness of four body features (general body surface, GBS; lateral stripe pattern, LSP; opercular flap spot, OFS; and dorsal fin-ray spot, DFRS) was observed during the formation of 16 pairwise status relationships among bluegill sunfish (Lepomis macrochirus Rafinesque). In eight cases (Method A) an intruder fish was placed into a 110 liter aquarium occupied for 7 previous days by a resident fish. In eight additional cases (Method B) two fish lived on opposite sides of a 110 liter aquarium divided by an opaque screen for 7 days prior to removal of the screen. Dominance was established by one of the fish in all but two cases (both were Method B) and aggressive encounters proceeded through five clearly identifiable stages: (1) a stage of inactivity just after pair formation, (2) a stage in which the fish oriented and approached each other tentatively and without further interaction, (3) a stage in which threat displays are emitted, often by both fish, but no actual body contact occurs, (4) a stage of vigorous attacks, and (5) a stage where one fish is clearly emitting all attacks and where the other fish is fleeing and/or hiding. Darkness of each feature in each fish was evaluated with a statistically reliable three-point scale at each stage during the contest. GBS increased in darkness equally for dominant and submissive fish. LSP, OFS and DFRS became darker in dominant fish and lighter in submissives, suggesting that these three features may have status-related signal value. Method of pair formation did not interact with these effects.