Eye color as an indicator of social rank in the fish Nile tilapia

Abstract

We investigated the association of eye color with the dominantsubordinate relationship in the fish Nile tilapia, Oreochromis niloticus. Eye color pattern was also examined in relation to the intensity of attacks. We paired 20 size-matched fish (intruder: 73.69 ± 11.49 g; resident: 75.42 ± 8.83 g) and evaluated eye color and fights. These fish were isolated in individual aquaria for 10 days and then their eye color was measured 5 min before pairing (basal values). Twenty minutes after pairing, eye color and fights were quantified for 10 min. Clear establishment of social hierarchy was observed in 7 of 10 pairs of fish. Number of attacks ranged from 1 to 168 among pairs. The quartile was calculated for these data and the pairs were then divided into two classes: low-attack (1 to 111 attacks -2 lower quartiles) or high-attack (112 to 168 attacks -2 higher quartiles). Dominance decreased the eye-darkening patterns of the fish after pairing, while subordinance increased darkening compared to dominance. Subordinate fish in lowattack confrontations presented a darker eye compared to dominant fish and to the basal condition. We also observed a paler eye pattern in dominants that shared low-attack interactions after pairing compared to the subordinates and within the group. However, we found no differences in the darkening pattern between dominants and subordinates from the high-attack groups. We conclude that eye color is associated with social rank in this species. Moreover, the association between eye color and social rank in the low-attack pairs may function to reduce aggression.

Stress responses of the fish Nile tilapia subjected to electroshock and social stressors

“…This is important to adequately characterize the social stress since the previous history of social interaction modulates the stress response in the Nile tilapia (12). The social ranks of the focal animals were also confirmed by qualitative inspections of the body (9) and/or eye (22) coloration (the eye and body of dominant Nile tilapia fish are pale while those of subordinates are darker).…”

“…However, they differ in that electroshock is restricted to the physical contact, with no cue whereby the fish can anticipate the stressor exposure, while a social stressor includes anticipatory psychological components. The presence of a dominant fish is an intimidating threat that can be permanently perceived by the subordinate fish without necessarily involving biting (16,22).…”

“…We chose the fish species Nile tilapia, Oreochromis niloticus, because it is an important fish model for studying the stress response (9)(10)(11)(12)(13)17,21,23,24) and has evident agonistic behavior (11,12,22,25,26), both aspects related to the proposed hypothesis.…”

Agonistic and reproductive behaviors in males of red hybrid tilapia, Oreochromis niloticus (Linnaeus, 1758) x O. mossambicus (Peters, 1852) (Osteichthyes: Cichlidae)

“…The behaviour of Nile tilapia, Oreochromis niloticus (Linnaeus, 1758), has been extensively investigated. Studies on this fish behaviour patterns considered several aspects, such as communication (Giaquinto and Volpato, 1997), social stress (Fernandes and Volpato, 1993;Alvarenga and Volpato, 1995;Correa et al, 2003;Volpato et al, 2003;Barreto and Volpato, 2004), social hierarchy (Barki and Volpato, 1998), aggression (Alvarenga and Volpato, 1995;Giaquinto and Volpato, 1997;Correa et al, 2003); reproduction (Gonçalves-de-Freitas and Nishida, 1998), learning (Barki and Volpato, 1998;Moreira and Volpato, 2004), and others. However, most of the studies on the Nile tilapia are related to fish production or fish management due to the importance of this species in tropical systems (Vijayan et al, 1997;Barcellos et al, 1999a,b;Gontijo et al, 2003).…”

“…Besides these, the eye color is also used as an indicator of social rank in the Nile tilapia, O. niloticus (Volpato et al, 2003), and in other fish species (Beaching, 1995;O´Connor et al, 1999O´Connor et al, , 2000Sutter and Huntingford, 2002). However, in the Nile tilapia, social and rank signals do not rely exclusively on visual cues.…”