Variability in phenotypic traits such as body size or shape is typically lower in animal groups than within their populations ̶̶ indicating that such groups are phenotypically assorted. Active group choice is one of the mechanisms by which such phenotypic assortment can be achieved. Several species of reef fish form mixed-species groups or “fish flocks” where two or more species associate for foraging or anti-predatory benefits. For attendant groups of reef fish which form primarily for foraging benefits – competition for resources between shoal members may result in phenotypically variable groups. Frequency dependent differential predation (predators preferring phenotypically odd individuals) and activity matching may drive phenotypically assorted grouping which is seen in shoaling groups of reef fish that form primarily for anti-predator benefits.

Variability in phenotypic traits such as body size or shape is typically lower in animal groups than within their populations ̶̶ indicating that such groups are phenotypically assorted. Active group choice is one of the mechanisms by which such phenotypic assortment can be achieved. Several species of reef fish form mixed-species groups or “fish flocks” where two or more species associate for foraging or anti-predatory benefits. For attendant groups of reef fish which form primarily for foraging benefits – competition for resources between shoal members may result in phenotypically variable groups. Frequency dependent differential predation (predators preferring phenotypically odd individuals) and activity matching may drive phenotypically assorted grouping which is seen in shoaling groups of reef fish that form primarily for anti-predator benefits.

Variability in phenotypic traits such as body size or shape is typically lower in animal groups than within their populations ̶̶ indicating that such groups are phenotypically assorted. Active group choice is one of the mechanisms by which such phenotypic assortment can be achieved. Several species of reef fish form mixed-species groups or “fish flocks” where two or more species associate for foraging or anti-predatory benefits. For attendant groups of reef fish which form primarily for foraging benefits – competition for resources between shoal members may result in phenotypically variable groups. Frequency dependent differential predation (predators preferring phenotypically odd individuals) and activity matching may drive phenotypically assorted grouping which is seen in shoaling groups of reef fish that form primarily for anti-predator benefits. We investigate the similarities in phenotypic traits (viz. body size, body shape, body markings and colours) in the association patterns for attendant and shoaling groups of reef fish in the Arabian and Andaman Seas. We find that shoaling groups in both the Lakshadweep archipelago and the Andaman Islands undergo phenotypic assortment (showing low phenotypic variability), possibly to overcome the oddity effect thus minimizing their predation risk. We also explore the environmental factors that drive the formation of shoaling groups of herbivorous reef fish by estimating resource abundance and relative predator abundance across a fishing gradient (within and outside Marine Protected Areas) in the Andaman Sea.