MEPS 176:25-38 (1999)  -  doi:10.3354/meps176025

Delayed metamorphosis of a tropical reef fish (Acanthurus triostegus): a field experiment

Mark I. McCormick*

Department of Marine Biology, James Cook University, Queensland 4811, Australia

ABSTRACT: Larval duration of a demersal fish is a product of its genotype, its larval environment and the capacity of the species to delay metamorphosis. Circumstantial evidence has led to the hypothesis that the lower age-limit for settlement is governed by the rate of larval development, while the upper age-limit is determined by the extent to which a delay of metamorphosis is possible. This study examined the capacity of a widely distributed reef fish, the manini Acanthurus triostegus, to extend its larval duration by delaying metamorphosis. Variation in larval duration was examined from 8 samples of manini collected in French Polynesia using crest nets over a 2 yr period. Variation in the overall age at colonisation determined from daily otolith increments was very low (CV 5.5%) given the pan-Pacific distribution pattern of the species. A field experiment was conducted to determine whether manini could delay metamorphosis. To enable its interpretation, the metamorphosis of the species was characterised morphologically. Metamorphosis involved a loss of transparency, a shortening of fin spines and a migration of the mouth from a terminal to ventral position over a 5 d period. To experimentally examine the capacity of the species to delay metamorphosis manini were caught at night as they colonised a reef and placed in 1 of 2 treatments: benthic cages in the shallow backreef, or fine monofilament cages suspended between 3 and 6 m in a 50 m water column on the outer reef slope. Fish in benthic cages completed metamorphosis within 5 d. In contrast, 24% of fish in pelagic cages (8 out of 34 fish) remained transparent (although some developed faint stripes) and retained the pelagic body shape. Fish that delayed metamorphosis still deposited a mark on their otoliths indistinguishable in structure from the settlement marks deposited on otoliths of their benthic-caged counterparts and reef-caught juveniles. This is the first experimental evidence that the settling stages of some demersal fish species can delay metamorphosis in the reef environment. As such it supports the conceptual model generally applied to the selective settlement of demersal fishes.

KEY WORDS: Coral reef fish · Metamorphosis · Recruitment · Settlement · Larval duration · Delayed metamorphosis · Growth · Otolith · Field experiment

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