The early life history of the brooding damselfish Acanthochromis polycanthus: effects of environment and ancestry

Kavanagh, Kathryn Diane (1996) The early life history of the brooding damselfish Acanthochromis polycanthus: effects of environment and ancestry. PhD thesis, James Cook University of North Queensland.

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In complex life-histories, the timing of transitions between stages is of central importance in any consideration of ecological or evolutionary responses to environmental pressures. Understanding these interactions between environment, growth rate, developmental rate, and the timing of stage transitions is useful in predictions of the direction of ecological or evolutionary change in animals with complex life histories, such as frogs and fishes.

The primary aims of this study were to examine environmental correlates of growth, development, parental investment, and mortality rates for one species of coral reef damselfish (Pomacentridae: Acanthochromis polyacanthus) whose unique life history permits such site-specific demographic correlations. From literature references and measurements obtained from developmental series, these characteristics are compared with several other species of coral reef damselfishes. Field studies were conducted on the exposed and lagoonal reef zones surrounding Lizard Island, Great Barrier Reef. Developmental series were obtained from captive-reared animals kept under controlled temperature conditions in the lab.

Acanthochromis polyacanthus is highly unusual because it lacks a pelagic larval stage and exhibits extended biparental care of eggs and juveniles, in many ways similar to that of the substrate-brooding cichlid fishes. Hatching occurred during midday, in contrast to all other damselfishes which hatch at sunset. Acanthochromis have large, relatively well-developed hatchlings (5 mm SL). Dispersal from the nesting area is initiated by parents chasing their offspring away, which occurred 3-4 months after hatch (approximately 50 mm SL) at Lizard Island.

Parental effort was significantly greater in exposed reef zones than in the lagoon, and was greater in younger broods. This pattern was apparent in the analysis of defensive chases by parents, with more chases occurring in exposed reef zones and from parents with younger broods. Intruding conspecific adult Acanthochromis were the most frequently chased taxon (31.6% of all chases). Juveniles in a brood were also "tended" (approached and/or herded back into the brood) significantly more often in exposed sites. Olfactory cues may be used by parents in detecting juveniles; a pilot study found that significantly more parents tested chose to spend time in water with the scent of their juveniles as opposed to plain seawater. Within a zone, parental care per juvenile did not change significantly as juveniles aged. Specialized juvenile behaviors included, most notably, glancing off the sides of the parents. Glancing rates did not differ between reef zones but significant differences between juvenile stages were found. Glancing rates were not significantly different in feeding and non-feeding juveniles, suggesting that this behavior is not a major source of nutrition. However, an experimental study indicated that ingestion of mucus did occur during glancing.

Production of broods was greater in front reef (exposed) sites but juvenile survival was greater in lagoon sites. There was a significant season/zone interaction with the greatest survival in the lagoon in early summer. Density of Acanthochromis adults was higher on front reefs. A significant positive correlation between adult density and brood density at all sites indicated that adult Acanthochromis prefer to live in front reef zones despite lower juvenile survival. The maximum size of Acanthochromis in exposed sites was consistently greater than in lagoon sites.

Under calm weather conditions, juvenile growth rates were generally lower in the lagoon than in exposed sites, however, growth rates of lagoon juveniles increased rapidly following strong winds (which potentially flush plankton-rich waters into the lagoon). Exposed sites showed no difference in growth rates between calm and strong wind conditions. In contrast to increased growth rate, ossification rate was slowed in the lagoon juveniles after strong winds, suggesting that there is a trade-off between growth and development. No differences were detected in growth or development rates of lagoon juveniles that were provided with supplemental food.

Characteristics of Acanthochromis suggest that predation is an important selective force that affects the energetics and survival of early stages of fishes living on the reef. On ecological and evolutionary scales, spatial variation in environmental factors must influence the vital rates (sensu Houde, 1987) of young reef fishes; the interactions of these rates with the timing of stage transitions may allow predictions of the direction of change.

Item ID: 33775
Item Type: Thesis (PhD)
Keywords: Acanthochromis polycanthus; behavior; behaviour; damselfishes; growth rates; juveniles; life cycles; life histories; Lizard Island; mortality rate; Pomacentridae; predation; reef fish
Date Deposited: 21 Jul 2015 04:31
FoR Codes: 06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060308 Life Histories @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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