Fish Grow Coral

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The planktivorous damselfish, Yellow-tail Dascyllus (Dascyllus flavicaudus), feeding above their branching coral host, Pocillopora eydouxi. The fish shelter among the branches of their coral host when threatened by predators during the day and throughout the night. Skeletal growth of Pocillopora colonies increases with increasing number (biomass) of sheltering damselfish, which is related to increased concentrations of nitrogenous wastes that are excreted by fishes and taken up by the coral.
Melissa H. Schmitt

There is an obvious relationship between coral and the tropical fishes that live in close association with them -- in general the more coral there is on a reef, the more fish will occur there. While it is clear that the amount of coral habitat on a reef influences the number of fishes present, MCR scientists have found that the reverse often is true as well -- the number of coral-dwelling fishes present can markedly enhance the amount of coral on a reef.

Scientists concerned with how disturbances and climate change will affect resilience and biodiversity of tropical reefs have focused largely on how coral-dwelling fishes and invertebrates will be affected by reduced growth and survival of their coral habitat. Until recently, little attention has been paid to whether the fishes and invertebrates that live with corals provide their cnidarian hosts with any substantive benefit. MCR investigators have discovered that coral-dwelling fishes and invertebrates markedly enhance the growth and survival of their hosts, as well as enable corals to persist in otherwise inhospitable areas.

MCR scientists have found three different pathways by which fishes and invertebrates can increase the growth and survival of their cnidarian hosts. One way is by their excretion of nitrogenous wastes that the dinoflagellate symbionts (Symbiodinium) of reef-building corals need to make the photosynthetic products that in turn corals use for tissue and skeletal growth. The effect of this local enrichment of nutrients by fishes can be as great as a doubling in the growth rate of the cnidarian host. A second general way some coral-dwelling fishes and crabs benefit their hosts is by defending them against predators of coral. Indeed, one of the most important habitat-forming corals on reefs in the central Pacific can only co-occur with its predators if the coral is defended by territorial damselfish.

The third way coral-dwelling animals were found to enable corals to thrive and persist is by providing essential housekeeping services. For example, certain crabs were found to sweep especially harmful sizes of sediments off the tissues of their coral host, which enables these corals to inhabit areas with otherwise lethal levels of sedimentation.

The idea that the abundance and diversity of animals on a tropical reef may simultaneously be a cause and a consequence of the amount of coral present is a novel hypothesis that contributes to our understanding of the complex dynamics of coral reefs, the most productive and diverse of all marine ecosystems.

Field experiments by MCR investigators revealed that planktivorous damselfish greatly enhance the rate of coral skeletal growth (panel a) and increases in colony volume (panel b) of habitat-providing corals among whose branches the fish shelter. Short-term (1 mo) experiments done in 2005 (open circles) and 2008 (closed circles) indicated that the rate of coral calcification (skeletal growth) increased with increases in the number of fish individuals that sheltered on a colony. A long-term field experiment revealed that the annual increase in volume of a branching coral was twice as great when damselfish sheltered on it compared to colonies where damselfish were kept away (data are mean ± 1 SE; treatments are statistically different at P < 0.05). Ammonium concentrations near coral branches were greatly enriched when damselfish were present on a coral (data not shown), and the ammonium excreted was incorporated into the tissues of the coral, which appeared to fuel photosynthesis by the coral's endosymbionts (Symbiodinium).
Modified from Holbrook et al. 2011.
For further reading: 
Holbrook, S.J. and R.J. Schmitt. 2004. Population dynamics of a damselfish: effects of a competitor that also is an indirect mutualist. Ecology 85:979-985.
Holbrook, S.J., A.J. Brooks, R.J. Schmitt and H.L. Stewart. 2008. Effects of sheltering fish on growth of their coral hosts. Marine Biology 155: 521-530.
Holbrook, S.J., R.J. Schmitt and A.J. Brooks. 2011. Indirect effects of species interactions on habitat provisioning. Oecologia in press.
For further information: 
Dr. Andrew J. Brooks
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