Coral Reefs: A Very Short Introduction by Charles Sheppard

Author:Charles Sheppard
Language: eng
Format: epub
ISBN: 9780191505232
Publisher: OUP Oxford
Published: 2014-05-10T04:00:00+00:00

Thus the quantity of living organic tissue on the reef at any one time is not necessarily very great but its rapid growth and consumption, its productivity, is immense.

Chapter 6

Reef fish and other major predators

The kaleidoscope in the water

Perhaps the most immediate visual impression for a snorkeler or diver visiting a healthy reef is the immense number of fish in a variety of shapes, sizes, and colours. Some, especially the smaller ones, are very timid, never venturing far from the protective branching corals while others, such as parrotfish, swarm over reefs in great schools scraping algae and other food material from both dead rock and living coral. On an unfished reef, thousands of fish hover in the water above the reef. No other natural habitat in the ocean shows this diversity and abundance. The diversity is so great that about a quarter of all marine species may be found on coral reefs, even though this habitat occupies only an estimated 1 or 2 per cent of the area of the earth. The exact area is hard to determine, since many reefs are deep, not mapped, and cannot be seen by satellite. Clearly, there are strong driving factors leading to the proliferation of these numerous and various species of differing size and body shape; and the variety and number of fish have great practical importance for humans, many of them providing food for the large and increasing human populations that live along tropical shorelines.

Many smaller species are coral obligates, meaning they can live only in, or feed only on, particular species of coral. This naturally constrains their overall abundance and distribution. When the corals on which they are dependent die, obligate fish species are the first to show population collapse. Although, some do survive, for example by swapping to new host corals, provided the corals contain roughly the same kind of branching structure. The association that any particular fish species has with a reef may be more or less permanent, or it may occur for only part of the lifecycle of the fish, and it might be food or habitat related.

As with the corals, there are major geographical influences on the diversity patterns of reef fishes: there are quite different assemblages in the Caribbean for a start, and there are many more species in Southeast Asia than in the Eastern Pacific. Relatively isolated areas such as the Red Sea have many endemic species not found anywhere outside that region, while some species range across entire oceans. There are patterns of diversity correlated with latitude too. Furthermore, on any one reef the diversity of the fish assemblage increases with the overall complexity or rugosity of that reef. Generally, the more complex the topography of habitats, the more complex and varied are the assemblages of fishes that utilize them. And again, as with corals, the various species have adapted to different depth ranges, with some thriving in the shallow surf zone, many requiring vast expanses of sand over which to forage, and some being restricted to deeper areas.

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