Cross-shelf patterns in the community structure of coral-dwelling Crustacea in the central region of the Great Barrier Reef. 11. Cryptofauna

Abstract

In a 4 yr study the crustacean cryptofaunal assemblages associated with live and dead Pocillopora verrucosa were sampled systematically from isolated coral heads placed at leeward and windward zones of an inner shelf, a mid-shelf and an outer shelf reef in the central region of the Great Barrier Reef. The spatial scales examined ranged from 1 m to 90 km. At all scales there was pronounced variation in cryptofaunal abundance. Prior to bioerosion of the corals, reef location was the dominant determinant of total abundance with subordinate, taxon-specific, effects of exposure. The cryptofaunal assemblages on living and dead corals on the mid-and outer shelf reef were dominated by copepods with maxlmum abundance on the back reef sites of the mid-shelf reef. Total abundance was significantly lower on the inner shelf reef, which had a distinctly different assemblage characterised by fewer copepods and greater numbers of ostracods, cumaceans and tanaids. At all locations, sediment accumulated on the corals but in greater amounts on dead ones. The type of sedment trapped by corals appeared to be an important determinant of cryptofaunal composition. Over the 4 yr period, there was a significant decline in cryptofaunal abundance at all locations; this was more pronounced on dead corals and coincided with bioerosion. The results indicate that factors determining crustacean crypto-fauna1 abundance operate at different scales. At the cross-shelf scale, higher abundance of the predominantly copepod assemblages on the mid-shelf reef may be indicative of elevated primary productivity in this region. At the reef scale, differences in sediment composition and rates of bioerosion among zones (windward and leeward sites) may influence assemblage structure. At the replicate scale, among individual coral heads, both the abundance and composition of crustacean cryptofauna may be influenced by microhabitat features.