Species composition and distribution of the comatulid crinoids of Heron Island and Wistari Reefs

Abstract

Species composition, community and population structure, and distribution patterns of comatulid crinoids were investigated at Heron Island and Wistari Reefs, in the southern region of Australia's Great Barrier Reef. The investigations were carried out on a series of field excursions over a two and a half year period.Thirty-six (36) species of crinoids in seven families were recorded from Heron Island and Wistari Reefs. The physical characteristics, colour patterns, habitat, occurrence and behaviour of each species were recorded. Sorteen (16) species are new records for the Heron Island area, and ten (10) species are additions to the fauna previously known from the southern Great Barrier Reef. Seven (7) species are new records for Queensland waters, and six (6) have not been previously reported in northern Australia. Forty-five (45) species are now recorded from Heron Island and Wistari Reefs, although this is probably an artificially high figure. The total is probably about 40 species. The biogeographical affinities of the comatulid crinoid fauna appear to be in agreement with those of other echinoderms from Queensland waters. However, on the available evidence, a latitudinal gradient of species richness within the Great Barrier Reef is not apparent. There are only two other data points (Lizard Island and Davies Reef) against which comparisons can be made, hence no firm conclusions can be drawn at this time.The crinoid community structure consists of a central core of ubiquitous species, overlain by several discrete species associations. The associations occur at a small scale and appear to be determined by substrate distribution and current flow. About 50% of the species occurring on Heron Island and Wistari Reefs are rare. These species are important in defining the community structure, in that they are often components of the small scale associations.Crinoid spatial distribution is strongly depth stratified. This stratification is primarily substrate controlled, rather than by a direct relationship with light or wave energy. More or less discrete associations of species (above) characterize different substrate types. The most commonly used substrates are branching hard corals, gorgonian fans, arborescent soft corals, caves and crevices and coral rubble. The overall crinoid density obtained was 0.107 crinoids m-2 . This represents the lowest density found in any large-scale quantitative survey. However, the area surveyed was almost five times greater than that of any previous survey. The maximum density of 1.81 crinoids m-2 over an area of 100 m2 is the highest yet recorded in any large-scale quantitative survey of coral reef comatulids.A discrete group of crinoid species shows strongly nocturnal behaviour. In general, the species in this group are smaller, less robust, and have fewer arms than continually active species. Predation appears to be the primary selective pressure for nocturnalism, in that smaller species may be unable to survive predation levels from day-active, visually-orienting predators that are sub-lethal to larger, multibrachiate crinoid species. Individuals of several species of comatulids were found to remain perched in the same locations for up to two years.Crinoid populations at Heron Island and Wistari Reefs appear to be relatively slow growing, with low levels of recruitment. Juveniles were only rarely encountered. Size-frequency analyses of four species revealed a single, broad-based, major size class, and an elongate tail or minor size class at the lower end of the distribution, representing the most recent recruitment. It is proposed that recruitment is controlled by predation and the availability of refuges from predation. Successful recruits are those individuals that have developed to a sufficient size and/or number of arms so that they can tolerate occasional loss of arms while living the exposed existence necessary for their method of filter-feeding.