Sediment utilization, feeding-niche breadth, and feeding-niche overlap of Aspidochirotida (Echinodermata: Holothuroidea) at Heron Island, Great Barrier Reef

Abstract

Aspidochirotida are abundant deposit-feeders in the lagoon (0.17plus/minus0.53 ind per square metre; mean plus/minus1 SEM) and on the reef flat (0.68plus/minus0.06 ind square metre) of Heron Island. Granulometry of sediment and faeces indicates minimal feeding-niche separation between species. Electivity (E) for grain sizes ranges from -0.74plus/minus0.14 to 0.43plus/minus0.19. Coarser grain sizes (<-0.5) tend to be excluded, and finer grain sizes (>0.3) included disproportionally in the diet. However, each species ingests all available sediment grain sizes and all species ingest each grain size in roughly the same proportions. Feeding-niche breadth (FT) for all species ranges from 0.97plus/minus0.01 to 0.99plus/minus0.01 and feeding-niche overlap (L) for each species pair ranges from 0.78 to 1.15. Spatial separation of species occurs. Holothuria atra and Holothuria leucospilota have aggregated distributions, with H. atra found more commonly on the inner reef flat, at a mean of 34plus/minus2% of the distance from the low tide line to the reef crest, and H. leucospilota more commonly found on the outer reef flat, at a mean relative distance of 67plus/minus3%. In the lagoon, H. atra and H. leucospilota forage at a distance (1.7 to 3.0 m) from coral patch reefs, while Holothuria edulis, Stichopus chloronotus and Stichopus variegatus forage nearer (0.8 to 1.3 m). However, active exclusion is unlikely. Coefficients of association (C subscript 7) for most species pairs indicate co-occurrence at frequencies predicted simply by distributions of these species. Only Holothuria impatiens and Stichopus horrens are significantly associated (C subscript 7 = 0.34) on the reef flat, reflecting their shared cryptic habitat. Only H. edulis and S. chloronotus are significantly associated (C subscript 7 = 0.76) in the lagoon. Total consumption by Aspidochirotida in the lagoon and reef flat (3.93 and 12.76 g dry sediment metres squared per day, respectively) represents only a small fraction of the available surface sediment (0.06% and 0.22%, respectively). Available surface sediment is not a limiting resource and therefore competition for food between co-occurring deposit-feeding Aspidochirotida is probably slight