ABSTRACT
Current understanding of modern, shallow-water, tropical crinoid ecology derives almost entirely from coral reef assemblages. Only Amphimetra and Heterometra have been observed regularly on unconsolidated low-energy bottoms. Our preliminary survey reveals that about a dozen comatulid species occur on a gentle sandy slope in 12-18 m off Lizard I., Great Barrier Reef, Australia. Dominant macroorganisms are chlorophyte algae, seagrasses and small unattached scleractinians. Several of the crinoids (e.g Comatella nigra) also occur on reefs. Several others appear restricted to unconsolidated substrates but cling to sponges or algae (e.g Amphimetra tessellata, Zygometra microdiscus), or hide beneath rubble or sponges (Heterometra crenulata). A. tessellata arranges its arms in a radial fan or funnel, while Zygometra spp. form parabolic filtration fans similar to those of Pontiometra andersoni on reefs. Comatula purpurea lies irregularly coiled but visible beneath sprawling branching sponges. Comatula rotalaria, which lacks anchoring cirri and bears 20 arms up to 0.3 m long, elevates its calyx above the sediment by flexing 5-7 of its long interior arms as shallow Vs; the shorter exterior arms orient more or less upward as a central tuft. Quantitative transects reveal substantial differences in species composition between 12-13 m and 16-17 m. C. rotalaria offers a potential model for substrate adaptations and arm postures in Late Cretaceous Uintacrinus socialis, which, though it bears fewer longer (1 m) arms, also lacks cirri and exhibits similar brachial morphology. A comparison with extant Alloeocomatella pectinifera suggests that U. socialis may have oriented its arms vertically despite their great length.
