Comatulids apparently rely on the direct interception of particles, which reflects the operation of their feeding apparatus as an adhesive fiber filter rather than a simple sieve. The latter would retain all particles larger than the mesh size while allowing all those smaller to pass through. Crinoids clearly capture particles smaller than the spaces between podia (Holland et al. 1986). The structure and function of adhesive fiber filters are best interpreted by aerosol filtration theory, which was first applied to biological filter feeders by Rubenstein & Koehl (1977). Since then, the theory has been widely applied to crinoids [including fossil stalked taxa (Ausich 1980, Kammer 1985, Kammer & Ausich 1987)] and explains a great deal about crinoid feeding biomechanics and behavior, including variations in length, spacing and posture of primary podia, pinnules and arms, and variations in habitat, all relative to current flow (Meyer 1979 1981 1982a b, Liddell 1982 Leonard et al. 1988, Leonard 1989). In particular, the theory predicts that different filter arrays will function optimally under different flow regimes (Baumiller 1997, Holterhoff 1997).


As a result, species subject to slower, multidirectional flow, orient pinnules and arms in a multidirectional posture (see next paragraph) and bear longer more widely spaced podia than species that occupy exposed perches, arrange arms in more planar arrays and bear shorter more closely spaced podia (Meyer, 1973 1979, Liddell 1982). Similarly, several studies demonstrate how different crinoids alter their filtration postures in response to variations in current velocity and pattern (Magnus 1963, Meyer 1973 1982a, La Touche 1978, Meyer & Macurda 1980, Byrne & Fontaine 1981, Meyer et al. 1984a, Leonard et al. 1988, Messing 1994, Baumiller 1997). In addition to reflecting flow conditions, the wide array of feeding postures and positions offer useful clues to field identification, particularly in the tropical Indo-West Pacific where single reefs may support dozens of species.



In microhabitats subject to weakly turbulent or continuous multidirectional flow, comatulids extend arms at random and arrange their pinnules in two or more planes along each arm. This multidirectional posture, mentioned above, is characteristic of semi-cryptic reef-dwelling comasterids such as Comanthus parvicirrus, C. gisleni, Phanogenia gracilis and Davidaster spp. (Meyer 1973, Macurda & Meyer 1980, Messing 1994), and antedonids such as cryptic Ctenantedon (Meyer 1972) and, under suitable conditions, Antedon (La Touche 1978).




Under conditions of directional horizontal flow, comatulids orient their arms in a variety of postures perpendicular to the current. Ambulacra face downcurrent, except when flow is a bidirectional surge, and arms and pinnules typically curve at least slightly concave upcurrent. Although the nomenclature has varied among authors, intergrades exist, and individuals change postures with variations in flow, several distinct forms are recognizable. The following is taken largely from Meyer & Macurda (1980) with modifications by Messing (1994) and a few more recent additions. In the first four, the pinnules orient in a single plane normal to flow along each arm, like barbs on a feather.


Arcuate fans consist of two layers of arms (biplanar) oriented normal to flow. They are typical of nocturnally active mariametrids that have 20-50 arms and that cling to exposed perches (e.g., Lamprometra, Liparometra, Stephanometra). In monodirectional currents, arms on the downcurrent side twist so that all ambulacra face downcurrent. These fans sometimes widen into funnels or bowls, the latter grading into the radial and parabolic fan postures. In the comasterid Oxycomanthus bennetti, which often has >60 arms, the fan is thicker, bushier and intergrades with the multilayered arrays below.



In a radial fan, typical of colobometrids, asterometrids and himerometrids that cling to arborescent cnidarians, and some smaller deep-sea stalked crinoids, arms orient roughly like wheel spokes (monoplanar) with the oral surface downcurrent (Meyer & Macurda 1980, Stevens 1989, Messing 1994). The fan is usually slightly concave upcurrent, so this posture grades into the next. Mariametrids (e.g., Stephanometra echinus) sometimes form such fans, or shallow bowls, spread across coral perches and not obviously oriented normal to flow. Some clinging species, particularly the comasterid Capillaster multiradiatus and the colobometrid Cenometra bella, often interrupt the radial fan with several irregularly arrayed or curled arms.







In parabolic fans, characteristic of most stalked crinoids, the oral surface orients downcurrent and arms recurve into the current. Among comatulids it occurs in species that achieve a functionally stalked existence either by clinging to narrow elevated perches such as sea lily stalks and sea whips [thalassometrids such as Stylometra spinifera (Messing 1985)], or via extremely long cirri (the colobometrid Pontiometra andersoni and the zygometrid Zygometra microdiscus [Meyer & Macurda 1980, Messing et al. 2005]).












In the simple arm fans of some comasterids (Comactinia echinoptera, Alloeocomatella pectinifera) and tropiometrids (Tropiometra), arms extend singly or in small groups from a crevice without forming a continuous filtration surface.


Multilayered arrays are typical of comasterids with >80 arms and vary from thick, irregularly bushy fans to more-or-less completely bushy masses. Some arms assist in anchoring. Under unidirectional flow, arms on the upcurrent side may form a more regular multilayered fan with some twisting of downcurrent arms. Stevens (1989) distinguishes a multidirectional ball posture in which the arms assume a meridional arrangement or form a tangled mass, with tips curled inward and overlapping (Comanthus alternans). This is an active feeding posture assumed under strong flow conditions and should be distinguished from the meridional posture described below under slack water. In the photo at left, Comaster audax has some arms raised in a multilayered fan and others coiled in the ball posture. Comasterids with 40-60 arms that assume a multidirectional posture when semicryptic, may form irregular multilayered bowls or irregular bushes when fully emerged (e.g., Comanthus mirabilis, Comatella stelligera).









A variation on the multilayered array occurs in the Australian Comatula rotalaria, which has only 27 arms at most (usually 20). As in several of the comasterids with >80 arms and few or no cirri, it supports itself above the bottom on curved arms, but only a few in this case. Unlike the others, it occurs only on unconsolidated substrates and has interior arms distinctly longer than exterior arms; C. rotalaria arranges the latter in an erect central tuft (Messing et al. 2005).






When subject to stronger surge, some form more-or-less arcuate fans or funnels (e.g., Clarkcomanthus littoralis, Comanthus wahlbergii). Some himerometrids (Himerometra spp.) also form multilayered bowls, fans and balls. With a few exceptions in which pinnule orientation is monoplanar (Oxycomanthus bennetti, Nemaster grandis and Himerometra spp.), successive pinnules along the arms of multilayered arrays are offset, by about 30° under high energy reef crest conditions and by up to 90° in quieter (deeper or sheltered) water.


Under slack water conditions, comatulids orient arms in a variety of ways. Antedon and Florometra assume a conical posture (La Touche 1978, Byrne & Fontaine 1981). Diurnally cryptic species curl up in crevices, under ledges or among coral branches. Exposed reef taxa assume a meridional posture with arms arched over the oral surface. The asterometrid Pterometra venusta (Stevens, 1989) and Pontiometra andersoni form a reversed meridional posture with arms curved aborally, a posture roughly similar to that of stalked isocrinids under similar conditions. However, Stevens (1989) observed P. andersoni to curl its arms orally into a dense ball under slack conditions.







Modified from Messing (1997).




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