Charles Messing's Crinoid Pages: Feeding Postures

In a fan posture (formerly called an arcuate fan), characteristic of feather stars that cling to a substrate, the arms project upward and to the sides in a fan-like array oriented across the current. Those with relatively few arms can array their arms in a single plane (monoplanar). Those with more arms (~20-50) tend to form two-layered fans. With a current from one direction, arms on the downcurrent side twist at their bases so that all ambulacra face downcurrent. Under a back-and-forth surge, the two fan layers face opposite directions. These postures are typical of nocturnally active mariametrids that cling to exposed perches (e.g., Dichrometra, Stephanometra). As arm number increases (e.g., in Anneissia bennetti with >60 arms), the fan becomes thicker, bushier and intergrades with the multilayered arrays below.

Leftmost image: Klunzinger's Feather Star, Lamprometra klunzinger at night at Abu Dabab Reefs, Red Sea, Egypt, Derek Keats, CC BY 2.0

Multilayered fans are typical of Comatulidae with >80 arms and vary from thick, irregularly bushy fans to more-or-less completely bushy masses. Some arms usually curve downward and assist in anchoring. Under unidirectional flow, arms on the upcurrent side may form a more regular multilayered fan with some twisting of downcurrent arms. A variation occurs in the Australian Comatula rotalaria, which has only 27 arms at most (usually 20), no cirri, and has interior arms distinctly longer than exterior arms (unique among living crinoids). It supports itself on sediment bottoms on a few curved longer arms with the shorter interior arms forming an erect central tuft (Messing et al. 2006). Species of Comatulidae with 40-60 arms that assume a multidirectional posture (see below) when semicryptic, may form irregular multilayered bowls or irregular bushes when fully emerged (e.g., Clarkcomanthus mirabilis, Comatella stelligera).

In a disk posture (formerly called a radial fan), typical of species in several feather star families (e.g., Colobometridae) that cling to elevated perches (e.g., sea fans, whip corals), and some deep-sea stalked crinoids, arms orient roughly like wheel spokes 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 tenuipinna) sometimes form such fans, or shallow bowls, spread across coral perches and not obviously oriented normal to flow. Some clinging species, particularly Capillaster multiradiatus (Comatulidae) and Cenometra bella (Colobometridae), often interrupt the radial fan with several irregularly arrayed or curled arms.

In the parabolic posture, characteristic of many stalked crinoids, the upper part of the stalk bends so that the oral surface orients down-current and arms recurve into the current. Among feather stars, 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. 2006]).

Conical postures range from narrow funnels to wide bowls open down-current and intergrade into both the disk and parabolic postures. Under weak flow conditions, the funnel or bowl may open upwards like the traditional posture supposedly relying on a rain of detritus, although the flow here is still horizontal. Both stalked crinoids and feather stars may assume similar non-feeding conical postures either under slack water or when the current is too strong to maintain a disk or parabolic posture for feeding (see below). Unless extended tube feet can be seen, it may be difficult to determine whether a crinoid in a conical posture is feeding or not.

In the independent arm posture of some feather stars (e.g., Alloeocomatella pectinifera (Comatulidae),  Tropiometra carinata (Tropiometridae)), arms extend singly or in small groups from a crevice without forming a continuous filtration surface, but the pinnules of each arm remain arrayed in a single plane like barbs on a feather.

In microhabitats subject to weakly turbulent or multidirectional flow, most feather stars belonging to family Comatulidae assume a multidirectional posture. They extend arms at random and arrange their pinnules in two or more planes along each arm, with successive pinnules offset by about 90º (but by as little as 30° under higher energy conditions). Examples include Comanthus parvicirrus, Phanogenia typica, and Davidaster spp. (Meyer 1973, Macurda & Meyer 1980, Messing 1994), and antedonids such as cryptic Ctenantedon kinziei (Meyer 1972) and, under suitable conditions, Antedon (La Touche 1978). When subject to stronger surge, some form more-or-less fan- or funnel-like arrays (e.g., Clarkcomanthus littoralis, Comanthus wahlbergii).

Under slack water conditions, crinoids orient arms in a variety of non-feeding postures. Diurnally cryptic species curl up in crevices, under ledges or among coral branches. Some exposed reef species assume a meridional posture with arms arched over the oral surface (roughly like lines of longitude, or meridians). The asterometrids Pterometra (Stevens 1989) and the colobometrid Pontiometra andersoni form a reversed meridional posture with arms curved aborally and food grooves facing outward.

Southern passion star - Ptilometra australis, John Turnbull, CC BY-NC-SA 2.0

Many stalked crinoids (e.g., Isocrinida, Phrynocrinidae) assume a wilted flower posture, with stalk more or less erect and distal arms drooping. Both stalked species and feather stars may form a funnel-like or conical non-feeding posture under slack water, either upright or drooping, depending upon local topography. However, they may also form a cone when a strong current collapses the arms orally in a trauma posture.