All living crinoids appear to be passive suspension feeders; they do not generate their own filtration current, but rely on extrinsic water movement to bring food particles to them. Until the 1970’s, understanding of their ecology was both sketchy and fanciful. Crinoids were long thought to spread their arms in an upturned bowl, subsisting on a slow rain of detritus from above (Hyman 1955, Nichols 1962). Fell (1966) reported that they entangled prey with rhythmic scooping motions of their arms. However, SCUBA, ROV and submersible observations have largely disproved both views (e.g., Meyer 1973, Macurda & Meyer, 1974) and have also shown that crinoid feeding is not completely passive. Crinoids are active participants to the extent that they modify arm and pinnule postures (and mobile comatulids and isocrinids seek preferred feeding stations) to take best advantage of prevailing and changing flow patterns and velocities (Meyer 1982a, Meyer et al. 1984a, Baumiller 1997). Although crinoids range from rheophilic "current-lovers" to species that prefer weak flow environments (rheophobic), even abyssal forms appear to depend on horizontal water movements for food rather than a rain of detrital particles.
Many ecological studies of living crinoids have been descriptions of feeding postures and strategies, because the food-gathering apparatus of arms and pinnules comprises such a large proportion of the animal's structure. However, in the comatulids and stalked isocrinids, which account for almost 90% of living species, arms and pinnules also function in locomotion, a dual obligation that requires a skeleton rigid enough to stand erect against a passing current, yet flexible enough to permit movement (Lawrence 1987, Messing & Dearborn 1990). The muscular articulations of arms and pinnules fit this requirement as follows: crinoid ligaments consist of mutable collagenous tissue (or catch connective tissue), the uniquely echinodermal material capable of altering between flaccid and stiffened states (Motokawa 1985, Wilkie & Emson 1988). Contraction of muscles on the ambulacral side of the fulcral ridge curls or rolls an arm inward toward the mouth and flexes pinnules toward the arm axis. When the muscles relax, the elasticity of the large antagonistic ligament on the aboral side of the ridge extends arms and pinnules outward. Once extended, stiffened ligaments allow the arm and pinnules to maintain an extended posture passively against a current for food gathering. Individual articulations have limited scope but an arm of over 200 segments or a pinnule with more than 50 may have great flexibility (see also Baumiller 1997).
Comatulids are the most mobile of extant crinoids and are active arm crawlers, in many cases creeping from cryptic daytime retreats to exposed nocturnal feeding perches (Meyer et al. 1984a, Lawrence 1987, Vail 1987). Also, at least some colobometrids, antedonids, atelecrinids and thalassometrids can swim with graceful and coordinated arm undulations although they appear to do so only rarely (Macurda 1973, Macurda & Meyer 1983, Shaw & Fontaine 1990). Such mobility likely contributed to the shallow-water survival of comatulids in the face of the late-Mesozoic radiation of durophagous predators that drove stalked crinoids into deeper water (Meyer & Macurda 1977, Meyer 1985, Schneider 1988, Oji 1996) (see "Predation" below).
The basic feeding
mechanism is well known although few species have been closely examined and
several significant details remain to be worked out. Much of the following is
taken from Meyer’s (1982a) and
Fine fingerlike podia (or tube feet), the terminal branches of the water vascular
system, occur in groups of three (triads) along both sides of the pinnular ambulacra. Each triad consists of a long, medium
and short (or primary, secondary and tertiary) podion.
In the photo at left, only the primary podia are visible, as rows of fine short
threads along the pinnules. A similar arrangement across five families examined
so far suggests that the arrangement is common to all comatulids, and probably
to all living crinoids (Nichols 1960, Meyer 1979, Byrne & Fontaine 1981 LaHaye & Jangoux 1985,
Holland et al. 1986). The primary podia, 0.43-0.85 mm in length (Meyer 1979,
Byrne & Fontaine 1981), alternate with flap-like lappets along the ambulacral margin and, when extended, project
almost at right angles outward from the central groove. The bases of secondary
podia are fused to the inner surface of the lappets; their contraction pulls
the lappets inward, covering the groove. In Antedon
bifida and Florometra serratissima,
secondary podia curl outward at an angle over the lappets (
When a suspended food particle comes in contact with a primary podion, the podion flicks, bends or curls rapidly inward, forcing the particle into the food groove. The shorter podia and lappets vary somewhat in function among species. In A. bifida and F. serratissima, the shorter podia (and, in F. serratissima, the lappets as well) scrape particles off the primaries and retain them in the groove; in A. bifida, secondary podia can also capture food (Nichols 1960, Byrne & Fontaine 1981, LaHaye & Jangoux 1985, Lawrence 1987). By contrast, the primary podia of O. serripinna perform all "conspicuous small-scale feeding acts" unassisted by secondary podia (Holland et al., 1986). Byrne & Fontaine (1981), Holland et al. (1986) and Leonard (1989) also describe the coordinated activity of multiple adjacent podia associated with capture of larger or mobile particles.
The primary podia at least are adhesive, but the role of mucus in food capture apparently varies among species. Several authors (Magnus 1963, Rutman & Fishelson 1969, Nichols 1960) describe food capture via entanglement in mucous threads and strands. Nichols (1960) describes the podia of A. bifida as forcibly ejecting mucous strands when contacted by a food particle (not a preformed mucous web), but La Touche contends that mucous threads "apparently do not occur in A. bifida" (personal communication in Byrne & Fontaine 1981, p. 17). F. serratissima produces mucous threads, but does not shoot them out upon particle contact. Although Byrne & Fontaine (1981) suggest that food collection via threads could be an important resource, perhaps during dense plankton blooms, they consider that direct impingement of particles on adhesive primary podia is the typical collection method, a conclusion also reached by Meyer (1982a) and Holland et al. (1986) for all comatulids. The latter authors found no mucous threads in O. serripinna.
Wiping of podia against each other and against the current generated by ciliary tracts on the groove floor wraps captured particles in mucous secretions and forms them into boluses which are then transported mouthward by the cilia (Nichols 1960, Byrne & Fontaine 1981, LaHaye & Jangoux 1985, Lawrence 1987). Holland et al. (1986) discuss three possible mechanisms of particle transport via ciliary action. Pinnule grooves run into arm grooves which converge like tributary rivers on the mouth.
The photo at right shows the oral surface, or disk, of a typical comatulid with ambulacral food grooves (AFGs) converging on the central mouth (M) and with the anal papilla (AP) off to one side.
[Modified from Messing (1997)]
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