What we know about crinoid diets derives from analyses of gut contents and fecal material. Diets include a variety of protists (e.g., diatoms, dinoflagellates and other unicellular algae, foraminiferans, radiolarians, tintinnid ciliates), invertebrate larvae (e.g., veligers), small crustaceans (copepods, ostracods), and detrital particles. However, these results likely represent an incomplete picture of what actually provides nourishment. Gut contents often contain clearly indigestible particles such as sediment grains and sponge spicules, indicating that particle capture is non-selective and may not reflect digestibility. Crinoids may also capture naked plankton such as oligotrich ciliates that may be removed by digestion or rendered unidentifiable in the feces. However, Holland et al. (1991) found that particles ingested by the colobometrid Oligometra serripinna travel rapidly through the gut and, by an hour after ingestion, accumulate in the extreme hind end of the intestine and rectum where most digestion apparently takes place. They also suggested that, at least in the generally low particulate organic carbon (POC) environment of coral reefs, gluttonous feeding during brief episodes of greatly elevated POC (e.g., during spawning of other invertebrates) might represent a significant component of crinoid nutrition.
Detritus, which constitutes a significant component of some crinoid gut contents (La Touche and West 1980, Featherstone et al. 1998), offers additional difficulties in assessing diets. Microbial populations in the detritus may provide substantial nourishment, but their contribution remains unquantified. Also, captured detrital material may not be distinguishable from fecal material produced by the crinoid. Uptake of dissolved nutrients has been documented in crinoids (West 1978, Smith et al. 1981), but its contribution to nutrition likewise remains unknown.
Identifiable dietary components vary substantially among crinoid species. As examples, ciliates, forams and radiolarians contribute 50-87% of Lamprometra klunzingeri (=L. palmata) gut contents (Rutman & Fishelson 1969, Meyer 1982b); chiefly fecal, re-suspended detritus makes up 53-85% of Antedon bifida food (La Touche & West 1980), and diatoms and dinoflagellates contribute 54-57% of fecal samples of Oxycomanthus bennetti and Pontiometra andersoni (Meyer 1982b). Differences may reflect variable availability (seasonality, locality and activity rhythms), tube foot morphology and spacing (Meyer 1979 1982a b), and ambulacral groove width (see below). In the stalked crinoids Neocrinus decorus and Endoxocrinus parrae, Featherstone et al. (1998) found detritus to contribute 59.2-69.0% by gut content area in all seasons sampled, far outweighing copepods (12.0-24.4%), the next most important category. Radiolarians were the most abundant food item by particle count excluding detritus (46.0-59.0%). Although these two species have apparently different filtration morphologies (N. decorus has fewer, shorter, more widely spaced arms and feeds higher above the substrate than E. parrae), their diets do not differ significantly. Still, Meyer (1982b) noted that significant variability exists among fecal samples from different individuals of the same species in a local population taken on a single dive, reducing the usefulness of interspecific comparisons. Rank abundance and presence/absence data remain useful, nevertheless.
Food particle size also varies among species with the great majority of items falling between about 20 and 150 μm. Leonard (1989) successfully fed Antedon mediterranea coccolithophores >11 μm across. Several authors have contended that ambulacral groove width sets the upper size limit of particles that crinoids can successfully retain (Fell 1966, Rutman & Fishelson 1969, La Touche & West 1980). Yet, numerous studies record items larger than groove widths. According to Rutman & Fishelson (1969), L. klunzingeri has a groove width of 162 μm, yet 26% of its gut contents measure >200 μm across with a few particles exceeding 500 μm. Unfortunately, these authors measured particles along their longest axis. It also appears likely that their groove measurement represents a contracted state. Meyer (1982b) recorded a 200-μm groove width in L. palmata with the maximum least dimension of virtually all particles <200 μm. This measurement provides a more accurate assessment of food particle size limits, because it accounts for accommodation in the groove of long thin objects. Nevertheless, in Capillaster multiradiatus, Meyer (1982b) also recorded items 350-550 μm in maximum least dimension, clearly larger than groove width. Similarly, La Touche & West (1980) noted that A. bifida (in aquaria) could convey particles substantially larger than groove width (to 1 mm diameter) to the mouth in still water. Stalked N. decorus and E. parrae have groove widths of 240 and 290 µm, respectively, and ~90% of food particles were ≤200 µm (Featherstone et al. 1998).
Meyer suggested that variations in length and spacing of primary podia among species contribute to dietary differences (1979, 1982a) and that longer primary podia may permit capture of larger particles (relative to co-occurring species with similar groove widths), or that ambulacra may stretch to accommodate larger particles in some species (1982b). Another possibility is that wider arm grooves in some species may capture a greater proportion of particles than in other species that use only their pinnules. The 50-µm difference in groove width between N. decorus and E. parrae, does not appear to contribute to dietary differences, however (Featherstone et al. 1998).
[Modified from Messing (1997).]
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