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).]
References
Featherstone, C.M., Messing, C.G. & McClintock, J.B. 1998. Dietary
composition of two bathyal stalked crinoids: Neocrinus decorus and Endoxocrinus
parrae (Echinodermata: Crinoidea: Isocrinidae). Pp. 155-160. IN: Mooi, R. & Telford, M. (eds.) Echinoderms:
Fell, H.B. 1966. Ecology of crinoids. Pp. 49-62. IN: Boolootian,
R. A. (ed.) Physiology of Echinodermata.
La Touche,
R.W. & West, A.B. 1980. Observations on the food
of Antedon bifida (Echinodermata: Crinoidea). Marine Biology 60:39-46.
Leonard, A.B. 1989. Functional
response in Antedon mediterranea (Lamarck) (Echinodermata: Crinoidea): the interaction of
prey concentration and current velocity on a passive suspension-feeder. J. Exper. Mar. Biol. Ecol. 127:81-103.
Messing, C.G. 1997. Living Comatulids. Pp. 3-30 IN: Waters, J.A. & Maples,
C.G. (eds.) Geobiology of Echinoderms. Paleontological
Society Papers 3.
Meyer, D.L. 1979. Length and spacing of the tube feet in crinoids (Echinodermata) and
their role in suspension-feeding. Marine Biology 51:361-369.
Meyer, D.L. 1982a. Food and
feeding mechanisms: Crinozoa. Pp. 25-42. IN: Jangoux, M. and Lawrence, J. M. (eds.) Echinoderm
Nutrition. Balkema,
Meyer, D.L. 1982b. Food composition and feeding behavior of sympatric species of
comatulid crinoids from the
Rutman, J. & Fishelson, L. 1969. Food composition and feeding
behavior of shallow-water crinoids at Eilat (
Smith, D.F.,
Meyer, D.L. & Horner, S.M.J. 1981. Amino acid uptake by the
comatulid crinoid Cenometra bella
(Echinodermata) following evisceration. Marine Biology
61:207-213.
West, B. 1978. Utilisation of dissolved glucose
and amino acids by Leptometra phalangium (J. Müll.).
Sci.
Proc. Royal