A feather star swimming. Three groups of arms (red, yellow, blue) move separately to achieve forward motion. (Illustration by Emily Damstra)
sibly facilitating the capture of food particles by the tube feet. Sea lilies assume a similar feeding posture, although they recurve their arms almost 270° upstream to form a parabolic filtration fan. The mouth may be oriented laterally downstream, with the food grooves also turned downstream, or it may be oriented upward in slack currents. Feather stars living in low-current areas use a radial feeding posture, orienting their arms in many directions with the pinnules extended radially in four rows. The radial feeding posture serves to maximize the surface area of the feeding structures so that more particles will settle on them.
The crinoid diet consists of phyto- and zooplankton and detritus, and varies with habitat and seasonal availability. The size of the particles captured depends on the width of the food groove. The primary podium (the largest in the group of three) collects particles in the water column and folds them back into the groove. Relatively large particles are captured by podia partly curling over them; small particles adhere to the mucous layer. The podia transfer the par ticles to the food groove by brushing them away with the ciliary tract or the tertiary podia. The secondary podia behave as the primary and secondary podia do, collecting particles in the water column and folding them back into the food groove.
All crinoid species are gonochoric (although some individuals may present hermaphroditism), and they probably do not reproduce asexually. Depending on the species, the ova vary in size from 0.004-0.012 in (100 to 300 pm). The maturing oocyte enters the ovarian lumen through a temporary opening in the layer of nongerminal cells in the inner epithelium, a process called ovulation. Inside the ovarian lumen, the oocytes undergo two maturation divisions and become ova. Crinoids take 12 to 18 months to reach maturity. The gametogenic cycle usually takes one year, although in some species it takes several months and in others takes almost three years. The spawning season, the period of the year during which gametes are released, varies among species and populations of each species and can last from one hour to many months.
Sperm are released directly from the testes into sea water. Females of most species also spawn freely into sea water, but in some feather stars, the ova are retained on the outer surface of the mother's genital pinnule. In these species, the ova may be kept for days and then released, or may enter into brood pouches (where such pouches exist). Almost all crinoids develop by lecititrophic larvae (short-lived, nonfeeding, planktonic larvae called doliolaria larvae) followed by a benthic, non-feeding, stalked stage that metamorphoses to a benthic stalked juvenile. Most crinoids have only doliolaria larvae, which are ovoid with four or five transverse bands of cilia and a tuft of apical cilia. Only one species is known to have internally brooded vitellaria larvae, which lack the ciliated bands.
No species are listed by the IUCN.
Significance to humans
1. Tropiometra carinata, 2. Rosy feather star (Antedon bifida), S. Oligometra serripinna, 4. Comactinia echinoptera, S. Orange sea lily (Nemaster rubiginosa). (Illustration by Emily Damstra)
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