Crinoids: Sea Lilies, Feather Stars: Often misidentified as brittle stars or entirely overlooked as hidden or decorative ornaments due to their plant-like appearance, immobile/anchored sea lilies and the related, mobile feather stars make up the most ancient class of the phylum echinodermata, the Crinoidea (cry-noi-day-ah).
Due to easy breakage, suspension feeding habits and shy and retiring behavior these animals are considered challenging to keep. Most are doomed from difficulties accumulated from the time of their collection to delivery to your outlet. Feather stars need not be impossible however, as you will see.
Classification: Taxonomy, Relation With Other Groups, Morphology
The scientific class name is derived from the Greek, krinon = lily & oidos = like. They are (for the phylum Echinodermata) typically radially symmetrical, resembling...Feather Duster Worms (!), with from 5 to 200 arms.
Crinoids are construed to be the most ancient class of spiny-skinned animals, with fossil forms dating back to the Paleozoic, before Urchins, Sand Dollars, Sea Cucumbers, Sea or Brittle Stars. They are further removed from the other living Classes of Echinoderms in having a mouth oriented away from the bottom (aboral), and having at least an early life history as attached juveniles. As young their bodies are composed of an attached stalk; this condition persists for Sea Lilies, but Feather Stars break off, become stalk-less and lead "free-living" (mobile) life styles. The internal skeletal ossicles make the stalk appear jointed. Many have cirri on the end of the stalk to grasp the substrate. Growth of the stalk is from the crown end where the numerous arms come together. (see images)
There are about 550 living species of Crinoids, 80 of the more primitive stalked Sea Lilies, and the bulk in the more modern branch of the living Class, the Order Comatulida, the Feather Stars.
Crinoids are found worldwide in marine habitats, sometimes in great numbers from intertidal zones to abyssal depths, primarily in Indo-Pacific and polar waters.
Selection: General to Specific
Most specimens will not "make it" at the penultimate (retailer) end-user source due to traumas encountered in collection and transport & starvation. These animals should not be touched or moved unnecessarily... it is possible to nudge them along or even move them by getting the specimen to "perch" on a wood dowel, net handle to move underwater in a sealed container.
Due to the vagaries of cost and current collection and transport techniques the stalked, attached sea lilies themselves are rarely offered, and when they do show up are generally already dead from being thrashed about. Therefore we are going to, & want to, run into free-living Feather Stars when we talk about crinoids.
What to look for Many and intact arms, actively feeding. Once a specimen has been selected, it should not be touched anymore than necessary. Tube feet, pinnules and whole arms snap off easily. With a shallow-depth net or spatula-shaped tool, coax/brush the animal into the shipping container (bag) underwater.
Free-living crinoids need hiding spaces, out of the light, to be comfortable. Many hide during the day, coming out to feed only at night. Some have been trained to day-feed.
I suggest varying the living space to allow for selection for depth & current. Most species are positively rheotactic, orienting themselves in the path of flow to effectively filter feed.
Proper water quality has been cited as a preventer of autotomy (self-breakage) of crinoid arms. High concentrations of organics, sudden changes of pH, salinity & temperature have been linked to loss of arm segments.
Mixing differing species and over-crowding of the same species is to be avoided.
Should be done in subdued lighting in well-established systems only. Leave lighting off for at least a day.
Feather Stars do not appear to have or to make intentional enemies; beware of curious "numbskull/autistic/will-try-anything-twice species, e.g. Crabs, Lobsters, Octopi, Triggers, Puffers...
Feather Stars are opportunistic sharers of space/habitats with other species, found on and in sponges, corals, all manner of slots and crannies. There may be a danger when/if uninitiated swimming species/individuals alight on stinging-celled landing pads.
Similar to Sea, Brittle and Basket stars, crinoids can regenerate lost arms, pinnules or cirri.
All species are separate sexes (dioecious with no visible structural differences (dimorphism) between males and females. There are no distinct gonads. When gametes are mature they rupture (yowch!) through the pinnule walls.
Crinoids have a non-feeding, free-swimming vitellaria larval phase similar to that of sea cucumbers. The larva settles to the bottom, attaches itself and develops into a miniature sea lily. Feather stars pass through this stalked stage as pentacrinoids for a few to several months before breaking off at the crown and becoming mobile.
Sea Lilies are sessile, attached firmly to the substrate, and can only move as much as their stalks can bend. Feather Stars can crawl and swim. When moving the oral surface is always up. Swimming varieties move by stroking their arms in various patterns by the species. Neato. Crawling is done by lifting the body off the substrate so only the tips of the arms are touching.
Feeding/Foods/Nutrition: Types, Frequency, Amount, Wastes
The arms/branchlets bear pinnules that convey food via seemingly twitching their tube feet toward and across an elaborate conveyor system. During feeding the arms and pinnules are outstretched and their podia are erect. The papillae along the length of the podia are secreting mucus, plankton becomes trapped in the mucus and the podia tosses it into the ambulacral groove where cilia carries it to the mouth. The mouth in turn leads to a short esophagus then intestine. The intestine makes one or more turn down around the aboral side of the animal then turns around and heads back up to a short rectum and anus. Waste is ejected as mucus balls which drop from the anal cone. The whole of this digestive tract is lined with undulating cilia.
Most species are nocturnal suspension feeders. You will want to have timers to cut off your particulate filter(s) during these feeding bouts. Best done by using timers to switch the filter pump motors off for ten to fifteen minutes.
Food material in the wild includes all manner of phyto- and zooplankton and general detritus. In public aquaria, cultured diets of brine shrimp nauplii, copepods and diatoms have proved effective.
Disease: Infectious, Parasitic
Other Biology of Interest:
Though the size of extant forms is a few inches to a couple of feet in dimension, that of some extinct crinoids was very great. One (Extracrinus subangularis had a stalk of nearly 21.5 meter! in length. Yowzah.
By any visit to your friendly neighborhood or national natural history museum you can gain an appreciation for how species-rich and dominant crinoids were in reef communities of the geological past.
The un-stalked, commercially available Feather Stars are oft brilliantly colored/marked and not impossible to keep. They are for the most part entirely non-obnoxious, being non-predatory, not-tasty, and disease-free; perfect reef tank candidates.
Thought to be extinct prior to dredging studies by the Challenger expeditions of the 1870's, free-living crinoids are difficult but not impossible aquarium specimens. Requisite are selecting healthy individuals, providing subdued lighting and hiding spaces and an accommodating crepuscular to night time feeding routine.
Barnes, Robert. Invertebrate Zoology. Saunders, Orlando. 1987. 5th ed.
Fenner, Bob. 1993. More spiny-skinned animals. Crinoids: Sea Lilies and Feather Stars. FAMA 10/93.
Fenner, Robert M. 1998. The Conscientious Marine Aquarist. Microcosm, VT. 432pp.
Herwig, Nelson. Starfish, Sea Urchins & Their Kin. RCM (FAMA) 1981
Kloth, Tom. 1981. Basic Invertebrate Anatomy & Terminology V. Echinodermata. FAMA 2/81.
Lau, Colin J. 1989. Lilies of the (Ocean) Valley: Crinoids in Aquaria. FAMA 5/89.
Lawrence, John A. A Functional Biology of Echinoderms. John Hopkins U. Press, Baltimore. 1987.