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Aquarium.Net Feather Stars Dec 96

This month Ron discusses Feather Stars, December 1996 Index for Aquarium Net, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

Feather Stars - Fragile Beauty

by Dr Ron Shimek

Undoubtedly, the most beautiful of the echinoderms are the feather stars and sea lilies, or crinoids. They all have a relatively small central body surrounded by 10 to 200 arms which have fine lateral branches giving the arms the appearance of feathers. Shallow-water crinoids are generally taxonomically placed in an group called the Order Comatulida of the Class Crinoidea in the Phylum Echinodermata. The comatulid central body, or calyx, has a few tendril-like cirri on the underside which the animal uses to grasp the substrate. A few comatulids are found in deep waters, but below depths of about 300 m, another type of crinoid, the sea-lily, is found.

All modern sea lilies are in the Order Isocrinida, and have a stalk extending from the undersurface of the body. Their cirri are found on the stalk and are also used to grasp the substrate, however because of the large size of the stalk relative to the rest of the animal, sea lilies are practically immobile. The sea lilies are exceptionally abundant in tropical waters, but are always found at relatively great depths so they are seldom seen. This is a pity, as not only are sea lilies beautiful, they really are living fossils and a representative of the fauna found in the ancient seas of the earth. This group reached its heyday about 100 million years BEFORE dinosaurs walked the Earth. Crinoid parts are particularly common in many rocks of Paleozoic Age, and commonly collected by amateur fossil hunters.

A sea lily or stalked crinoid collected by a research submersible from a depth of 300m off of the Bahamas

For a good overall look at crinoids including fossils, explore these links: (this is a commercial site, but has illustrations of superb crinoid fossils).

There are about 600 species of modern crinoids, and about two-thirds of them are comatulids. Feather stars are common reef inhabitants and are particularly diverse in the Indo-Pacific, where they are found in relatively high densities, but they are also found commonly in the Caribbean, although there are only a few species found there. Although uncommon on the Atlantic coast of North America, one species, Florometra serratissima , is plentiful in spots along the Pacific Coast where aggregations of several thousand can sometimes be found, particularly along the coast of British Columbia.

. The NE Pacific comatulid or feather star, Florometra serratissima , this species only has 10arms, and the cirri used to grasp the substrate are visible.

Generally coral-reef crinoids are moderate in size organisms, with a span across the extended arms that ranges from 30 to 60 cm. They hang on to various reef substrata with their cirri and extend their arms out in the current to catch plankton. They are good echinoderms and possess the water-vascular system and tube feet that are characteristic of this phylum; however, unlike most other echinoderms, they do not use the tube feet for locomotion. If they need to move they generally do so slowly, by the deliberate grasping of the substrate by the cirri. If they need to move more rapidly, perhaps to escape a predator, they release the grip of the cirri and, using a particularly beautiful undulatory motion of the arms, they propel themselves up into the water, swimming for short distances.

This swimming is undirected and results in random movement, as they lack eyes to see where they are going. Crinoids, like all other echinoderms, lack even a rudimentary brain, and the nervous system is difficult to see. Nonetheless, they have good sensory capabilities, with millions of small sensory cells located throughout there skin. How the information that they receive is processed, and utilized, however, remains unclear.

Feeding - The Reason for Reef-keepers' Problems.

In a reef-aquarium, crinoids often seem to behave normally, at least for a while. Then, almost invariably, they gradually disintegrate, from the tips of the arms inward, until they finally die. These animals are simply starving to death. Maintenance of many suspension-feeding organisms is difficult in reef aquaria. All suspension-feeding animals, ranging from feather-duster worms to flame scallops to tunicates have specific requirements for their food. In some instances, the hobbyist is able to provide either the appropriate food or an acceptable analogue. In most cases, however, this is not the case and such animals often perish in our systems. Crinoids have a peculiar feeding mechanism, and seem exceptionally "picky" about their food. Consequently, they are exceptionally difficult to maintain in captivity.

Feather-stars feed by capturing minute particles of planktonic food. The natural diets have been investigated in only in a few species, and in these most of the food seems to consist of invertebrate larvae, microscopic animals, and protozoans. As is common in so many filter-feeding organisms, they utilize a combination of cilia and mucus to assist in either food capture or movement of the food to the mouth.

Ciliary-mucous suspension-feeding is probably the most widespread feeding type in the marine environment; almost all phyla have some representatives that feed in this manner, and many species only can feed in this way. Mucus is secreted by gland cells, and cilia are used to move this mucus. Mucus is often taken for granted, but this material is so ubiquitous, that it can truly be said that without mucus, life itself would not be possible... Chemically mucuses are called glycoproteins, or chemical combinations of sugars and proteins. Generally mucuses are sticky and viscous, although the latter property can often be varied by varying the ph of the medium.

Cilia are small, undulating processes located on the surfaces of cells. Although they are often called hair-like, they are in point of fact, many thousands of times smaller than a human hair. Their motion results from the movement of slender protein fibrils located in them. In feather stars and many other ciliary-mucous filter feeding organisms, cilia can be found lining depressions of the body called food grooves. Mucus is secreted by the glandular cells lining the food grooves and moves in flowing stream toward the mouth, with the movement of the mucus due directly to the underlying cilia.

The food transport system of the crinoids extends out on to the arms, or the "feathers," of the feather star. It subdivides and continues out into the lateral small projections off the main arm and consists of a mucous-filled ciliated food groove which connects all the way to from the finest arm branch to the mouth on the upper surface of the body. Out on the fine branches, or pinnules, of each arm, the food groove has a row of tube feet on each side of it.

Close up of the rays of a tropical crinoid, the tube feet are visible extending upward from some of the pinnules or side branches of the rays

Crinoids utilize their tube feet in a unique feeding method to catch their food. In all other echinoderms, each tube foot works independently of its neighbors and is used in locomotion. Crinoid tube feet neither work as independent units nor are they used in locomotion. Rather they are used only for feeding, and they work as groups of six, divided into two groups of three tube feet each, on opposite sides of the food groove. These tube feet extend directly up from the animal's surface into the water.

Each of the groups of three tube-feet consists of a long, a medium, and a short tube foot. Each individual tube foot has a different feeding function. When a long tube foot is hit by an acceptable food item (generally a larva or a ciliate), it flicks, knocking the item toward the other side of the tube foot group. The middle tube foot on the other side also flexes or flicks knocking the item toward the groove and finally, the small tube foot on the original side flexes to knock the item into the food groove, where it is conveyed to the mouth by the sticky mucus. All six tube feet work together to ensure that food particles get knocked down into the groove.

This active feeding method is significantly different from most other suspension-feeding organisms which passively filter food from the water and then sort it somehow after it is collected. Crinoids can be considered to make an active choice to feed on each particle that impacts against a tube foot. Presumably, these choices are made based on the particle size, density, and chemical composition or flavor. In any case, however, it means that these animals can be exceptionally selective in their choice of food.

A tropical diurnal feather star from the shallow waters of Palau.

The selectivity of the feeding process ensures that the animal gets the maximum benefit for the minimum expenditure of energy. However, it also means that an aquarist who wishes to maintain these beautiful animals has to provide a food that passes all of the crinoid's criteria for food. And, it appears in most cases, this may well be impossible. Unlike the other suspension feeders, which can utilize substitute food, crinoids generally do not even recognize the substitutes as food.

There may be a solution to this problem, however. Culturing of microplanktonic organisms such as rotifers might provide a useable food source for at least some of the crinoids. Additionally, annelid, bivalve, and gastropod larvae are available commercially for industrial use in aquaculture and for environmental bioassays. These are temperate or subtropical larvae that might be acceptable substitutes for tropical species that are eaten by crinoids. These larvae would also be of benefit to many other reef organisms that feed on microplankton, such as small-polyp scleractinian corals. Unfortunately, with the exception of rotifers, none of these other microplanktonic organisms are accessible to the average hobbyist.

Other Aspects of Crinoid Biology

Supplying appropriate food and in appropriate quantities is the key for maintenance of all animals, but can be especially difficult with regard to crinoids and some other echinoderms. Another particular aspect of their biology has to be considered, and that is the necessity of maintaining them under full strength sea-water salinity. Compared to corals and some other animals kept by aquarists, crinoids have a relatively high metabolic rate. Most of this metabolic energy is spent in maintaining an acceptable internal ionic balance in their water-vascular system, and their energy needs in this regard can be substantial. Crinoids are not able to cope with low salinity conditions, and must be kept in aquaria where the specific gravity is no lower than 1.025, and preferably about 1.026. If they are kept in lower salinity water, they will be unable to get enough energy, even if well-fed, to regulate their internal ionic balance, and they will die.

Maintaining crinoids in captivity should be fairly easy once the problem of finding an acceptable food source is solved, and in fact breeding them in captivity should also be relatively straight-forward. Crinoids produce non-feeding larvae. After a short planktonic existence, these larvae metamorphose into miniature sea lilies. After a brief period of growth, the stalk breaks off, and the small comatulids take up life as almost microscopic versions of the adults, and they eat the same foods as adults. Given the appropriate foods, there is really no reason why the complete life cycle could not occur in a hobbyist system.

There are a wide variety of crinoids that might live in reef aquaria, but the choice of the appropriate ones will have to be done with some knowledge of the animal's normal life style. Activity periods of these animals are variable. As with fishes and some other reef-dwelling animals, there appear to be three types of crinoids; day-active species, night-active species, and those species that are crepuscular and active around dawn and dusk. Night-active crinoids tend to seek shelter and will remain largely invisible during the day, and the crepuscular species will only be visible for a few hours each day.

A yellow comatulid species that becomes active about an hour before dusk. During the day this individual was found curled up into a ball.

The same species as above, photographed in late evening. This individual was about 60 cm across the arms.

Generally, crinoids prefer vigorous currents, and will tend to seek out those areas of the reef tank where the strongest currents are found, even to the extent of climbing onto the openings of power heads. In nature, they are often found on projections and objects that extend well up off the bottom into areas of currents beyond the benthic boundary layer of sluggish water movement. Because of this behavior, they should be kept in deep tanks.

A multicolored day active crinoid from Yap Island. It is about 60 cm across the arms.

Additionally, while they are common coral reef animals, they are not generally found in the areas of brightest illumination. Some temperate echinoderms, such as the Northeast Pacific basket star, Gorgonocephalus eucnemis , seem to be exceptionally sensitive to bright lights and will disintegrate in brightly lit aquaria. Some tropical crinoids may do the same, and will probably do best in aquaria that are not brightly illuminated.

Cenometra bellis , a white feather star from Palau, attached to a whip coral. This animal was about 30 cm across

Like all other animals, feather stars are subject to parasitism. The most common parasites are rather strange, highly modified annelid worms. These bizarre little worms are called myzostomids. Myzostomes are discoidal and generally no bigger than a millimeter or two in diameter. They have ten or so small hooks on their bottom surface which they use to attach to crinoid arms. They have a long tubular proboscis, and live by straddling a food-groove and slurping up the collected goodies coming past on the ciliary-mucous conveyor belt. In nature, one seldom finds more than one or two of these on a feather star, and they probably do not harm the host much. In the food limited environment of the reef aquaria, they might be able to siphon off enough food to cause severe problems. Myzostomes can be found on the feather star by examining the animal in a shallow bowl, using a 10x hand lens or magnifying glass. They can be removed by using fine-tipped or watch-maker's forceps.

Three different comatulid species attached to a gorgonian off a wall in Palau. Each of the crinoids was about 45 cm across

For some other crinoid illustrations, examine the images at these links:


Byrne, M. and A. R. Fontaine. 1981. The feeding behaviour of Florometra serratissima (Echinodermata: Crinoidea). Canadian Journal of Zoology. 59:11-18.

Macurda, D. B., Jr. and D. L. Meyer. 1983. Sea lilies and feather stars.American Scientist. 71: 354-365.

McClintock, J. B., J. L. Cameron, and C. M. Young. 1990. Biochemical and energetic composition of bathyal echinoids and an asteroid, holothuroid, and crinoid from the Bahamas. Marine Biology 105: 175-183.

Meyer, D. L. 1982. Food and feeding mechanisms: Crinozoa. In: Jangoux, M., and J. M. Lawrence (Eds.): Echinoderm nutrition. A. A. Balkema, Rotterdam. pp. 25-45.

Meyer, D. L., C. A. LaHaye, and N. D. Holland. 1984. Time-lapse cinematography of feather stars on the Great Barrier Reef, Australia: Demonstrations of posture changes, locomotion, spawning, and possible predation by fish. Marine Biology. 78:178-184.

Mladenov, P. V. and F. S. Chia. 1983. Development, settling behaviour, metamorphosis and pentacrinoidal feeding and growth of the feather star, Florometra serratissima . Marine Biology. 73:309-323.

Rutman, J. and L. Fishelson. 1969. Food composition and feeding behavior of shallow water crinoids at Eilat (Red Sea). Marine Biology. 3: 46-57.

Young, C. M. and R. H. Emson. 1995. Rapid arm movements in stalked crinoids. Biological Bulletin 188: 89-97.

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Last modified 2006-11-23 01:37