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Sea Stars By Ron Shimek Aquarium.Net July 1997

Ron Shimek writes on starfish that are suitable for reef aquariums, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

Sea Stars

By Ron Shimek

One of the enduring images of the marine environment is that of a starfish or sea star on a beach or in a marine vista. Sea stars are found only in the marine environment and really are characteristic of that environment. Consequently, very few marine aquarists can refuse the lure of having a starfish in their aquaria as such an animal really DOES define a marine aquarium. This is a pity, as relatively few stars are good aquarium animals.

Sea stars are members of the class Asteroidea in the Phylum Echinodermata. Echinoderms are rather bizarre animals, and the sea stars are good representatives of the wackiness of the group. Unlike most advanced animals which are bilaterally symmetrical and have a head at one end, and an anus at the other, echinoderms are radially symmetrical without a head at all. The mouth is found centrally on the underside of the center part of the body, called the oral surface, and the anus is found more-or-less centrally on the top of the animal which is referred to as the aboral surface. As these animals have no head, they have no "front" end, and can move in any direction that whim takes them.

There are about 1500 species of sea stars. Asteroids always have a flexible body, however there is a lot of variation in the relative flexibility. Stars are characterized by an oral-aboral flattening and are generally considered to be strongly pentaradial, that is having a symmetry dominated by a pattern based on five. I think this consideration of strong pentraradiality is expressed only by biologists who have studied the rather pathetic asteroid fauna found in the North Atlantic. In the N. E. Pacific, there are about 35 species of relatively common sea stars, there are species that have 6, 7, 8, 9, 11, 13, 15, and 22 to 27 arms (Lambert, 1981; Kozloff, 1987). I have seen populations of one supposedly five-armed species, the bat star, Asterina miniata, from southern Vancouver Island, where individuals with 3, 4, 5, 6, 7, and 8 arms were all about equally abundant.

A Bit of Physiology

Along with no head, sea stars lack a lot of the standard structures that most folks associate with a head, such as good sensory organs and a brain. Echinoderms, in general, are the most brainless of the advanced invertebrates. Their nervous system is comprised mostly of a network of very fine nerve cells located throughout the body. A few larger specialized nerves are found, mostly associated with the locomotory system. Nonetheless, no ganglia, swellings, or associative regions are known. That having been said, it is amazing that they do have relatively complicated behavior. One echinoderm researcher has postulated that while no one structure in their nervous system is a brain, perhaps the whole nervous system functions as a brain.

The main feature that sets all echinoderms apart from all other animals is the presence of a relatively high pressure hydraulic system that they use in locomotion and food capture. This system is called the hydrovascular or ambulacral system. Basically it is a set of internal pipes and valves (all constructed out of very fragile tissues) that use water to extend and contract structures called tube feet. Tube feet are extensions of the final branches of the hydrovascular system and they are covered with the body wall and some musculature. They are used in feeding or in locomotion.

The presence of the hydrovascular system is probably one of the major limitations for keeping asteroids in the home aquarium. Most echinoderms simply cannot tolerate much fluctuation in the salinity of their environment and, furthermore, most of them are adapted to living in fully marine conditions, with the salinity at 35 to 36 parts per thousand (or about a specific gravity of 1.026 at a normal reef temperature of 84° F).

Water does not enter the water vascular system passively. It is actively pumped into the water-vascular system through a complex and still incompletely understood metabolic pathway. In doing so, it moves through a rather tortured pathway, crossing several delicate membranes, and passing through some rather filmy tissues. Fluctuations in salinity can cause these membranes and tissues to be ruptured or torn, and this will kill the animal... often very slowly, and often several days to weeks after the damage has occurred.

Additionally, the aboral surface of stars are often covered with small delicate projections called dermal gills which are considered to the primary site of gas exchange. These are thin, filmy projections of the main body cavity out of the animal through holes in the body wall. They are covered with thin tissue and are filled with fluid from the body cavity which is moved into and out of them by cilia. These structures are also subject to damage during periods of salinity fluctuation.

Consequently, although sea stars can be exceptionally tough animals under certain conditions, they simply cannot tolerate fluctuations of salinity at all well.

Many asteroids have spines on their aboral surface, probably the best example of this is the Crown-of-Thorns star, Acanthaster planci . Additionally, many, but not all, sea stars have structures called "pedicellariae" (the singular of this awful term is "pedicellaria"). These are modified spines that have two to four ossicles that are modified to be jaw-like pinchers. In those asteroids that have them, they are generally found on the upper (aboral) surface and seem to be used to pinch animals that touch or land on the surface of the animal. The pedicellariae of a few species, for example, Stylasterias forreri , are highly modified and derived to form miniature jaws with recurved fang-like projections. Stylasterias can use these jaws to catch small animals that settle near the pedicellariae. The unfortunate prey item, often a small fish, is carried slowly over the surface of the star, being passed from pedicellaria to pedicellaria and is finally stuffed into the mouth (Chia and Amerongen 1975).

When asteroids ingest food, the prey are processed by a relatively simple gut. Inside the mouth is a relatively spacious bag-like portion of the stomach, called the cardiac stomach, where most digestion occurs.

A close-up of the aboral surface of Pycnopodia helianthoides. Note the pincer-like pedicellariae and the frilly dermal gills.

Digestion is mostly extracellular in echinoderms.

Next in line, moving aborally, is found the pyloric stomach, which is an area where the digested food is moved into storage organs called pyloric caeca which extend down into the arms. There are generally two pyloric caeca per arm, but they arise from a single tube extending from the pyloric stomach into each arm. Food is stored in these organs as fats and complex sugars. The short intestine leaves the pyloric stomach and connects to the small rectum which gives rise to the anus. The intestine seems to primarily concerned with fecal compaction. The anus is generally located near the center of the aboral disk. There are small intestinal or rectal glands of uncertain function located at the junction of the intestine with the rectum. A few sea stars have an incomplete gut, and need to expel the uneaten food residue out the mouth.

Asteroids have legendary properties of injury repair and healing, being able to reproduce, as the legend goes, from a single arm tossed into the sea. Well... Like a lot of legends, this one has its basis in some fact, but their is a great deal of variability in asteroid regenerative capabilities. While many can repair injuries relatively easily, and some do have significant regenerative capabilities, most badly damaged asteroids die, succumbing to infection or predation. What asteroids do have though, is immortality. They have no old age or senescence and barring accidents, predation, starvation, and disease, they can live forever.

This brief account of starfish anatomy can be supplemented by the treatments in invertebrate zoology textbooks such as that by Kozloff (1990) or Ruppert and Barnes (1994).

Several downloadable images, in the .gif format, showing anatomical features and internal organs are available at the following site: /catquery.htm?Class=Asteriodea&Sujet=zoo

Some Natural History and Ecology

Most asteroids are carnivores and there is no more ecologically important group on any of the world's ocean bottoms. Wherever they have been studied, asteroids have been found to regulate and structure the benthic community. Many of them are so-called "keystone" predators (Paine, 1966, 1974; Mauzey, et al. 1968; Engstrom, 1974; Paine and Levin, 1981; Suchanek, 1979; Duggins, 1983). Keystone predators are predators whose abundance and actions can alter the community from one stable extreme to another. An example of such action would be predation by the Crown-of-Thorns star, Acanthaster planci .

At one time, the outbreaks of Acanthaster were looked upon with severe alarm. This predator eats coral, and a large population of the stars has the potential of removing all living coral from large portions of reefs. Acanthaster has often found in great numbers and it was hypothesized that these numbers were due to pollution or some other human action and it was feared they would literally eat the reefs away. It has been now shown that Acanthaster predation doesn't devastate reefs; the animals tend to eat a small area of coral and wander some distance before eating again. When there are large aggregations, they do eat a lot of coral - but, they leave a lot uneaten as well. This action has the effect of opening a lot of patches on the reef for other corals to recruit into by the settlement of their larvae.

The corals that the star eats are often the dominant competitors for space on the reef and by removing them, the star preserves and maintains coral diversity. The large numbers of Acanthaster have been shown, in an elegant paper by Chuck Birkeland of the University of Guam, to be caused by periods of heavy rainfall. These heavy rains cause significant runoff and nutrient enrichment in coral lagoons. This, in turn, causes a plankton bloom, which feeds Acanthaster larvae swimming in the lagoon. It generally takes the stars about three years to grow from larval size to the size where they will come out and forage during the day, and be noticed. Most Acanthaster outbreaks can be shown to occur about three years after a period of abnormally high rainfall (Birkeland 1982). So, it can be said, that coral reef diversity is due to heavy rains - as mediated and modified by sea stars. Incidentally good evidence of has been found in cores drilled in coral atolls indicating Acanthaster outbreaks have been occurring for over 5 million years... and we still have reefs.

In addition to the active predation by the use of pedicellariae as mentioned above, there are several other ways in which asteroids can feed. The first of these is called cardiac stomach extension. Here the star extends its stomach into a prey item such as bivalve and digests it, on the half shell, as it were. This is done by the common intertidal and shallow subtidal sea stars on both the Atlantic and Pacific coasts. Pisaster on the Pacific coast and Asterias on the Atlantic coast both feed on clams in this manner. Many people think that this is the only way that sea stars feed.

There are many other ways in which sea stars feed. For example, prey canalso be taken internally. One North-Eastern Pacific star, Pycnopodia helianthoides , eats anything it can catch by stuffing it into its stomach. And this star can catch almost anything (Mauzey et al., 1968)!

The giant N. E. Pacific sunflower star, Pycnopodia helianthoides . This species can reach 1.5 m (about 5 feet) in diameter. Definitely not for the home aquarium.

This particular species gets large; it has been claimed that Pycnopodia helianthoides are the largest sea stars in the world. They have been documented with a diameter greater than 1.5 m, and most of them are over 80 cm in diameter. I have found them eating mussels (one had over 200 small mussels in it), sea urchins (the star had eaten the urchin with spines and all - and this was an urchin over 40 cm across the spines), fish, crabs, and diving sea birds!! I have no doubt, by the way, that the star caught the bird, held on to until it drowned and then ate it.

Pycnopodia can easily move over the substrate at speed exceeding 10 cm per second (roughly 18 feet per minute). When moving at high speed, the animal has its leading arms, about 8 of them (this species typically has 24-28 arms) leaned up and reflected back over its aboral surface. On contact with a prey item it throws those arms down on top of the prey and hangs on. If such a star contacted a diving bird that was groveling in the bottom, such as a common merganser, the bird would find itself being held onto by upwards of 8,000 tube feet anchoring it to a 15 kg sea star. The odds are that the bird would drown. Incidentally, the birds are not watchful when feeding. I have been able to swim up to one such bird and tap it on the tail feathers whilst it had its head down under ledge. It gave a good impression of a submarine launched missile... A bird caught in the same situation by the star would become starfish food.

Information about many temperate sea stars can be gotten from these links. Gen_Critters/OchreStarfish/och_star.html Gen_Critters/SunStar/sunstar.html Gen_Critters/batstarfish/bat_star.html

Still, other stars extend their extruded cardiac stomach over substrate and then proceed to digest what is in contact with the stomach. This is how Acanthaster feeds. Apparently Acanthaster gets stung, and stung rather severely, by the nematocysts of its preferred prey, various SPS corals.

The star will extrude its cardiac stomach, and hold it out in front of itself with its tube feet and then lurch itself over its coral prey. When it does this, it appears to take some care to avoid touching parts of the prey with anything other than the stomach. It then digests the coral out if its skeletal cups.

Acanthaster planci , the crown-of-thorns star. This individual was about 45 cm across and was eating Acropora.

Still other stars, such as the blood stars of the genus Henricia will sit with their arms raised off of the substrate, and secrete mucus strands from the grooves between the rows of tube feet. After a short period, they use their tube feet to

This small Indo-Pacific star ( Mithrodia sp.) is rather common in some areas in Palau. The diet is unknown, and it is probably not suitable for home aquaria.

collect the mucous, with any adherent plankton, which they then eat.

The diversity of foods used by, and the variety of the methodology of feeding in, asteroids appears to be limited only by the number of researchers studying them. From the intertidal to the depths of the abyss, sea stars appear to often be THE dominant animals in their ecosystems.

Choriaster granulatus , the dough-boy star. This Indo-Pacific star is also a predator on corals.

This property is also the major downside to trying to keep sea stars in reef aquaria. They are significant predators and they will need a lot of food. Additionally, they are generally rather picky about what they eat and will only eat a limited variety of foods.

So reef aquarists need to be able to know what their charges might eat, and need to be able to provide for it. This is not as easy as it seems. As a general rule sea stars will not eat carrion, and so they need to be provided with some live food. Many of the common

Looking a lot like a deflated football, Culcita novaeguinae , is yet another coral-eating star.

coral reef sea stars other than the "Crown of Thorns" eat corals and unless the aquarist has an unlimited supply on corals, it is unlikely that such stars will be a hit with aquarists

Others, such as the common Oreaster in the Caribbean, are predators on sponges, and will find a sponge, climb on it and literally digest it down to the substrate (Wulff 1995). For many others, we simply do not know their diets. Although an aquarist may luck out and be able find an acceptable food for these animals, this is generally not the case.

Oreaster reticulatus . This common Caribbean star eats sponges

A few stars that are acceptable for reef aquarists include those in the genus Linkia . These animals appear to be browsers on small animals and adherent debris found on the surface of the substrate. Animals from the genus Linkia have some other advantages, too.

Linkia laevigata , this is an non-selective surface grazer common in the shallow waters of the Indo-Pacific and can survive on a diet of aquarium detritus and debris. This is a good star for home aquaria when small, but it can reach diameters of 30 cm or more.

Some species remain rather small and will cause little disturbance whilst roaming around the tank. Others will reproduce by dropping one arm. This arm will grow a new central disk and other arms.As the developing arm and disk bears a resemblance to medieval drawing of a comet, this is referred to as cometary reproduction.

Other information about tropical asteroids may be obtained from the following link:

So, although most sea stars are not suitable for a marine reef aquarium, a few species are. Fortunately, these few are brightly colored and have interesting shapes. And for the hobbyist, they can bring the icon of the marine world, the sea star home to watch and enjoy.

Linkia multiflora , this star may be acceptable for home aquaria, and appears to reproduce by shedding arms which grow back the remainder of the animal. It reaches diameters of about 10 cm.


Birkeland, C. 1974. Interactions between a sea pen and seven of its predators. Ecological Monographs. 44:211-232.

Birkeland, C. E. 1982. Terrestrial runoff as a cause of outbreaks of Acanthaster planci (Echinodermata: Asteroidea). Marine Biology. 69:175-185.

Chia, F. S. and Amerongen, H. 1975. On the prey-catching pedicellariae of a starfish, Stylasterias forreri . Canadian Journal of Zoology. 53: 748-755

Duggins, D. O. 1983. Starfish predation and the creation of mosaic patterns in a kelp-dominated community. Ecology. 64:1610-1619.

Engstrom, N. A. 1974. Population dynamics and prey-predation relations of a dendrochirote holothurian, Cucumaria lubrica , and sea stars in the genus Solaster . Ph.D. Dissertation, The University of Washington. Seattle. 144 p.

Kozloff, E. N. 1987. Marine Invertebrates of the Pacific Northwest. University of Washington Press. Seattle. 511 pp.

Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. 866 pp.

Lambert, P. 1981. The sea stars of British Columbia. British Columbia Provincial Museum. Handbook no. 39. Victoria, B. C. 153 pp.

Mauzey, K. P., C. Birkeland, and P. K. Dayton. 1968. Feeding behavior of asteroids and escape responses of their prey in the Puget Sound region. Ecology. 49:603-619.

Paine, R. T. 1966. Food web complexity and species diversity. American Naturalist. 100:65-75.

Paine, R. T. 1974. Intertidal community structure. Experimental studies on the relationship between a dominant competitor and its principal predator. Oecologia 15:93-120.

Paine, R. T. and S. A. Levin. 1981. Intertidal landscapes: disturbance and the dynamics of pattern. Ecological Monographs. 51:145-98.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. Saunders College Publishing. Philadelphia. 1056 pp.

Suchanek, T. H. 1979. The Mytilus californicus community: studies on the composition, structure, organization, and dynamics of a mussel bed. Ph. D.

Dissertation. The University of Washington. Seattle. 286 pp.

Wulff, J. L. 1995. Sponge-feeding by the Caribbean starfish Oreaster reticulatus. Marine Biology. 123: 313-325.

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Last modified 2006-11-19 01:15