We Owe A Lot To Brainless Dominance

It has been said, and rightly so, I think, that the icon for the coral reef aquarium hobby is a clownfish nestled down inside an anemone. However, if you asked any person at random to pick one animal that characterizes the marine environment, I suspect they would answer that it is a sea star or star fish. Sea stars are the animals everybody brings home from the beach to dry out and put on a shelf, there to remind them of the ocean while turning into a pathetic petrified reminder of its former glory.

Sea stars are members of the group of animals that scientists call the Class Asteroidea of the Phylum Echinodermata and they are certainly characteristic of marine environments. Indeed, they are found nowhere else, and throughout the ocean bottoms they are often the dominant animals. Their dominance is not often expressed in vast numbers, although they may be very abundant, but rather in the fact they are often keystone predators. Keystone predators are those predators whose activities control and structure their biological environment. Indeed, the term keystone predator was coined for the changes in the intertidal environment caused by the most common sea star on Pacific Coast of North America, the ochre star, Pisaster ochraceus (Figure 1).

Figure 1. Pisaster ochraceus, the Ochre star, this individual is about 5 inches in diameter. Of course, it isn't ochre colored. There are two common color varieties, and this is the most attractive, so that is what I took the picture of.

Anyone who has been to a rocky seashore is probably familiar with the intertidal zonation of organisms on those shores. Zonation, in this sense, means that if you look at the shore from a distance, the life on it seems to form discrete layers all parallel to sea level. On the coast of the N. E. Pacific Ocean, the plants and animals characteristic of the terrestrial environment are not found down to the water's edge, but their distributions end several feet above sea level, creating a zone of substrate where neither terrestrial nor fully marine animals live. This is the intertidal zone, found between the highest and lowest tides. Typically the highest intertidal zone is a barren area of little evident life; it is often an area of bare rock or rock. It gets too much salt spray for terrestrial animals to tolerate, but not enough water coverage to keep marine life alive, only a hardy golden crustose lichen is found there. In areas exposed to a lot of wave action, the next lower zone is dominated by small California mussels, Mytilus californianus. Where the wave action is not as extreme, some different species of mussels are found. These are in the Mytilus edulis/trossulus/galloprovincialis complex of species; all of which are virtually identical in appearance. Just below this will be one or two zones covered in barnacles, with different barnacle species being the predominant space-filling organism in each layer. Descending further down in these intertidal areas, one often finds a zone dominated by clonal zooxanthellate sea anemones, Anthopleura elegentissma. Finally, below this are several zones dominated by various algae.

Rocks in these zones are often found at steep angles and there are lots of cracks, crevices and channels distributed throughout the areas. In the bottom of the surge channels, large green sea anemones, Anthopleura xanthogrammica, are often found, and along the vertical surfaces are purple or orange sea stars, the lead actor in this little play, Pisaster ochraceus. These stars typically are relatively robust animals, often about 6 inches in diameter, but thick for their size and very rugged.

About 40 years ago, in a series of interesting experiments, a researcher from the University of Washington wanted to determine the effects of sea star predation in this environment. Over a period of a couple of years, he visited his study sites on the exposed coast of Washington on every low tide period and removed all the Pisaster he could find. Over a couple of years, some extraordinary changes occurred on the beaches. These normally small, one to three inches long, mussels increased in abundance tremendously and grew rapidly, and got far larger than they normally did, reaching lengths of eight to fourteen inches. They grew over and smothered the barnacles. Similarly, the kelps that were found only in the lower areas of the beach started moving upward on the rocks into shallower water. After about four years, the barnacle zones had disappeared, and the rocky beach belonged only to the mussels and kelps.

The Pisaster were selective predators, and preferred mussels as prey. The lower limit of the previous mussel zone coincided with the maximum tidal height that the stars could forage up to without dying from desiccation when the tide went out. With no stars around, the mussels settled out of the plankton in the high intertidal and moved downward where they grew much faster than normal, as they were covered with water more frequently each day and received a lot more food. Below them, the barnacles, which had previously been eaten by the stars when they did not have mussels to eat, went uneaten and grew much larger than normal. They became functional attachment sites for kelp sporelings and the kelps covered them and smothered them. Basically, without the sea star cleaning out mussels and barnacles, the dominant competitors for the space in the system took over, and changed the community significantly. The researcher, Robert T. Paine, referred to the sea star as a keystone predator, for without it the rocky intertidal community in that region collapses, much as an arch collapses without its keystone (Paine, 1966, 1974).

What does this have to do with coral reef aquaria, you ask? Well, on coral reefs, as in the rocky intertidal of Washington, much of the observed diversity comes from sea star predation. Here the sea star that is the most important predator is the oft-reviled "Crown of Thorns," Acanthaster planci. The higher parts of reef platforms are dominated by rapidly growing and competitively dominant coral species, in several genera such as Acropora, Pocillopora, Seriatopora, and Montipora. Aquarists, for no good reason, refer to animals in these and some other genera small-polyped-scleractinians. As we all are aware, if given enough food and enough light these animals are rapidly growing and are often capable of overgrowing other animals and killing them. In the game of biological competition, second place wins you the silver medal of death awarded by the grim reaper. With these species, it is not how you play the game, it is whether you win or die.

Figure 2. Acanthaster planci, the Crown of Thorns sea star. This individual was about 15 inches in diameter.

The question you might be asking yourself then is, "Given that these species are often rapidly growing and competitively dominant corals, why are there so many different kinds of corals found in these areas?" The reason for a lot of the coral diversity is the same reason that may be given for zonation on rocky intertidal beaches of Northwestern North America; starfish predation on the competitively dominant animals allows other species to occupy space. Acanthaster is a major coral predator in these areas, but it is by no means the only one.

These stars move through the reef, and they eat corals. But, they don't eat all corals, nor do they eat continuously. What a star seems to do is eat a coral, and then wander around a bit and then eat another coral. If there are a lot of stars eating corals in an area, as there are during "Crown of Thorns "Plagues,"' the net result is not a reef flat barren of corals, but an area polka-dotted with small patches of dead corals.

Such a reef looks awful……..ly inviting for coral larvae to settle into. These open patches are often the only places where newly settled coral larvae can thrive. On a reef, corals are spaced much farther apart than they are in a reef aquarist's tank. Generally, the distance between "hand-sized" coral heads is on the order of a foot or so. If they are any closer together than that, they fight. They are fighting - competing - for space; and the loser dies. The reason that they are spaced like that is that any coral larva that lands within about 6 inches or so of a moderately sized colony is out competed by the larger colonies surrounding it and cannot survive.

The open patches left after a crown of thorns outbreak are open spaces for the next settlement period of new corals. Coral larvae, called planulae, which are about the size and shape of the small brown flatworms visible in many reef tanks, swim through the water. Corals only make one choice in their life, and that choice, where to settle and spend the rest of its life, is made by the mature planula. Corals spawn into the water and the planula is formed by the cellular division that occurs during the early embryonic development that occurs after the egg is fertilized. Corals have relatively big eggs packed with yolk, and the larvae develop from the egg using the yolk as food. They do not feed, and after a few days, they look like small flatworms. This small wormlike creature swims along the bottom, and every few minutes it will swim to the surface and touch its front end to the surface. In effect, it "tastes" the surface for the appropriate chemical flavor; the right flavor indicates a good place to settle and grow. If there are alot of corals in the area, these small larvae end up becoming just so much food for these plankton-feeding animals. But, if there are some open places where bacteria and maybe the "right" kind of algae have settled, the surface tastes "right," and the little worm sticks itself to the surface. Over the next few days, it changes into a small single coral polyp and starts to grow. And the only reason it had room to begin was due to the coral-eating predator that had passed that way maybe as much as a couple of years before. After a few years, the reef is a diverse place with no evidence of the Acanthaster predation, and then the cycle repeats. In this way, the coral diversity of a reef is maintained.

So, What is A Sea Star and How Do They Do It?

Sea stars are categorized as being in the PHYLUM ECHINODERMATA, a group of about 6,000 species. They are all marine, most are moderate size, relatively few are tiny and none are truly microscopic. Most of them live on, or in, the marine benthos or bottom environment.

There are six major living subgroups within the Echinodermata, called classes:

•the Class Crinoidea, or feather stars,
•the Class Asteroidea or sea stars,
•the Class Ophiuroidea or brittle, serpent, and basket stars,
•the Class Echinoidea, the sea urchins,
•the Class Concentricycloidea, the sea daisies, and
•the Class Holothuroidea, or sea cucumbers.

Additionally, there are a large number of fossil groups that allow us to know a lot about the evolution of the Echinoderms. As a group, the Echinoderms are really odd animals when compared to just about any other group of animals. Externally, they are distinguished by the lack of a defined front or back end; rather they are radially symmetrical, similar in some regards to sea anemones and corals. This radial symmetry is an evolved or derived state; however, they start life as a bilateral animal with a front and a back end, and change into a radial animal through a relatively drastic metamorphosis. Some species, particularly among the sea cucumbers, but also among the sea urchins, have become quite bilaterally symmetrical and have left and right sides and a back and front. Most echinoderms are radially symmetrical, and as most of them have appendages that occur in multiples of five, they are referred to as having pentaradial symmetry.

They totally lack a head, brain, and large apparent sensory structures. The nervous system in many of them is so diffuse that, except for a few very large nerves, most of the nerves are so small that one needs electron microscopy to even observe them. The basis for behavior in any echinoderm is poorly known, at best. With no brain, much of the behavioral responses seen are assumed to be simple reflexes, however, many of them have complex behaviors, and how these are mediated is unknown.

They have an internal skeleton of calcium carbonate, often with a magnesium component. Except for the spines of pencil urchins, all of the skeletal structures are internal; so all the spines in most sea urchins and all other echinoderms are covered with tissue. One characteristic that is very important to aquarists, but which is totally invisible from the outside, is that the bodies of echinoderms are largely hollow and filed with cavities lined with very thin tissue.

Sea stars are perhaps the classical Echinoderm, and they are likely the animals most folks think of when they consider that group. They are categorized as belonging to the Class Asteroidea, which has about 1500 species. Sea stars always have a more-or-less flattened, flexible body, although in some it is pretty stiff. Under each arm or ray is a groove lined with either two or four rows of tube feet.

Figure 3. Choriaster granulatus, the Doughboy star, this individual is about 12 inches across and, contrary to its "puffy" appearance, it is quite stiff and rigid. This species is also a predator on corals.

The tube feet are the external manifestations of an organ system unique to echinoderms, the ambulacral or water-vascular system. This is a hydraulic system that consists of thousands of internal pipes, tubes and valves. Each tube foot is connected by a lateral tube to a radial canal running down the center of each arm. Fluid is pumped into each tube foot, and valves may close isolating it. Inside the sea star's arm, a small balloon-like structure called the ampulla extends up from the tube foot. The tube foot looks like an eye-dropper with a flexible tube and no opening. When the eye-dropper bulb (= ampulla) is closed by muscle contraction, the fluid in the ampulla is pushed into tube, thereby extending it. When those ampullar muscles relax and muscles running along the length of the tube foot contract, the foot is pulled back in and shortened, and the fluid is moved back into the ampulla inflating it. The foot can pivot at the point it joins the body, due to muscles connecting it to the body wall. The tip of the foot terminates, in most sea stars, in an adhesive pad, which sticks to the substrate by the use of a temporary glue. So when a sea star moves, the tube foot is pivoted in the direction of locomotion, extended, and pivoted back in a swinging "walking" step. When it contacts the substrate, a glue fastens it to the substrate (Hermans, 1983). As the walking step continues the star moves above the foot, just like you move above your foot when your leg pivots. At the end of the pivot cycle, the glue is released, and the tube foot extends off the substrate. It is contracted and pivoted back to the initial position. Now, this is pretty easy to visualize for one tube foot, and controlling this might be pretty easy if you only have a few tube feet, but some stars have as many as 40,000 tube feet and can move very rapidly across the substrate! All this is done with no brain to control or co-ordinate any part of the locomotion, and how the locomotion is co-ordinated is not known. The neuronal basis for any behavior is not known for any echinoderm.

Figure 4. A Choriaster, drawn as if cut open to show the internal structures. There are two pyloric caeca and two gonads in each arm, although only one of each is shown for clarity. From this vantage point, the gut is visible in the center part of the animal, but the mouth is seen from the insides. The gut structures are shown in various shades of red, pink, purple, orange or brown. The gonad is yellow, and the ambulacral system is in shades of blue.

Internally, they have a short gut that primarily runs from the central mouth directly through the animal terminating at the anus, which is generally in the center of the upper part of the body. The mouth faces down, and immediately above it is a short esophagus. The first part of the stomach is located above this, and it is called the "cardiac" stomach. This may be extruded in some, but not all, stars to initiate digestion outside the animal. The cardiac stomach is connected to the "pyloric" stomach. This "stomach" is basically the central section of a food storage organ, and it sends two large sack-like extensions called pyloric caeca into each arm or ray. These function in food storage and can vary a lot in size. From the pyloric stomach, a short intestine extends to the anus. There is a pair of rectal caeca of uncertain function attached to the rectum.

Above or adjacent to the pyloric caeca are the gonads which discharge through an external opening on each side of the ray near the top. There is no sexual dimorphism, and the sexes are externally identical.

Sea stars feed in a number of different ways. Many of them, including Pisaster ochraceus and Acanthaster planci extend their cardiac stomachs outside the body and digest much of their prey externally. Some of them, such as the burrowing sand stars, sold erroneously in the hobby as sand sifters, take their prey internally and eat it there. One of the largest stars is Pycnopodia helianthoides found in the Northeastern Pacific, and it eats its prey internally. In an examination of these stars to see what they were eating, I have found two of them with the remains of complete diving ducks inside them, and I have no doubt that they could have captured the birds, drowned them, and eaten them. Other stars such as Linckia laevigata, the blue star imported for the aquarium hobby, and species of Pteraster, eat by extending their stomach over the substrate, and they basically try to digest the world. They eat sponges, small microorganisms, small sessile organisms, and whatever else doesn't walk away. Finally some stars, such as the blood stars, Henricia species, found commonly in many temperate seas, extend their rays up into the water and extend mucus from them. Plankton adheres to the mucus, and the stars eat both the mucus and the plankton.

Figure 5. Pycnopodia helianthoides, the sunflower star. This species reaches diameters of more than 5 feet and takes its prey internally to digest it. It is a major predator in many subtidal areas of the North Eastern Pacific.

More detailed information about sea stars may be obtained by consulting some of the standard invertebrate zoology text books (Kozloff, 1990; Ruppert and Barnes, 1994).

Aquarium Concerns

Sea stars are often delightful animals to watch in the ocean. However, they are very seldom desirable in our reef aquaria, simply because they are large and predatory. Many of the larger ones found on coral reefs are at least potentially predatory on corals, and having one of these animals in a tank can become an exercise in watching expensive dining. Other moderately-sized stars may be kept for a while, but generally will starve to death in our systems; included in this group are species of Fromia, most of which appear to require specific sponges and tunicates as prey.

A few species of stars may be kept successfully in aquaria. Probably the most ubiquitous of these are several (?) small species of cushion stars, possibly in the genus Asterina. These small stars are gray, white, or sometimes mottled with green, and are about one half inch across. They reproduce by fission, and are seldom seen with a complete array of arms. There appear to be three distinct types, which may be different species, found in reef aquaria. The most common variety is one that appears to eat algae and surface films. The second most common variety (although it is quite rare) eats zoanthids and soft corals. The rarest variety of these small white stars eats stony corals. Fortunately, aquarium control of them is pretty easy. They are not the speediest of animals, and if you find you have a type that is causing problems, periodic starfish safaris can generally rid a tank of them.

Larger species of stars have generally poor success rates in our tanks. Stars such as the chocolate chip star are simply too predatory to maintain, as are the so-called sand-sifting stars. Sold to "sift sand," these latter species will devastate a live sand bed in short order. About the only larger stars that are successfully kept are species of Linckia, both the large Linckia laevigata, and the smaller Linckia multifora. Both are both surface deposit feeders and eat algal films, and many small sessile animals, but tend to leave corals and other ornamental animals alone.

The major concern for aquarists with any sea star is, or should be, salinity. These are animals that do not tolerate changes in salinity at all well, and also seem to suffer from transport stress. They need full strength sea water at 35 ppt to 37 ppt salinity. They also do best at temperatures in the 80º F to 84º F range. They must be acclimated very slowly; often an acclimation of six hours or more is called for. Even so, the survival rate of these beautiful animals is abysmal. Probably less than one tenth of the ones imported for the hobby survive a week in a hobbyist's tank. If they survive more than a few days, however, they will likely do well for a long time.

We depend, in the long run, on sea stars and their predatory activities for many of the corals seen in our tanks, and they are often beautiful and interesting animals. Unfortunately, most of them are NOT suitable as reef aquarium animals. Even for those few that are suitable, however, our success rate is pretty poor, and more care needs to be taken in the shipping, handling, transport, and general acclimation of them, to ensure their survival in our tanks.

If you have any questions about this article, please visit my author forum on Reef Central.

References Cited:

Hermans, C. O. 1983. The duo-gland adhesive system. Oceanography and Marine Biology: an Annual Review. 21:283-339.

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

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.

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

References of Interest:

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

Chess, J. R., E. S. Hobson and D. F. Howard. 1997. Interactions between Acanthaster planci (Echinodermata, Asteroidea) and Scleractinian Corals at Kona, Hawai'i. Pacific Science. 51:121-133.

Death, G. and P. J. Moran. 1998. Factors affecting the behaviour of crown-of-thorns starfish (Acanthaster planci L.) on the Great Barrier Reef: 2: Feeding preferences. Journal of Experimental Marine Biology and Ecology. 220:107-126.

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

Flammang, P. 1995. Fine structure of the podia in three species of paxillosid asteroids of the genus Luidia (Echinodermata). Belgian Journal of Zoology. 125:125-134.

Jaeckle, W. B. 1994. Multiple modes of asexual reproduction by tropical and subtropical sea star larvae: An Unusual adaptation for genet dispersal and survival. Biological Bulletin (Woods Hole). 186:62-71.

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.

Okaji, K., T. Ayukai and J. S. Lucas. 1997. Selective feeding by larvae of the crown-of-thorns starfish, Acanthaster planci (L.). Coral Reefs. 16:47-50.

Williams, S. T. 1999. Species boundaries in the starfish genus Linckia. Marine Biology. 135:137-148.

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

Links of Interest:

Asteroidea -- general information.

This is site is always a good one to begin with for any animal group and their treatment of sea stars is up to their usual standard.


A diverse site, with a lot of good information is found at this link:


Internal Structures - Photos of Dissections. - what one really looks like inside.


Great Pictures, good info --- Antarctic Sea Stars


Sea star larvae - beautiful pictures


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We Owe A Lot To Brainless Dominance