Evolution and systematics
The class Asteroidea is a highly diverse group comprised of seven orders, 35 families, and an estimated 1,600 known living species, although their precise phylogenetic relationship and hence classification still proves challenging to taxonomists.
Asteroids belong to a major group of other bottom-dwelling animals called echinoderms. Collectively this group includes echinoids (sea urchins), holothurians (sea cucumbers), crinoids (feather stars), and ophiuriods (brittle stars), the latter group closely resembling sea stars. All echinoderms share similar pentamerous radial symmetry and spiny skin characteristics, although sea stars differ slightly because they have five or more arms large enough to contain space for digestive and reproductive glands. Another group of animals thought to belong to echinoderms are concentricycloids, or sea daisies. These small disc-shaped animals discovered in the abyssal seas off New Zealand and Bahamas in the late 1980s are considered an evolutionary forerunner to asteroids.
Sea stars have an ancient linage that shows embryologi-cally they are not too distantly related to the phylum Chordata (back-boned animals). The fossil record places a form of asteroid over 300 millions years before the dinosaurs, sharing a common ancestry with ophiuroids, yet within 50 million years of their appearance they became clearly differentiated. Their evolutionary path has included some bizarre taxa that have been hard to classify, yet this successful group has persisted and remain ecologically important to many marine communities worldwide.
Sea stars vary considerably in size, shape, and color, even within the same populations. Their diverse forms reflect evo lutionary adaptation to the cosmopolitan habitats they occupy. Despite this diversity they all share similar physical characteristics. All are star-shaped (stellate) with a central body or disc that has symmetrically projecting arms with rows of tube feet running along the lower surface of a V-shaped furrow called the ambulacral groove. Typically, the number of arms is five, but some species such as the coral-eating crown-of-thorns Acanthaster planci can have up to 30. Their size ranges from the 0.4 in (1 cm) arms of the cushion star Patiriella parvivipara, which gives the appearance of a nearly spherical body, to the long skinny arms of Novodinia antillen-sis, which span almost 3 ft (91 cm) in diameter. In most species, the arm tips carry an optic cushion of red-pigmented and light-sensitive cells that sense changes in the prevailing environment.
The skeleton of a sea star consists of small calcium carbonate plates called ossicles. These are often studded and spiny, and provide a firm but flexible skeleton of connective tissue. Flexibility enables a variety of postures to be adopted without muscular effort, thus providing an effective means to capture and handle prey and allow individuals to closely follow irregular substrates in search of food. Alternatively, their flexibility can enable sea stars to upright themselves if overturned.
The surface of a sea star looks and feels rough because of the numerous small and transparent sacs called papulae that cover the body, which provide a respiratory surface for exchanging oxygen. The upper and lower body surfaces also contain pincer-like structures called pedicellariae, which come in a variety of forms from simple modified spines to highly specialized opposing hooks. Their function is to rid areas around the papulae of small organisms and debris, and in some species capture prey by detecting their presence. These are tube feet anus madreporite stone canal optic cushion anus
■ tube foot optic cushion nerve ring canal radial canal pedicellaria spine digestive caecum gonad moveable' spine nerve
Sea star anatomy. (Illustration by Patricia Ferrer)
usually small fish or shrimp-like crustaceans on which the sea star feeds. The shape of pedicellariae is an important characteristic for asteroid taxonomy.
Sea stars have an unusual way of moving. Water is taken in through the madreportite, a small, perforated plate on the upper surface of the disc, and into the water vascular system, a canal of tubes connected to the tube feet. Following muscular contraction water is directed under pressure to the tube feet, which then extend under its force. Movement is achieved through a coordinated stepping motion where, on muscle contraction, the feet adhere to the sediment surface, pushing the individual forward. Depending on the species, tube feet have suckers that help stick the sea star to hard surfaces or assist in prying open the shells of its mollusk prey. Besides involvement in prey capture; tube feet also have a respiratory function. Species with more than five arms and reproduce asexually will have numerous madreportites.
The greatest diversity of sea stars occur in coastal regions, although as a group, they are well represented globally from the Antarctic, Pacific, Atlantic, and Indian Oceans. They inhabit wave-exposed inter-tidal zones of coastal waters to the calm sandy pavements of the deepest oceans. The Ben-thopectinidae family of sea stars, for example, lives exclusively in the deep-sea of the Atlantic and Pacific Oceans, whereas the species Odontaster validus, which belongs to the family Ganeriidae, are found only in the Antarctic Ocean. Perhaps the most well-known and ubiquitous group of sea stars belongs to the order Forcipulatida. This group includes the genus Asterias, a veracious predator of mussels and oysters in many coastal waters around the world. In the Far East, the species Asterias amurensis has extended its normal range into New Zealand and Australia, where as a non-native species it has caused extensive economical and ecological damage to the shellfish industry.
As a group, sea stars live in virtually every habitat found in the sea, ranging from tidal pools, rocky shores, sea grass and kelp beds, beneath rock rubble, on coral reefs, sand, and mud. In some species a broad and flattened body may act as a snowshoe when foraging on very soft mud. In the upper shore, they are periodically exposed by the retreating tide, resulting in extended periods of desiccation. The only refuge is cover in moist crevices beneath rocks. By contrast, in the deep sea at depth greater than 29,530 ft (9,000 m) they are found inhabiting sandy bottoms and steep cliffs.
They are prominent seafloor predators. Perhaps their success and influence comes from a unique combination of attributes. These include indeterminate growth, a morphology and digestive system generalized enough to capture, handle, and ingest many different prey types and sizes, and a sensory ability sophisticated enough to respond quickly to the presence of prey and changes in the prevailing environment. Moreover, their flexible bodies and suckered tube feet enable them to adhere firmly to the seabed whilst manipulating prey, thus enabling them to survive in high stress environments by withstanding the full force of crashing waves.
Sea stars have a "central nervous system," or diffuse nerve net, but lack anything identified as a brain. Despite this, they are sophisticated enough to adapt to change based on previous experiences (conditioning), whereby behavior that is persistently unsuccessful, usually a feeding one, is stopped.
They are not considered social animals, yet many species tend to aggregate or swarm in large numbers during certain times of the year. These events tend to be triggered during spawning periods, feeding frenzies, or seasonal migrations to deeper waters offshore. Some sea stars show avoidance behavior to other species or attraction towards members of the opposite sex. Feeding is perhaps the most common cause of aggregation, where sea stars can appear in thousands to prey on mussels, oysters, or coral.
The daily activity patterns in many sea stars are synchronized to changes in light intensity, usually around dawn and dusk. Such activity may help to avoid predators and coincide sea star foraging activity with the activity of their preferred prey. In others such as Astropecten irregularis, daily activity patterns are synchronized to periods of slack water on a high and low tide when velocities are low enough to optimize foraging success.
Sea stars are carnivorous, preying on sponges, shellfish, crabs, corals, worms, and even on other echinoderms. Most are generalists, feeding on anything that is too slow to escape, such as mussels and clams, whilst others are specialized feeders preying exclusively on sponges, corals, bivalves, or algae. Prey is located by the chemical odors emanating from its waste products or by small movements that betray its presence when detected by a sea star. Food preferences can change depending on availability of prey, which change geographically and seasonally. Even weather conditions in temperate species and reproductive state (usually during gonad growth) affects dietary requirements.
Feeding strategies can be divided into those that are scavengers, feeding mainly on decaying fish and invertebrates; those that are deposit feeders, filling their stomachs with mud from which they extract microscopic organisms and organic matter; and those that are suspension feeders, filtering prey and food particles from the water (e.g., Novodinia antillensis).
Depending on the species, sea stars have two different feeding methods. Intra-oral feeders ingest their prey into their stomach alive, sometimes distending or rupturing their disc in the process. The burrowing sand star Astropecten irregularis, for example, can swallow hundreds of live juvenile mollusks during one foraging period. In some cases prey such as clams
and snails resist digestion by keeping their valves or opercu-lum plate tightly closed, forcing sea stars to take weeks to digest them. Extra-oral feeders devour their prey (usually oysters and mussels) by pulling the shells apart using their tube feet and arms. Digestion occurs once the sea star's stomach is everted through its mouth and brought into direct contact with soft tissue. Often they take advantage of imperfections in the seal of the prey's shell and squeeze their stomach into 0.1 mm-wide gaps. Some animals such as clams, worms, and crustaceans avoid predation by co-existing as commensals. The worm Acholoe astericola, for example, lives within the arm grooves of its host, the burrowing starfish As-tropecten irregularis, but when threatened the worm seeks refuge inside the sea star's stomach. In coral eaters, such as the crown-of-thorns Acanthaster planci, the stomach is applied directly to the coral and a patch digested. Sea stars can go without food for months. Pisaster ochraceus, for example, can survive for 18 months without eating, losing an estimate 35% of its body weight.
They have few predators as adults due to their armored spiny skeleton and rigid nature. In less heavily armored and juvenile sea stars, protection from predators comes from having a cryptic coloration. Other defensives include toxic spines or skin (e.g., Crossaster papposus and Acanthaster planci) and predator avoidance by burrowing beneath the sediment surface (e.g., Astropecten irregularis and Anseropodaplacenta). Some crabs, fish, birds, and other echinoderms are known to prey on sea stars. Usually, they feed on arm tips, as their calcified bodies are difficult to eat and not very nutritious.
Most sea stars have separate sexes with no visible differences between them. Internally, each arm contains a pair of gonads that become almost filled with eggs or sperm, depending on the sex, at the time of breeding. The majority of species are broadcast spawners where eggs and sperm are released into the water column to be fertilized. To increase the chances of fertilization, sea stars aggregate when they are ready to spawn. These events usually rely on environmental cues, such as day length, to coordinate timing and may use chemical signals to indicate readiness. The crown-of-thorns sea star, for example, releases a potent chemical into the water column to attract the opposite sex. Fertilized eggs rapidly develop into free-living bipinnaria and later brachiolaria larvae that are planktonic. Eventually, they undergo metamorphosis and settle on the seabed to grow into adults. This type of reproductive strategy is known as indirect-development.
Some sea stars brood their young, where females hold their fertilized eggs in a brood space under the arm (e.g., Asterina phylactica), in the stomach (e.g., Leptasterias hexactis), or incubate them in the gonads (e.g., Patiriella parvivipara). In the last two cases, young develop internally and escape through small openings the female's body wall called gonopores. Many brooding sea stars inhabit polar and deep-sea regions. Some brooding sea stars, however, produce unguarded egg masses that they attach to the seabed (Asterina gibbosa).
Asexual reproduction is another method of development that involves either fission or regeneration of entire animal from arm parts. Almost a dozen species divide through their disc, producing clones with identical genetic makeup (e.g., Linckia laevigata). Seven species are known to voluntarily pinch off one or more arms (autotomous asexual reproduction) that subsequently regenerate a complete new disc and arms; these species tend to be very small. Even sexually reproducing animals can show asexual characteristics at different stages of their life cycle. For example, larvae can pinch off body structures capable of growing into another independent feeding larvae.
No asteroid is listed in the IUCN Red List of Threatened Species. Some species are protected, however, at local levels, particularly in tropical destinations where souvenir hunters have lead to a decline in numbers. In the Caribbean, for example, the sea star Oreaster reticulatus has protection.
Various sea stars cause significant ecological and commercial impact, particularly to harvested shellfish throughout the world. On the North Atlantic coasts, Asterias forbesi feed intensely on oysters, mussels, and scallops, with sea star aggregations causing massive damage to shell fisheries. In New Zealand and Australia the accidental introduction of Asterias amurensis has caused extensive damage to both commercial fisheries and endemic communities. Their arrival is thought to be from ballast water discharged before docking by visiting ships and has triggered a nationwide strategy to halt their progress. Coral reefs often fall victim to the destructive feeding power of some sea stars that invade these globally important habitats. In Indo-Pacific regions Acanthaster planci is an infamous coralivore of coral reefs, causing devastating infestations and major management problems.
However, sea stars do have some commercial value. In Denmark, Asterias are used as an ingredient in fish meal, which is fed to poultry. The ancient Indians of British Columbia and the Egyptians used them as fertilizer. Some companies collect sea stars for biological supplies to schools and collectors. Their multi-rayed image is emblematic of the sea, making their dried bodies a valuable commodity to the souvenir trade.
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