Starfish

Starfish, also known as sea stars, constitute a class of marine invertebrates typically characterized by a star-polygon morphology. (In common usage, these appellations are frequently extended to ophiuroids, which are more accurately identified as brittle stars or basket stars.) These organisms are additionally termed asteroids, a designation derived from their taxonomic classification within the class Asteroidea (). Approximately 1,900 species of starfish inhabit the seabed across all global oceans, ranging from warm, tropical environments to cold, polar territories. Their distribution extends from the intertidal zone to abyssal depths, reaching up to 6,000 m (20,000 ft) beneath the surface.

Starfish or sea stars are a class of marine invertebrates generally shaped like a star polygon. (In common usage, these names are also often applied to ophiuroids, which are correctly referred to as brittle stars or basket stars.) Starfish are also known as asteroids because they form the taxonomic class Asteroidea (). About 1,900 species of starfish live on the seabed, and are found in all the world's oceans, from warm, tropical zones to frigid, polar regions. They can occur from the intertidal zone down to abyssal depths, at 6,000 m (20,000 ft) below the surface.

As echinoderms, starfish typically exhibit a central disc from which usually five arms radiate, although certain species possess a greater number. The aboral, or dorsal, surface can present as smooth, granular, or spiny, and is enveloped by overlapping plates. Many species display vibrant coloration, encompassing various hues of red or orange, while others appear in shades of blue, grey, or brown. Starfish utilize tube feet, powered by a hydraulic system, and possess a mouth situated centrally on their oral, or ventral, surface. They are characterized as opportunistic feeders, predominantly preying on benthic invertebrates. Specialized feeding strategies observed in several species include gastric eversion and suspension feeding. Their life cycles are complex, enabling both sexual and asexual reproductive modes. The majority demonstrate the capacity to regenerate damaged or lost arms, and can autotomize arms as a defensive mechanism.

The taxonomic class Asteroidea fulfills several crucial ecological functions. Certain species, including the ochre sea star (Pisaster ochraceus) and the reef sea star (Stichaster australis), function as keystone species, exerting a disproportionate influence on their ecosystems. The tropical crown-of-thorns starfish (Acanthaster planci) is recognized as a prolific coral predator across the Indo-Pacific, while the Northern Pacific seastar is classified among the Worst Invasive Alien Species.

The fossil record of starfish is ancient, extending to the Ordovician period approximately 450 million years ago, yet it remains relatively sparse due to the post-mortem disintegration of these organisms. Consequently, only the ossicles and spines are typically preserved, rendering fossilized remains challenging to discover. Owing to their distinctive symmetrical form, starfish have featured prominently in literature and folklore. They are occasionally gathered as curiosities, incorporated into design or logos, and consumed in certain cultures.

Anatomy

The majority of starfish exhibit five arms radiating from a central disc; however, this number varies across different groups. Some species possess six or seven arms, while others may have between 10 and 15 arms. In the Antarctic species Labidiaster annulatus, the arm count can exceed fifty. Gene expression studies suggest that the starfish body externally corresponds to a head (with lips associated with the tube feet) and internally to a torso. Starfish are equipped with two distinct vascular systems: one facilitating water transport for locomotion and other physiological processes, and another dedicated to blood circulation.

Body Wall

The body wall comprises several layers: a delicate cuticle, a single-layered epidermis, a substantial dermis composed of connective tissue, a slender coelomic myoepithelial layer for musculature, and a peritoneum lining the body cavity. Within the dermis resides an endoskeleton formed from calcium carbonate elements termed ossicles. These ossicles are honeycomb-like formations, consisting of calcite microcrystals organized in a lattice structure. Their morphology ranges from flat plates to granules and spines, collectively covering the aboral (dorsal) surface. Specialized ossicular structures include the madreporite (serving as the entrance to the water vascular system), pedicellariae, and paxillae. Paxillae are umbrella-shaped structures characteristic of starfish inhabiting buried substrates. The margins of contiguous paxillae converge to create a false cuticle, beneath which a water cavity protects the madreporite and fragile gill structures. Even externally visible ossicles are situated beneath the epidermal layer.

Many starfish taxa, such as Valvatida and Forcipulatida, are characterized by the presence of pedicellariae. These specialized, scissor-like ossicles are located at the spine tips and function to dislodge organisms attempting to settle on the starfish's surface. Certain species, including Labidiaster annulatus and Novodinia antillensis, additionally employ their pedicellariae for prey capture. Furthermore, papulae, which are delicate, thin-walled extensions of the coelomic cavity, may project through the body wall into the ambient water, primarily facilitating respiration. A three-dimensional network of collagen fibers, oriented perpendicularly to each other, provides structural support for these elements, with ossicles and papulae situated within its interstices. This intricate organization allows for both flexible arm movement and the rapid development of stiffness and rigidity, which are essential for specific stress-induced actions.

Water Vascular System

The starfish's water vascular system constitutes a hydraulic network of fluid-filled canals, integral to locomotion, adhesion, food manipulation, and gas exchange. Water enters this system via the madreporite, a porous, often prominent, sieve-like ossicle located on the aboral surface. This madreporite connects, via a calcareous-lined conduit known as the stone canal, to a ring canal encircling the oral aperture. From the ring canal, several radial canals diverge, with one extending along the ambulacral groove within each arm. Short lateral canals branch alternately from both sides of each radial canal, each terminating in an ampulla. These bulbous ampullae are connected to the external tube feet (podia) by short linking canals that traverse ossicles within the ambulacral groove. Typically, two rows of tube feet are present; however, in certain species, the alternating long and short lateral canals result in the appearance of four rows. The entire internal surface of the canal system is ciliated.

When the longitudinal muscles within the ampullae contract, they force water into the tube feet and simultaneously close the valves in the lateral canals. This action causes the tube feet to extend and make contact with the substrate. Despite their superficial resemblance to suction cups, the adhesive mechanism of the tube feet relies on chemical adhesion rather than suction. Release from the substrate is facilitated by the secretion of other chemicals and the relaxation of the ampullae. The tube feet adhere to surfaces and propel the animal through a wave-like motion, wherein one arm section attaches while another detaches. To expose their sensory tube feet and eyespots to external stimuli, some starfish elevate the tips of their arms during locomotion.

Starfish, having evolved from bilateral ancestors, can exhibit bilateral movement, especially during predation or when threatened. During crawling, specific arms function as leading appendages, with others trailing. Should a starfish become inverted, two adjacent arms and an opposing arm press against the substrate to elevate the remaining two arms; the opposing arm then disengages as the starfish rights itself and resumes its typical orientation.

Beyond their role in locomotion, the tube feet also serve as accessory gills. The water vascular system facilitates the transport of oxygen to, and carbon dioxide from, the tube feet, as well as the distribution of nutrients from the gut to the locomotor muscles. Fluid circulation within this system is bidirectional and propelled by ciliary action.

Digestive System and Excretion

The starfish's digestive tract occupies the majority of the central disc and extends into each arm. The mouth is centrally located on the oral surface, enveloped by a robust peristomial membrane and sealed by a sphincter. A brief esophagus links the mouth to a stomach, which comprises an eversible cardiac region and a more diminutive pyloric region. The cardiac stomach is glandular and sacculated, anchored by ligaments connected to ossicles within the arms, enabling its retraction after eversion. The pyloric stomach features two extensions into each arm, known as the pyloric caeca. These are elongated, hollow tubules lined with numerous glands that secrete digestive enzymes and absorb nutrients from ingested food. A short intestine and rectum extend from the pyloric stomach to the anus, situated at the apex of the disc's aboral surface.

For primitive starfish, including genera such as Astropecten and Luidia, prey is ingested whole and initially processed within the cardiac stomach, with indigestible hard materials like shells being expelled. The resulting partially digested fluid then proceeds to the caeca for further digestion and nutrient absorption. In contrast, more evolved starfish species possess the ability to evert their cardiac stomach externally to envelop and digest food, subsequently transferring it to the pyloric stomach. The dynamic processes of cardiac stomach retraction and contraction are regulated by the neuropeptide NGFFYamide.

Ammonia constitutes the primary nitrogenous waste product, eliminated through diffusion across the thin-walled surfaces of the tube feet, papulae, and other permeable regions. Additional waste substances include urates. The organism's coelomic fluid contains phagocytic cells, termed coelomocytes, which are also present in the hemal and water vascular systems. These coelomocytes encapsulate waste matter and subsequently migrate to the distal ends of the papulae, where a segment of the body wall is detached and expelled into the ambient aquatic environment.

Starfish maintain their internal body fluids at an osmotic equilibrium with the surrounding seawater. This absence of a dedicated osmoregulation system is likely the reason for their exclusion from freshwater habitats and their infrequent occurrence in estuarine environments.

Sensory and Nervous Systems

Despite lacking numerous highly specialized sensory organs, starfish are capable of perceiving tactile stimuli, light, temperature fluctuations, spatial orientation, and the ambient water conditions. Their tube feet, spines, and pedicellariae exhibit sensitivity to touch. Furthermore, the tube feet, particularly those located at the distal ends of the rays, possess chemosensory capabilities, allowing starfish to identify chemical cues, such as food sources. Ocelli, or eyespots, are situated at the terminus of each arm, with each ocellus comprising 80 to 200 simple photoreceptive units composed of pigmented epithelial cells. Discrete photoreceptor cells are also distributed across other regions of their bodies, responding to light. The behavioral response to light, whether advancement or retreat, is species-dependent.

Although starfish do not possess a centralized brain, they exhibit a sophisticated nervous system characterized by a circumoral nerve ring and a radial nerve extending along the ambulacral region of each arm, parallel to the radial canal. The peripheral nervous system comprises two distinct nerve nets: one situated within the epidermis, serving as the sensory system, and another lining the coelomic cavity, functioning as the motor system. Neurons traversing the dermis connect these two networks. Both the nerve ring and radial nerves contribute to locomotion and sensory processing. The sensory component receives input from various sensory organs, while the motor nerves regulate the tube feet and musculature. Should a single arm detect an appealing stimulus, it can exert temporary dominance over the other arms, initiating directed movement towards the stimulus.

Circulatory and Gas Exchange Systems

The body cavity houses the circulatory, or hemal, system. This system consists of vessels arranged into three distinct rings: the hyponeural hemal ring encircling the mouth, the gastric ring surrounding the digestive apparatus, and the genital ring positioned near the aboral surface. A heart, beating approximately six times per minute, is located at the apex of a vertical axial vessel that interconnects these three rings. Starfish blood lacks respiratory pigments like heme, but is presumed to facilitate nutrient transport throughout the organism. Gas exchange primarily occurs via specialized gills, known as papulae, which manifest as thin-walled protrusions along the aboral surface of the arms. Oxygen is transferred from these papulae into the coelomic fluid, which then circulates gases throughout the body.

Secondary Metabolites

Starfish synthesize a diverse array of secondary metabolites, primarily lipids, which encompass steroidal derivatives of cholesterol and fatty acid amides of sphingosine. These steroids predominantly consist of saponins, specifically termed asterosaponins, along with their sulfated counterparts. Their composition exhibits interspecific variation, generally comprising up to six sugar moieties (commonly glucose and galactose) linked by a maximum of three glycosidic chains. Long-chain fatty acid amides of sphingosine are frequently observed, with certain variants demonstrating established biological activity. Furthermore, starfish possess various ceramides and a limited quantity of alkaloids. These endogenous chemicals likely serve roles in both defense mechanisms and intercellular communication within starfish. For instance, some act as feeding deterrents, effectively discouraging predation. Additionally, certain compounds function as antifoulants, augmenting the action of pedicellariae to inhibit the settlement of other organisms on the aboral surface of the starfish. A subset of these substances comprises alarm pheromones and escape-eliciting chemicals; their release elicits responses in conspecific starfish and frequently induces flight in potential prey. Global research endeavors are currently investigating the efficacy of these compounds for potential pharmacological or industrial applications.

Life Cycle

Sexual Reproduction

The majority of starfish species exhibit gonochorism, characterized by distinct male and female individuals. Conversely, some species are simultaneous hermaphrodites, capable of producing both eggs and sperm concurrently; in a subset of these, a single gonad, termed an ovotestis, generates both gametes. Other starfish species display sequential hermaphroditism. For instance, protandrous individuals of species such as Asterina gibbosa commence their lives as males, subsequently transitioning to females as they mature. In certain species, including Nepanthia belcheri, a large female can undergo fission, producing male offspring that later revert to females upon reaching sufficient size.

Each arm of a starfish houses two gonads, which release gametes via gonoducts situated on the central disc, positioned between the arms. While fertilization is typically external, a limited number of species exhibit internal fertilization. For the majority of species, buoyant eggs and sperm are freely released into the water (a process known as free spawning), with the subsequent embryos and larvae developing as planktonic organisms. Conversely, in some species, eggs may adhere to the ventral surfaces of rocks. Certain starfish species demonstrate maternal brooding, where females either envelop their eggs or retain them within specialized external or internal bodily structures. Starfish that brood by "sitting" on their eggs typically adopt a humped posture, elevating their central discs from the substrate. For instance, Pteraster militaris broods a portion of its offspring while dispersing the excess eggs, which are too numerous to be contained within its pouch. In these brooding species, the eggs are comparatively large, provisioned with yolk, and typically undergo direct development into miniature starfish, bypassing an intermediate larval stage; this developmental mode is termed "lecithotrophic." In Parvulastra parvivipara, an intragonadal brooder, the developing starfish acquire nutrients by consuming other eggs and embryos within the brood pouch. Brooding behavior is observed in species inhabiting colder waters and in smaller species that produce a limited number of eggs.

Spawning periodicity can be modulated by various environmental factors, including photoperiod, water temperature, and food availability. Individuals may aggregate for synchronous gamete release, employing pheromones for mutual attraction. In certain species, a male and female may form a temporary pair. This pairing often involves pseudocopulation, wherein the male crawls upon the female and fertilizes her gametes as they are expelled.

Larval Development

Starfish embryos generally emerge from their eggs as blastulae. Subsequent development involves invaginations: the initial invagination, originating from the blastopore, forms the anus, while a secondary invagination within the ectodermal layer establishes the mouth. The archenteron then extends towards and connects with the mouth, thereby forming the digestive tract. Externally, a ciliary band develops. This band subsequently expands, encircling the larval surface and eventually extending onto two nascent arm-like projections. At this developmental stage, the larva is designated as a bipinnaria. These cilia facilitate both locomotion and feeding, with their rhythmic beating propelling phytoplankton towards the mouth.

The subsequent developmental phase is the brachiolaria larva, characterized by the emergence of three short ventral-anterior arms, each equipped with adhesive tips encircling a central sucker. Both the bipinnaria and brachiolaria larval forms exhibit bilateral symmetry. Upon reaching full development, the brachiolaria larva descends to the seabed, securing itself via a short stalk formed from its ventral arms and sucker. This attachment initiates metamorphosis, involving a profound reorganization of larval tissues. During this transformation, the larva establishes an oral surface on its left side and an aboral surface on its right. Although the gut persists, the mouth and anus relocate to novel anatomical positions. Certain body cavities regress, while others differentiate into the water vascular system and the visceral coelom. Consequently, the developing starfish adopts a pentaradially symmetrical body plan. Finally, it detaches from its stalk, emerging as a free-living juvenile starfish, typically measuring up to 1 mm (0.04 in) in diameter.

Asexual reproduction

Certain starfish species exhibit asexual reproduction in adulthood, achieved either through the fission of their central discs or by the autotomy (self-amputation) of one or more arms. Individual arms capable of regenerating into a complete organism are designated as "comet forms." Furthermore, the larvae of various starfish species can reproduce asexually prior to reaching maturity. This larval asexual reproduction occurs via the autotomy of specific body parts or through budding. Larval asexual reproductive rates escalate in response to abundant food availability. While this process incurs energetic and temporal costs, potentially delaying maturation, it enables a single larva to produce multiple adult individuals under favorable environmental conditions.

Regeneration

Many starfish species possess a remarkable capacity for regeneration, enabling them to regrow lost arms and, over time, develop an entirely new limb. While some species can reconstitute a complete central disc from a solitary arm, others necessitate the presence of at least a portion of the central disc attached to the severed part for successful regeneration. This regrowth process can span several months, during which starfish are particularly susceptible to infections in the initial phases following arm loss. Beyond fragmentation employed for reproductive purposes, body division can also serve as a defensive mechanism. The detachment of body parts is facilitated by the rapid softening of a specialized connective tissue, triggered by neural signals. This particular tissue, known as catch connective tissue, is prevalent across most echinoderms. Research has identified an autotomy-promoting factor that, upon injection into another starfish, induces the rapid shedding of arms.

Lifespan

Starfish species exhibit substantial variability in their lifespans. For instance, Leptasterias hexactis attains sexual maturity at a weight of 20 g (0.7 oz) within two years and typically lives for approximately ten years. In contrast, Pisaster ochraceus reaches maturity at 70–90 g (2.5–3.2 oz) over a five-year period and possesses a documented maximum lifespan of 34 years.

Ecology

Distribution and habitat

Starfish inhabit marine environments globally, encompassing both tropical and polar regions. Primarily benthic organisms, they reside on sandy, muddy, and rocky substrates. Their bathymetric distribution extends from shallow intertidal zones to the deep-sea floor, reaching depths of at least 6,000 m (20,000 ft). These echinoderms are most frequently encountered in coastal areas.

Diet

The majority of starfish species function as generalist predators, consuming microalgae, sponges, bivalves, gastropods, and other small invertebrates. The crown-of-thorns starfish specifically preys on coral polyps, whereas other species are detritivores, subsisting on decomposing organic matter and fecal material. A limited number of species are suspension feeders, collecting phytoplankton; for instance, Henricia and Echinaster frequently associate with sponges, exploiting the water currents generated by these organisms for feeding. Furthermore, several species are capable of absorbing dissolved organic nutrients directly from the ambient water, which can constitute a substantial component of their nutritional intake.

Specialized anatomical features can facilitate feeding and prey capture; for example, Pisaster brevispinus, commonly known as the short-spined pisaster found along the West Coast of America, employs specialized tube feet to burrow deeply into soft substrates to extract prey, typically bivalves. Upon securing a bivalve, the starfish gradually pries open the shell, overcoming the clam's adductor muscle, and then inserts its everted stomach into the resulting aperture to digest the soft tissues internally. Remarkably, the required gap between the clam's valves for stomach insertion is merely a fraction of a millimeter.

Ecological impact

Starfish are recognized as keystone species within their marine ecosystems. Their ecological significance stems from their considerable size, varied diets, and adaptability to diverse habitats. The concept of a "keystone species" was, in fact, coined by Robert Paine in 1966 to characterize the starfish Pisaster ochraceus. Paine's research on the low intertidal coasts of Washington state revealed that predation by P. ochraceus was a primary determinant of species diversity. Experimental removal of this apex predator from specific shoreline segments led to a reduction in species diversity and the subsequent dominance of Mytilus mussels, which effectively outcompeted other organisms for spatial and nutritional resources. Comparable findings emerged from a 1971 investigation involving Stichaster australis along the intertidal coast of New Zealand's South Island. This study demonstrated that S. australis could eliminate most transplanted mussels within two to three months; conversely, in areas where S. australis had been eradicated, mussel populations proliferated dramatically, leading to ecological overgrowth and a threat to biodiversity.

The foraging behavior of the omnivorous starfish Oreaster reticulatus in sandy and seagrass environments near the Virgin Islands significantly influences the composition of microbial communities. These starfish ingest sediment accumulations, thereby removing surface films and algae attached to particles. This disturbance leads to the displacement of sensitive organisms by species capable of rapidly recolonizing the cleared sediment. Furthermore, the foraging activities of these starfish generate varied concentrations of organic matter, potentially attracting larger detritivores such as fish, crabs, and sea urchins that consume sediment.

Starfish can occasionally exert detrimental impacts on ecosystems. For instance, outbreaks of crown-of-thorns starfish have inflicted considerable damage upon coral reefs in Northeast Australia and French Polynesia. A Polynesian study documented a drastic reduction in coral cover, plummeting from over 40% to less than 5% within four years following the arrival of migratory starfish in 2006. This decline subsequently triggered cascading effects on both sessile benthic fauna and reef fish populations. Asterias amurensis represents an uncommon instance of an invasive echinoderm. Its larvae are believed to have been transported to Tasmania from central Japan in the 1980s through ship ballast water discharge. The population of this species has since expanded significantly, posing a threat to crucial bivalve fisheries in Australia. Consequently, they are classified as pests and are included on the Invasive Species Specialist Group's roster of the world's 100 most detrimental invasive species. Furthermore, certain species that prey on bivalve molluscs are capable of transmitting paralytic shellfish poisoning.

Threats

Starfish are susceptible to predation by a variety of organisms, including conspecifics, sea anemones, other starfish species, tritons, crabs, fish, gulls, and sea otters. Their primary defensive mechanisms involve saponins embedded within their body walls, which impart an unpalatable taste. Certain starfish, such as Astropecten polyacanthus, also contain potent toxins like tetrodotoxin, while the slime star is capable of exuding substantial amounts of repellent mucus. The crown-of-thorns starfish is characterized by sharp spines, toxins, and vivid warning coloration.

Several starfish species are occasionally afflicted by a wasting syndrome attributed to Vibrio bacteria. A more pervasive sea star wasting disease periodically results in widespread mortalities. Research conducted in 2025 on starfish populations off the central British Columbia coast indicates that individuals inhabiting fjords exhibit enhanced survival rates during disease outbreaks, likely due to the lower temperatures and elevated salinity characteristic of these environments.

The protozoan Orchitophrya stellarum is recognized for infecting and impairing starfish gonads. Starfish are also susceptible to elevated temperatures. Experimental data indicate that the feeding and growth rates of Pisaster ochraceus significantly diminish when their body temperatures exceed 23 °C (73 °F), with mortality occurring at 30 °C (86 °F). This particular species possesses a distinctive capacity to absorb seawater for thermoregulation when exposed to sunlight during a receding tide. Furthermore, it appears to utilize its arms for heat absorption, thereby safeguarding its central disc and essential organs.

Starfish and other echinoderms are susceptible to marine pollutants. The common starfish is recognized as a bioindicator species for marine ecosystems. A 2009 study indicated that P. ochraceus demonstrates a lower susceptibility to ocean acidification compared to other marine animals possessing calcium carbonate structures. In other groups, such structures are prone to dissolution under reduced pH conditions. Researchers observed that when P. ochraceus were subjected to 21 °C (70 °F) and 770 parts per million of carbon dioxide (levels exceeding projections for the next century), they exhibited remarkable resilience. Their survival is attributed to the nodular composition of their skeletal framework, which enables them to mitigate carbonate deficits through increased fleshy tissue development.

Evolution

Fossil Record

The earliest fossil echinoderms originate from the Cambrian period, with the initial asterozoan representatives (a clade encompassing starfish and brittle stars) being the Somasteroidea, which display characteristics common to both taxa. Starfish are rarely preserved in the fossil record, potentially due to the disarticulation of their rigid skeletal elements post-mortem. Despite this, specific localities, known as Lagerstätten or "starfish beds," contain intact, in-situ fossilized skeletal assemblages.

By the late Paleozoic, crinoids and blastoids constituted the dominant echinoderm taxa, with their fragmented remains constituting nearly exclusive fossil evidence in certain limestone formations. During the two major extinction events of the late Devonian and late Permian, blastoids were extirpated, and only a limited number of crinoid species persisted. Numerous starfish species also became extinct during these events; however, the surviving few species subsequently underwent rapid diversification over sixty million years, spanning from the early to mid-Middle Jurassic. A 2012 study revealed that starfish speciation can proceed with considerable rapidity. Over the last 6,000 years, a notable divergence in larval developmental strategies has emerged between Cryptasterina hystera and Cryptasterina pentagona, with the former adopting internal fertilization and brooding, while the latter maintains a broadcast spawning reproductive strategy.

Diversity

The scientific name Asteroidea was conferred upon starfish by the French zoologist de Blainville in 1830. Its etymology traces to the Greek aster, ἀστήρ (meaning "a star"), and the Greek eidos, εἶδος (meaning "form, likeness, or appearance"). Starfish are classified within the subphylum Asterozoa, alongside brittle and basket stars (order Ophiuroidea), distinguished by their adult morphology, which features a stellate body plan with multiple arms radiating from a central disc. The arms of asteroids are skeletally integrated with the central disc via ossicles embedded within the body wall, whereas ophiuroids, conversely, possess distinctly articulated arms.

The starfish constitute a substantial and diverse class, encompassing more than 1,900 extant species. This class is subdivided into seven extant orders: Brisingida, Forcipulatida, Notomyotida, Paxillosida, Spinulosida, Valvatida, and Velatida. Extant asteroids, designated Neoasteroidea, exhibit morphological and evolutionary divergence from their Paleozoic predecessors. While their classification has remained largely stable, the taxonomic placement of Paxillosida remains a subject of ongoing debate. The deep-water sea daisies, although clearly Asteroidea and currently included in Velatida, do not readily conform to established phylogenetic lineages. Phylogenetic analyses indicate they might represent a sister group, Concentricycloidea, to the Neoasteroidea, or alternatively, that the Velatida itself could constitute a sister group.

Living Groups

Brisingida (2 families, 17 genera, 111 species)

Members of this order are characterized by a compact, rigid central disc and 6 to 20 elongated, slender arms, which facilitate suspension feeding. They possess a singular series of marginal plates, disc plates fused into a ring, a reduced count of aboral plates, the presence of crossed pedicellariae, and multiple series of extended spines along their arms. They primarily inhabit deep-sea environments, though some species are found in shallow Antarctic waters. Certain species exhibit tube feet with rounded apices, devoid of suckers.

Forcipulatida (6 families, 63 genera, 269 species)

Members of this order possess characteristic pedicellariae, featuring a short stalk terminating in forceps-like tips, alongside tube feet equipped with flat-tipped suckers typically organized in four rows. This order comprises prominent species found across temperate and cold-water regions, inhabiting environments from intertidal to abyssal depths.

Notomyotida (1 family, 8 genera, 75 species)

These deep-sea starfish exhibit exceptionally flexible arms, characterized by prominent musculature along their dorsal lateral regions. Notably, certain species within this group lack suckers on their tube feet.

Paxillosida (comprising 7 families, 48 genera, and 372 species)

This primitive order is distinguished by members that do not evert their stomachs during feeding and lack both an anus and suckers on their tube feet. Their aboral surface features papulae, and they are equipped with marginal plates and paxillae. These organisms predominantly occupy soft substrates. Furthermore, their larval development does not include a brachiolaria stage. The comb starfish (Astropecten polyacanthus) exemplifies a species within this order.

Spinulosida (encompassing 1 family, 8 genera, and 121 species)

The majority of species within this order are characterized by the absence of pedicellariae. All members possess a delicate skeletal structure, featuring either small or entirely absent marginal plates on their disc or arms. Their aboral surface is adorned with numerous clusters of short spines.

Valvatida (consisting of 16 families, 172 genera, and 695 species)

The majority of species in this order typically exhibit five arms and possess two rows of tube feet equipped with suckers. Prominent marginal plates are observable on both their arms and disc. Additionally, some species feature paxillae, and in certain instances, their primary pedicellariae are clamp-like and embedded within the skeletal plates.

Velatida (comprising 4 families, 16 genera, and 138 species)

This order primarily encompasses deep-sea and other cold-water starfish, frequently exhibiting a global distribution. Their morphology is typically pentagonal or star-shaped, featuring five to fifteen arms. Characteristically, their skeleton is underdeveloped.

Extinct Taxa

Extinct groups classified within the Asteroidea include:

• † Calliasterellidae, represented by the type genus Calliasterella, which originated in the Devonian and Carboniferous periods.
• † Palasteriscus, a genus from the Devonian period.
• † Trichasteropsida, including the Triassic genus Trichasteropsis (comprising at least two species).

Phylogenetics

External Relationships

Starfish are classified as deuterostome animals, sharing this characteristic with chordates. A comprehensive 2014 analysis, based on 219 genes across all echinoderm classes, yielded the subsequent phylogenetic tree. The divergence times of the clades are indicated beneath their respective labels in millions of years ago (mya).

Internal Relationships

Resolving the phylogeny of the Asteroidea has presented significant challenges, primarily because visible morphological features have proven insufficient. Furthermore, the applicability of traditional taxa as true clades remains a subject of debate. The phylogenetic framework proposed by Gale in 1987 is as follows:

Conversely, the phylogeny suggested by Blake in 1987 is:

Subsequent research, incorporating molecular evidence either independently or in conjunction with morphological data, had by 2000 not definitively resolved this phylogenetic debate. In 2011, building upon additional molecular evidence, Janies and colleagues observed that echinoderm phylogeny "has proven difficult" and that "the overall phylogeny of extant echinoderms remains sensitive to the choice of analytical methods." They subsequently presented a phylogenetic tree exclusively for extant Asteroidea, employing traditional starfish order names where feasible and indicating "part of" in other instances. This phylogeny is depicted below, notably separating the Solasteridae from the Velatida and disaggregating the former Spinulosida.

Human Interactions

Research Applications

Starfish have served as valuable subjects in reproductive and developmental investigations. Female starfish yield abundant oocytes that are readily isolated; these can be preserved in a pre-meiotic state and subsequently induced to complete cell division through the application of 1-methyladenine. The suitability of starfish oocytes for such research stems from their ease of manipulation, viability in seawater at ambient temperatures, transparency, and rapid developmental rate. Asterina pectinifera, frequently employed as a model organism for these studies, demonstrates resilience and is easily propagated and sustained under laboratory conditions.

Another significant research domain involves the remarkable regenerative capacity of starfish to replace lost body parts. Given that adult human stem cells exhibit limited differentiation potential, insights gained from understanding the regrowth, repair, and cloning mechanisms in starfish could hold substantial implications for human medicine.

Starfish possess an uncommon ability to dislodge foreign objects from their bodies, a characteristic that complicates their tagging for research and tracking initiatives.

Cultural Significance and Folklore

An Aboriginal Australian fable, recounted by Welsh headmaster William Jenkyn Thomas (1870–1959), describes a scenario where various animals required a canoe for an ocean crossing. The Whale possessed a canoe but declined to share it. Consequently, the Starfish engaged the Whale with stories and grooming, removing parasites, while other animals surreptitiously took the canoe. Upon discovering the deception, the Whale severely punished the Starfish, a beating that, according to the fable, accounts for the Starfish's current appearance.

In 1900, scholar Edward Tregear documented The Creation Song, characterizing it as "an ancient prayer for the dedication of a high chief" in Hawaii. The starfish is notably mentioned among the "uncreated gods" described in the early verses of this song.

Georg Eberhard Rumpf's 1705 work, The Ambonese Curiosity Cabinet, details the tropical species of Stella Marina or Bintang Laut, which translate to "Sea Star" in Latin and Malay, respectively, found in the waters surrounding Ambon. Rumpf notes that the Histoire des Antilles records an observation that when sea stars "see thunder storms approaching, [they] grab hold of many small stones with their little legs, looking to ... hold themselves down as if with anchors."

Culinary Uses

Starfish are occasionally consumed in certain regions, including China, Japan, and Palau. It is important to note that some species, such as the crown-of-thorns starfish, are toxic. Georg Eberhard Rumpf observed limited use of starfish as food within the Indonesian archipelago, primarily as bait in fish traps. However, on the island of "Huamobel" [sic], inhabitants reportedly process them by cutting them, extracting the "black blood," and cooking them with sour tamarind leaves. After a resting period of one to two days, the outer skin is removed, and the pieces are then prepared in coconut milk.

Collection and Trade

Starfish are sometimes removed from their natural habitats and marketed to tourists as souvenirs, ornamental items, curiosities, or for display in aquariums. Notably, Oreaster reticulatus, due to its accessible habitat and vibrant coloration, is extensively collected in the Caribbean. During the early to mid-20th century, this species was abundant along the West Indian coasts; however, sustained collection and trade have significantly reduced its populations. In the State of Florida, O. reticulatus is classified as endangered, rendering its collection illegal. Despite this, the species continues to be sold both within and beyond its natural range. A comparable situation is observed in the Indo-Pacific for species like Protoreaster nodosus.

References

Bibliography

Mah, Christopher L. (24 January 2012). "The Echinoblog". This blog, dedicated to sea stars, is authored by a highly engaged and expert specialist in the field.

• Mah, Christopher L. (24 January 2012). "The Echinoblog".Source: TORIma Academy Archive