Mel White wrote in National Geographic: “The sea star — often called a starfish, though it's no more a fish than it is a sheepdog — ranks with the most spectacular creatures of the diverse menagerie inhabiting the” sea. “.Big (sometimes a foot across) and obstinately colorful (some are orange, some are purple; no one knows why), the sea star is usually found in a rock fissure sprawled like a discarded toy.
Sea stars (Starfish) get their names for their resemblance to stars. They were initially called fish perhaps because they lived in the sea. Sea stars do not have gills, scales, or fins as fish do They are echinoderms and multi-armed invertebrates. There are approximately 1,900 species of sea star, all of which live in saltwater marine waters. Instead of blood, saltwater pumps nutrients through their bodies via a 'water vascular system.' Sea stars range in size from two centimeters (three-fourths of an inch) to more than one one meter three and a half feet (one meter) in diameter. Some are brown or dull yellow or orange in color but many are bright orange, blue, red, purple, green or a combination of colors. [Sources: Demver Zoo, NOAA; Fred Bavendam, Smithsonian magazine]
Sea stars are also known as asteroids because they belong to the class Asteroidea. They divided into seven different orders — 1) Brisingida, 2) Forcipulatida, 3) Paxillosida, 4) Notomyotida, 5) Spinulosida, 6) Valvatida and 7) Velatida. Sea Stars come in a variety of shapes. Some are thick and have short stumpy arms. Others are thin and have long tentacle-like arms.
Sea Stars are related to sand dollars, sea urchins, and sea cucumbers, all of which are echinoderms, meaning that they have five-point have radial symmetry (symmetry around a central axis),. However, this does not mean that all sea stars have five arms and species with 10, 20, or even 50 arms exist! If one of these arms is lost, a sea star has the amazingly ability to regenerate it. Most sea stars do have five arms.
Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se; Encyclopedia of Life eol.org; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems ; Monterey Bay Aquarium montereybayaquarium.org ; MarineBio marinebio.org/oceans/creatures; Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) coris.noaa.gov ; International Coral Reef Initiative icriforum.org ; Coral Reef Alliance coral.org ; Global Coral reef Alliance globalcoral.org ; Global Coral Reef Monitoring Network gcrmn.net
Echinoderms (meaning "spiny skinned") are a phylum of animals that includes sea urchins, starfish and sea cucumbers. They are invertebrates with no head. Their internal organs — and often their outer organs — are arranged in five symmetrical parts around a central stomach. The creatures have no front or back. Humans and other mammals are bilaterally symmetrical with nearly identical left and right sides and distinct front and backs.
There are around 7,000 different species of echinoderm. Most echinoderms have tough skins and flexible spines and/or tube feet. They also have unique groups of hydraulic organs that serve several functions and work in conjunction with muscles to power the tube feet. The tube feet have three main functions: 1) they help they animal move with powerful suction that grasps onto things and muscles that expand and retract; 2) they can take in oxygen from the seawater and serve as gills; and 3) they are packed with sensory neurons and help the animal sense its surroundings.
Echinoderms “do not have a ‘heart’ or anything analogous to it,” Chris Mah, a marine invertebrate zoologist at the Smithsonian National Museum of Natural History in Washington, told National Geographic. They don’t have blood either. Instead, they have millions of tiny, hairlike structures called cilia that beat constantly, pumping seawater via “a system of internal pipes and bags,” Mah says. Their internal cavity also has “all the various cells needed for transporting nutrients, immune cells, and so forth.” [Source: Liz Langley, National Geographic, February 13, 2016]
Sea Star Habitats and Ecosystem Roles
Sea stars live exclusively on ocean floor and live in almost every ocean habitat. They can be found in all the world's oceans — from warm, tropical waters to frigid, polar seas, and from from shallow intertidal zones down to abyssal depths of over 6,000 meters (20,000 feet) below the surface. Only a few animals feed on adult sea stars because of their tough, non-nutritious calcified skin. Some large tritons, birds, crabs sea otters, crustaceans and fish do eat them, though. [Source: Wikipedia]
Sea stars are not endangered. The populations of most species appear to be healthy except those affected by diseases and damage to coral reefs and other habitats. They play important ecological roles. In some places where they have been removed there has been an overpopulations of mussels and sea urchins which has prevented other sea creatures and kelp forests from thriving. Sea stars are sometimes harvested, ground up and sold for fertilizer and poultry feed.
Sea stars in the genus Pisaster are a keystone predators in the rocky intertidal zone off the Pacific Coast. They maintain diversity in tidal regions by keeping the strongly competitive bivalves at a low enough population level that they could not monopolize all the resources and form a monoculture. Seastars are most vulnerable to predation at the larval and juvenile stages, and are presumably preyed upon by fish or other echinoderms. [Source: Kim Chau, Animal Diversity Web (ADW)]
Sea stars can also can be a scourge. Overpopulations of sea stars can also cause problems. Crown of Thorn sea stars have ravaged coral in the Great Barrier Reef and other places. They can be serious threats to mussel and oyster beds as well as coral reefs. One sea star can devour over 50 young clams in a week. Sea stars also compete directly with commercial and recreational fishermen. If there is an overpopulation of sea stars, they are hard to get rid of because if they break, they regenerate, and there were more of them than before.
Sea Star Characteristics
Sea stars are ectothermic (use heat from the environment and adapt their behavior to regulate body temperature), heterothermic (have a body temperature that fluctuates with the surrounding environment) and have radial symmetry (symmetry around a central axis). They have a complex nervous system but no brain. Their size depends on the amount of food they eat not their age. Sea stars detect light with small eyespots located at the end of each arm. They have a groove extending from the mouth into each arm. Along the groove there are 2-4 rows of small tubular projections called tube feet tipped with muscular suction cups. [Source: Denver Zoo]
The mouth is on the ventral (oral) side and the anus is on the dorsal (aboral) side. The mouth opens into a primitive saclike stomach, whose folds extend out into the arms. Around the mouth is a circulatory system of water tubes and blood vessels, reproductive organs and other organs. Sea stars' primitive eyes, known as eyespots, are positioned on the end of each arm. They allow sea stars to react to light. Chemical sensors that help them locate prey. Forbes’ sea stars responds to chemosensory stimuli and can locate the source of odors. They use different orientation paths for different odors.
Sea stars bodies are covered by thousands of stony plates, called ossicles, that are imbedded in the skin but not connected. The ossicles are bound by connective tissue so that they move like flexible joints. This means they have surprising flexibility. You can jam a starfish into almost any space and given enough time it can usually extricate itself.
There is a small hard spot on the dorsal surface of the sea star body called the madreporite or sieve plate. It is perforated and is where seawater enters into the vascular system. Pores allow it to filter the water before it enters the vascular system. The madreporite feeds into the ring canal in the middle of the sea star. Attached to the ring canal are radial canals that branch off. Each radial canal runs down an arm. The radial canals are surrounded by ampullae and tube feet on each side. Ampullae are the bulbs at the top of the tube feet. [Source: Kim Chau, Animal Diversity Web (ADW) /=\
Sea Star Tube Feet and Locomotion
The bottom of the sea stars is covered by hundreds of tube feet with a mouth at the center. Sea star tube feet have suckers that allow them to cling to objects. The feet are operated using a unique vascular system that relies on hydraulic pressure to create or release a vacuum. Faster ones travel at 30 inches a minute.
The tube feet are used for feeding, respiration and sensory functions as well as locomotion. The network of water vessels in each arm draws in water and channels it to the tube feet enabling them to move. During locomotion the tiny tube feet perform a coordinated “grip and pull” action. The arms can bend and twist allowing sea stars to move over irregular surfaces, grasp prey or even flip themselves over. [Source: Denver Zoo]
According to Animal Diversity Web: Forbes’ sea stars can move at a rate of 15-20 centimeters (6 to 8 inches) per minute in unthreatened ocean water, but when under attack, can speed up to 25-35 centimeters (10 to 15 inches) per minute. They rely heavily on chemoreception to orient themselves and get around. When there is prey around, Forbes’ sea stars walks faster and at more direct angles to the food. During times of great wave action in shallow water, sea stars cling to rocks, flattening themselves against the rocks with all the power their tube feet can exert. [Source: Kim Chau, Animal Diversity Web (ADW) /=]
If a sea star is turned on its back, it turns one arm so that it grips the ground surface with its tube feet. The arm turns until the body has completed a slow somersault into its normal position. Sea stars do not move like wheels. They move in straight lines with one arm in advance when they are moving their fastest. /=\
Sea Star Feeding Behavior
Some sea stars feed on detritus on the ocean floor. Others feed on sea urchins, sea anemones, clams, other mollusks, other sea stars and other invertebrates. Some catch prey, swallow it hole and digest it in their bodies. Some catch their prey with tiny tentacles that pass the meal toward the sea star's mouth. Many starfish feed on corals by producing digestive fluids which they squirt into the polyp compartments and extract the polyp as a soupy liquid. Many sea stars are venomous. They use their toxin to stun prey Others trap their prey with mucus.
Sea stars can be ravenous carnivores with a special adaptation for consuming prey outside their bodies. Most sea stars have the ability to push their stomachs out through their mouths and digest whatever it touches, often a mollusk. This ability allows sea stars to consume a variety of prey larger than their mouthes. To eat shellfish, they use their powerful suckers to pry open a clam or oyster shell, then push their stomach out through their mouth and insert it inside the shell where they digest and absorb the soft inner tissues leaving an empty shell behind. [Source: Denver Zoo]
Mollusk-eating sea stars envelope mollusks with their tube feed and slowly wrench their shells open and feed on the flesh inside or find a small crack in the shell, as small as a tenth of millimeter, and inject their stomachs into the shell and eat the animal. Digestion, which may be aided by toxic juices, occurs inside the shell. The mollusk is turned into liquid and is guided into the sea star's mouth by the cilia on its arms. The is said a sea star will generally win in a battle against the bivalve because of its muscle endurance and its ability to insert its stomach through thin openings [Source: Kim Chau, Animal Diversity Web (ADW)]
Mel White wrote in National Geographic: “Despite its apparent lethargy, Pisaster ochraceus serves as a top predator of the intertidal zone — tiger of the tide pool — though it lacks anything like a brain. Sarah Ann Thompson, a marine biologist from the Farallon Institute in Petaluma, California, stoops to pick up an orange star. In a bizarre adaptation right out of a superhero movie, Pisaster can, in the span of a heartbeat — or what would be a heartbeat, if it had a heart — rigidify the "mutable tissue" in its normally limp body to transform itself into a structure as solid as bone. It then employs an internal hydraulic system and hundreds of suckerlike feet to grab the shells of a mussel and summon enough force to pull them apart. "This Pisaster has already killed the mussel," Thompson says, holding the sea star and the deceased in one hand and separating the mussel's shells a bit with the other. "Pisaster has everted its stomach out through its mouth, and it's digesting the mussel externally." So that creamy goo inside the mussel … ?"Yes, that's Pisaster's stomach. When it's finished eating, it pulls its stomach back inside itself and goes on its way." [Source: Mel White, National Geographic, June 2011]
The sea stars' habit of feeding on mollusks has made sea stars unpopular with fishermen who raise and collect clams, scallops and oysters. Studies of the effect of temperature upon predation rates by sea stars concluded that sea stars higher success rates in warmer water because the effectiveness of sea scallops' escape response decreases with temperature.
Sea Star Reproduction and Development
Sea stars generally reproduce by laying a great number of small eggs. The egg are surrounded by a protective covering, which in turn are covered by a layer of jelly. In the early stages of fertilization, the sperm attaches itself to the jelly. Chemical signals cause the protective covering to rupture and allow the sperm to fertilize the egg. A single female may produce over 2.5 million eggs in a single spawn. However since many marine animals feed on both eggs and larvae, few survive. Maybe one in a million makes it to adulthood.
Sea stars in general have separate sexes. There are sex organs in each arm on the ventral side. Individual sea stars are either male or female but they are capable of both sexual and asexual reproduction. Fertilization occurs outside the body when eggs and sperm are released into the water. To increase the chances of fertilization sea stars gather in groups when they are ready to spawn. [Source: Denver Zoo]
It is believed that environmental and chemical signals coordinate spawning. When one female sheds her eggs, other females in the area are stimulated to shed their eggs and then males are stimulated to shed their milt. Fertilized eggs form into tiny swimming larvae that develop bilateral symmetry. These bipinnaria larvae endure for about three weeks before settling on the ocean bottom. Once the larvae settle on the bottom they undergo metamorphosis changing into the radially symmetrical adult form. Asexual reproduction occurs by fragmentation when the animal breaks into two parts and each half forms a complete new sea star or by regeneration of parts of the animal. Life span is 3-5 years.
Sea Star Limb and Body Regeneration
Most sea stars have the ability to regenerate lost arms or even regenerate a whole new sea star from a single arm attached to a portion of the central disc. Sea stars can completely regenerate as long as they retain one fifth of the central disk and one arm. If a sea star is cut in half each side will grow into a new sea star. Fishermen used to try to kill starfish by chopping them in half, resulting in a doubling of the sea star population.
Sea star arms can be lost to predators or accidents. Sea stars can also purposely detach the appendages themselves to confuse or evade predators. Sometimes these the lost arms gives birth to new sea stars. Regeneration is possible because each of the arms contains parts of the vital organs including the digestive tract and reproductive organs. Regeneration is a slow process and may take a year for complete body part or body to reform. [Source: Denver Zoo]
Sea stars can can regenerate their body parts because they house their vital organs in their central disc, and because they have what scientists call ‘indeterminate stem cells’. On how these cells evolved and work, the National Science Foundation website says: “During embryonic development cells of most animals take on a particular identity —they become blood cells, lung cells, bone cells, etc. and keep these identities forever.” A few don’t. [Source: Roundglass Sustain, August 5, 2021]
These indeterminate stem cells have the ability to divide and create cells of any kind — an extraordinary and valuable trait to possess. Humans have such stem cells, but only during the foetal stage. Sea stars retain indeterminate stem cells throughout their lives, enabling them to regrow most parts of their bodies, even vital organs. This process can take anywhere between months to years, depending on the extent of the damage and the availability of resources such as food.
Sea Stars Can Regenerate Their Nervous Systems
In 2022, Carnegie Mellon University biologists announced the had discovered a mechanism that underlies the regeneration of neurons in starfish, showing for the first time that starfish can regenerate their nervous system. Their findings, published in eLife, could provide a pathway to future research on neuronal regeneration in humans and hope for people with serious spinal and brain injuries. According to the university: Some animals are known for their ability to regrow – or regenerate – body parts following dramatic injury. Starfish are among the most famous and dramatic examples, being able to grow an entire new body from just a single arm. Starfish also have the ability to regenerate neurons, something not seen in many animals. And other species, including humans, have little or no ability to regenerate. [Source: Jocelyn Duffy, Carnegie Mellon University March 17, 2022]
Starfish, humans and other vertebrates share a number of similarities in their early development, genome organization and gene content. Carnegie Mellon biologist Veronica Hinman studies starfish and their mechanisms of development, and her lab was curious to find out what it was in the cells and genes of starfish that allowed them to regenerate. “If we know what lies within starfish that allows them to regrow whole new bodies from limbs, we can compare that with what is in the cells and genes of human to see what is similar and different,” said Hinman, the Dr. Frederick A. Schwertz Distinguished Professor of Life Sciences and Head of the Department of Biological Sciences at Carnegie Mellon.
While working to better understand the science of regeneration in starfish, Minyan Zheng, then a graduate student in Hinman’s lab, and researcher Olga Zueva were able to show that starfish could not just regenerate their body, they could correctly regenerate their nervous system — something that very few animals can do. The researchers found that when neurons were injured in the starfish, they began to express the gene sox2, which caused cells to re-enter the neurogenesis program seen during development and form differentiated neurons in their brain. This demonstrated that starfish revert to developmental programs rather that use novel regeneration pathways to regrow neurons. The involvement of sox2 in neuronal regeneration is significant because this gene also is implicated in coaxing mature human cells into induced pluripotent stem cells (IPS) in cell culture.
Image Sources: Wikimedia Commons, NOAA
Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated May 2023