STARFISH: CHARACTERISTICS, TYPES, HABITAT, RESPIRATION - BIOLOGY - 2025

The starfish are echinoderms marine belonging to the class Asteroidea. Among its most notable features are its arms, which give it its distinctive star shape. Many species have five rays, however, they could have up to 40, as is the case of the sun star. These structures are radiated from the central disc, a circular area located in the center of the animal's body.

The upper or aboral area is covered by overlapping plates, which can give it a spiny, granular or smooth texture. In terms of coloration, they are bright, with orange, red, brown, blue or gray tones. They have tube or tube feet and an oral cavity, located on the lower surface.

Starfish. Source: pixabay.com

Starfish are distributed in all oceans globally, so it can be found in the Pacific, Atlantic, Arctic, Indian and Antarctic. In these they live from the intertidal areas to the abyssal zone, at depths greater than 6000 meters.

As for their diet, they are generalist predators. Within its diet are sponges, bivalves, corals, snails and even other echinoderms. Also, they could be detritivores or scavengers.

Regeneration

Several species of starfish have the ability to regenerate their arms, should they break. Thus, over time, another limb can grow back. Because this process can take several months, the area is exposed to severe infections.

On the limb that was separated, a mouth and a disk could grow. As this happens, the nutrients are obtained from those that were stored in the arm.

Fragmentation also constitutes an asexual way of reproducing, however, the loss of a part of the body can occur as a result of the action of a predator. In addition, the starfish could be detached from it, as an escape response to a threat.

Ecological impact

User: (WT-shared) Pbsouthwood at wts wikivoyage In studies carried out off the coast of Washington, specialists identified the enormous influence of P. ochraceus on the diversity of species in that area.

In a controlled manner, the population of this echinoderm was decreased in said region, resulting in the dominance in space and resources of the Mytilus mussels.

The behavior of the Stichaster australis, off the coast of New Zealand, was very similar. This consumed most of the existing mussels in the area, while in the area where it had been removed, mussels increased overwhelmingly, even threatening biodiversity.

Likewise, the foraging of migrating starfish creates new areas of organic matter, which causes a variation in the abundance and distribution of some organisms that feed on these sediments, such as crabs, fish and sea urchins.

Threat to the ecosystem

The North Pacific starfish is an invasive species that originally inhabited Japan. In the mid-1980s, Asterias amurensis larvae arrived in Tasmania, probably forming part of the waters contained in the boats.

Since then, its growth has been uncontrolled, to the point of representing a threat to the bivalve communities, a very important element within the region's economy.

Because of this, these starfish are considered pests and are among the 100 worst invasive species worldwide, according to the Group of Invasive Species Specialists.

On the other hand, crown of thorns (Acanthaster planci) shoots cause damage to coral reefs located in French Polynesia and Australia. Investigations revealed that coral cover decreased drastically since 2006, with the arrival of this migratory species.

Thus, in a period of three years, its percentage in that region decreased from 50% to 5%. This affected the fish in whose diet reefs are a fundamental part.

characteristics

1-Pyloric stomach. 2-Intestine. 3-Rectal gland. 4-Stone channel. 5-Madreporito. 6-Pyloric canal. 7-Pyloric blind. 8-Cardiac stomach. 9-Gonad. 10-Ambulacral sulcus. 11-Ampulla of the ambulacral foot. Source: © Hans Hillewaert. Wikiimedia commons

Body

The vast majority of starfish have five rays or arms, which project from a central disk. However, some members of the Solasteridae family have 10 to 15 rays. Even the Labidiaster annulatus could have between 40 and 45 rays.

The body wall is a thin cuticle. It has an epidermis made up of a layer of cells. The dermis is thick and is made up of connective tissue. In addition, it has a coelomic myoepithelial lamina, where the circular and longitudinal muscles are found.

In the dermis is the endoskeleton, formed by ossicles. These are composed of calcite microcrystals, arranged in a manner similar to a honeycomb.

These marine animals may have granules, spines, tubers, or tube plates. The patterns in which these structures are arranged, their location and characteristics are used to differentiate the various groups that make up the Asteroidea class.

Anatomy

Among the elements that make up the anatomy of the starfish is the madreporite plate. This is porous in nature and is connected, by means of a calcified channel, to the vascular system of the central disc. Its function is to supply additional water to meet the needs of the animal.

In relation to the anus, it is located outside the disk, close to the madreporite plate. On the oral surface, the ambulacral groove runs down each arm. On either side of this, there is a double row of unfused ossicles.

The tube feet are extended through notches and are internally connected to the aquifer vascular system.

On the surface of the body are pedicellars, which are valve-like. In some species they are grouped at the bases of spines, while in others they are scattered.

Its function is related to food, defense or the elimination of organisms that are established in the outer part of the starfish. Thus, Labidiaster annulatus has large pedicellars, which it uses to capture krill, one of the prey that forms its diet.

Shapes

Mary and. villegas Although the common name of the group of asteroids is starfish, the body shape of these animals is very varied. Thus, there are spherical ones, such as the Podosphaeraster, pentagonal, such as the Sphaeriodiscus and others with long arms and a small disk, an example being the Zoroaster.

The body can be flattened dorsoventrally, but they are also inflated and cushion-shaped, characteristic of the cushion star (Culcita.novaeguineae).

Locomotion

The starfish, like other echinoderms, move using a vascular system of water. Thus, water enters the body through the madreporite. Then it transits from the stone channel to the ring channel and the radials.

These radial channels carry water to the ampoule, providing suction to the feet of the tube. At the moment when the muscles of the ampulla contract, the valves of the lateral channels close and the water is forced out towards the feet of the tube.

Although the valves are similar to suction cups, the binding to the substrate occurs by chemical action, rather than by the effect of suction. Thanks to this, the starfish does not exercise its muscles when moving, avoiding extra energy consumption.

Thus, they can latch on to different substrates and move, with a movement similar to that of a wave. In this way, one part of the body sticks to the surface, as the other part is released.

Special features

Some starfish raise the tips of their arms when in motion, allowing maximum exposure of the eye spot and tube feet to external stimuli.

Although the vast majority of these animals do not move quickly, some burrowing species, such as those belonging to the genus Luidia and Astropecten, move quickly and progressively, sliding on the seabed.

Water vascular system

This is a hydraulic system made up of a network of channels filled with water, which take part in the process of locomotion, feeding, adhesion and gas exchange.

The water enters this system through the madreporite and is formed by a set of channels, lined with cilia, which connect it to an opening around the mouth.

Likewise, there are some canals that branch alternately on each side of the radial canal, ending in a blister. These bulb-shaped organs are attached to the tubular feet.

Excretory system

The starfish does not have excretory glands. Due to this, ammonia, as a nitrogen waste product, is eliminated by a diffusion process, through the papules and the feet of the tube.

Numerous phagocytic cells, called coelomocytes, are found in body fluid, which are also in the water vascular system. These envelop the waste and then migrate towards the papules, where the wall opens and they are expelled.

Also, some residues could be excreted through the pyloric glands and emptied together with the feces.

Moreover, so far, studies have not identified a mechanism for osmoregulation. So your body fluids are kept in the same saline concentration as the water where you live.

Some species could tolerate a low level of salt in the water, however, lacking a regulation system, they cannot inhabit fresh water bodies.

Sensory systems

In the starfish the sensory organs are not well defined. However, they are very sensitive to light, touch, changes in temperature and spatial orientation.

The spines and tubular feet are tender to the touch. They also pick up the chemical signals, thus allowing it to detect its prey.

At the end of each arm there are ocular points, made up of simple ocelli, in a number between 80 and 200. These pigmented cells respond to light and are covered by a transparent and thick cuticle that protects them. In addition, this membrane contributes to the focus of light.

Likewise, some have photoreceptor cells in various parts of the body. These have the ability to respond to visual stimuli, even if the eye spots are covered.

Nervous system

Although the starfish lacks a centralized brain, its nervous system consists of a ring around the oral cavity and a radial nerve. This runs through the body throughout the ambulacral area of ​​each arm. These have motor and sensory elements, which coordinate the balance of the star.

As for the peripheral nervous system, it has two nerve networks. One is a system of sensors located in the epidermis and the other network is located in the lining of the coelom cavity. The sensory nerves connect with their respective organs, while the motors control the musculature and the feet of the tube.

Circulatory system

The circulatory system is located in the body cavity. The vessels make up three rings, one around the mouth, another in the digestive system, and the third is located near the genital ring.

In relation to the heart, it beats about 6 times per minute and is located at the apex of the axial vessel that connects the 3 rings. In the area of ​​the base of each arm are the gonads.

Also, from the genital ring to the end of the arm is a lateral vessel. This has a blind end and the fluid that is inside does not have a fluid circulation.

Said liquid lacks pigment and is not directly related to gas exchange. Its usefulness could be related to the transport of nutrients throughout the body.

Taxonomy

-Animal Kingdom.

-Subreino Bilateria.

-Inferior Deuterostomy.

-Filum Echinodermata.

--Subfilum Asterozoa.

-Class Asteroidea.

-Order Velatida.

Caymanostellidae family.

Korethrasteridae family.

Myxasteridae family.

Family Pterasteridae.

--Superorden Forcipulatacea.

Order Brisingida.

Order Forcipulatida.

-Superorden Spinulosacea

Order Spinulosida Perrier.

-Superorden Valvatacea.

Order Notomyotida.

Order Paxillosida.

Valvatida order.

-Infraclass Concentricycloidea.

Peripodida Order.

Types

Pbsouthwood

Extinct groups

† Calliasterellidae, which included the genus Calliasterella, from the Carboniferous and Devonian periods.

† Trichasteropsida, made up of the genus Trichasteropsis, which lived in the Triassic. This group included at least two species.

† Palastericus, with a genus that inhabited the Devonian period.

Living groups

-Brisingida

This is made up of 2 families, 17 genera and 111 species. Species in this grouping have a small, inflexible disk. In addition, it has between 6 and 20 thin and long arms, which they use to feed.

On their bodies they have a single row of marginal plates, a unified ring of disc plates, and long spines on their arms. Likewise, the tubular feet lack suction cups and could have rounded tips.

- Forcipulatide

Made up of 6 families, 63 genera and 269 species. This order has distinctive valves, consisting of a short stem with 3 skeletal oscillations. Its body is robust and the tube legs have suction cups, arranged in four rows.

They are distributed in temperate regions of the North Atlantic, as well as in abyssal and cold waters.

-Notomyotida

To this group belong 1 family, 8 genera and 75 species. These starfish inhabit deep salty waters and their arms are flexible. On the inner dorsal surface of each arm they have longitudinal muscle bands. Some members lack suction cups on the tubular legs.

-Paxilloside

This group of primitive stars is made up of 7 families, 48 ​​genera and 372 species. They are characterized because their tubular legs do not have suction cups and because their cardiac stomach does not leave the body to feed. In addition, they have abundant papules on the aboral surface.

They generally live in sandy or soft-bottomed areas. An example of this order is Astropecten polyacanthus.

-Spinulosida

It is made up of 1 family, 8 genera and 121 species. Most stars in this order are valveless, but have small plates on the arms and on the disk. Likewise, on the aboral surface they have several groups of short spines. The red starfish Echinaster sepositus is a representative of this group.

-Valvatida

This grouping is made up of 16 families, 172 genera and 695 species. A large group of these animals have 5 arms and 2 rows of tubular legs with suction cups. Also, its suction cups are shaped like tweezers and are embedded in skeletal plates.

Some examples are the cushion star (Oreaster reticulatus) and sea daisies, belonging to the genus Xyloplax.

-Velatida

This order of starfish is made up of 4 families, 16 genera and 138 species. They live in deep waters or in cold waters, with a global distribution. They are pentagonal in shape with a number of arms that can range from 5 to 15.

In relation to its morphology, the body has a poorly developed skeleton, with spiny valves and papules, widely distributed in the aboral region.

Habitat and distribution

User: (WT-shared) Pbsouthwood at wts wikivoyage Starfish inhabit the Atlantic, Antarctic, Pacific and Indian oceans globally. However, there is greater diversity in some regions in the Indian-Pacific and in the Atlantic.

In this ocean, they extend from the European coasts to the islands of Cape Verde, including the Mediterranean Sea.

They live at various depths, from the intertidal area to the abyssal. Thus, they are also included in tropical coral reefs, tide pools, sand and mud, sea grasses, rocky shores and sea beds up to 6000 meters. However, the greatest diversity occurs in areas of coastal waters.

On the upper shore, they may be exposed when the tide recedes, which can occur during periods of desiccation. In that situation, the crevices under the rocks provide the only shelter. On the contrary, in the deep sea they inhabit steep cliffs and sandy bottoms.

Habitats

Of the 36 families that make up the Asteroidea class, 23 of these tend to live exclusively, or for most of their lives, in regions of cold water. In relation to tropical waters, 7 families develop in these and 6 families in salty water bodies in temperate zones.

Asteroid taxa that are distributed in cold temperate and cold water environments inhabit deep waters and high latitude regions. Some genera in this group are Ceramaster and Evoplosoma.

As for those who live in temperate waters, they constitute a minority. However, almost all families have some representation within this group. In some regions there is an overlap between these waters and tropical or cold environments.

In the genus Valvatida, there are several families that are found in tropical waters. An example of these are Acanthasteridae, Asteropseidae, Archasteridae, Mithrodiidae, Asterodiscididae, Ophidiasteridae, Oreasteridae and are all members of Valvatida,

Coral reefs

Coral reefs are one of the preferred habitats for some species of starfish, especially the crown of thorns (Acanthaster planci). This is characterized by having more than five arms and by being carnivorous, like others of its species.

When a group of these stars live on coral reefs, they could cause damage to the ecosystem. This is because these animals look for their prey in the soft tissues of the coral, which are abundant in the reefs. This attracts the stars, increasing their population, but decreasing that of the corals.

Oceans coasts

These marine animals can thrive easily in shallow ocean waters, including local beaches and rock wells. The proximity to the shore could expose the starfish to the threat of predators.

However, this represents less of a problem for this group, compared to other species, due to the starfish's ability to regenerate lost limbs.

Breathing

Respiration in starfish occurs through the tubular legs and papules, known as dermal gills. In addition, the coelom also intervenes, a set of channels that fill with water and that are connected to the tubular feet.

The physical phenomenon that governs the exchange of gases in this process is osmosis. In this, the oxygen and carbon dioxide molecules, dissolved in the water, move through a semipermeable membrane, without requiring energy expenditure.

Process

The hole in the upper part of the body, known as madreporite, allows water to enter. In this way, the cavity located in the center of the body is filled with fluid. This is carried to the tubular feet, where the gas exchange takes place.

In this process, the carbon dioxide flows through the thin skin of the tube feet, into the sea water where the star is located. Along with this, the oxygen dissolved in the water passes through the membrane and enters the body.

The vascular system is responsible for carrying oxygen from the feet of the tube to the rest of the body and collecting carbon dioxide and carrying it to the feet. The circulatory system could also play a role in this phase of respiration.

The gas exchange also takes place in the papules. These are bumps that exist on the walls of the top of the disc and on the arms. Oxygen is transferred from these structures to the coelom, where its fluid acts as a medium to transport the gases.

Reproduction

Sexual reproduction

Most species of starfish have separate sexes. Because the gonads are difficult to observe, it is not easy to distinguish the male from the female.

Some of the species are simultaneous hermaphrodites, since the body produces sperm and eggs at the same time. Also, it can happen that the same gonad produces sperm and eggs.

Other asteroids are sequential hermaphrodites, so their sex may change over the course of their lives. Thus, the Asterina gibbosa begins its life as a male and as it develops it changes to a female.

The situation is different in Nepanthia belcheri, since an adult female can divide and all the offspring are male. When they are adults, they become females.

The two gonads of the starfish are located in his arms. These glands have holes called gonoducts, through which the gametes are released.

Fertilization

Regarding fertilization, in the vast majority of cases it is external. However, in some species it occurs internally.

Generally, sperm and eggs are released into the water column, in order to be fertilized. To increase the chances of this happening, the starfish could group together and use chemical signals. For example, Acanthaster planci releases a substance into water that attracts males.

In those species that develop externally, the larva of the first stage is known as bippinaria. This lives free, forming part of the zooplankton. It is characterized by having a body covered with cilia and a pair of short arms.

When three more arms develop, it becomes a brachiolaria. However, in some cases it could evolve directly to the adult stage, as occurs with the species of the Paxillosida order.

The brachiolaria sinks to the seabed, adhering to the substrate. After this, the metamorphosis begins until reaching the adult stage. It is from this, when the arms grow and develop, while the larvae degenerate and disappear.

Incubation

In certain species, the females incubate the eggs, being able to keep them in specialized structures. Thus, it can be done in bags on the aboral surface or inside the gonads, as in Patiriella parvivipara.

Also, there are stars in which the larvae develop inside the pyloric stomach, as occurs in Leptasterias tenera. Others are known as hatchers, as they "sit" on the eggs, keeping their discs raised from the substrate.

Pteraster militaris incubates its eggs, which are large and have yolks. The developing offspring are called lecithotrophic, because they feed on the yolk. Generally the egg develops directly to the adult stage.

Asexual reproduction

Asexual reproduction involves fission or regeneration of the animal, starting from a piece of the arm. In relation to fission, the atomic nucleus splits into two or more fragments. Occasionally, the same animal can cause this rupture, releasing chemicals that facilitate it

Some, like Linckia laevigata, divide by disc, with offspring of identical genetic makeup. Other starfish, usually very small in size, have autotomic asexual reproduction. In this, the animal pinches one or more arms, which will later create a disk and the arms

Even some starfish that reproduce sexually may eventually present asexual characteristics at some stage of their life. For example, the larvae could shed some of their body structures, which will turn into another larva.

Feeding

Most starfish are generalist predators. Thus, they consume micro algae, sponges, snails, shellfish, coral polyps, worms and even other echinoderms. However, others are specialized, feeding almost exclusively on algae or bivalves.

Also, they can be scavengers or detritivores, thus feeding on decomposing organic material and fecal matter.

To find their prey they use the smells that they emanate, a product of their organic waste, or by the movements they make. Feeding preferences may vary, due to seasonal availability and geographic variations of the species.

Digestive system

The intestine occupies much of the disc and expands into the arms. As for the mouth, it is located in the central part of the oral surface. There it is surrounded by a peristomial membrane and has a sphincter, which closes it.

This opens, through a short esophagus, to a stomach. This organ is divided into a pyloric and a cardiac portion. In addition, it has a short intestine that extends from the pyloric stomach to the anus.

The digestion

Primitive starfish, such as Luidia and Astropecten, ingest their prey whole, beginning their digestion in the cardiac stomach. Those elements that it does not consume, such as the carcasses, are expelled through the mouth.

The semi-digested material reaches the pyloric stomach, where digestion continues and nutrients are absorbed.

In more developed species, the cardiac stomach can leave the body to swallow and digest food. In the case that the prey is a clam, the starfish slightly separates the two valves with its tubular feet.

Then, it inserts a small part of its stomach into the other animal's body, which secretes enzymes to start the digestive process. Subsequently, the stomach, together with the semi-digested mass, retracts into the body and passes into the pyloric stomach.

Due to the ability to digest its prey outside the body, the starfish can hunt animals larger than its mouth. Thus, it can consume arthropods, oysters, small fish and mollusks.

However, some can be herbivorous or can trap food particles in the water.

Behavior

Starfish are considered unsocial. However, at certain times of the year, they form groups.

This behavior tends to originate on various occasions, such as in the spawning stage, in feeding around corals or in seasonal migrations, directed to deeper waters in the high seas.

Daily patterns of activity are in sync with variations in light intensity. In this way, the vast majority of activities are carried out at dusk and dusk. Thus, you can avoid threats from predators.

Also, this synchrony manages to coincide foraging with the activity of its prey, thus being able to capture it more easily.

Despite lacking a central nervous structure, like the brain, it has a diffuse nerve network and a sensory system in the skin. This allows it to capture light stimuli, variations in ocean currents, and chemicals. Thus, they can perceive the proximity of both a prey and a predator.

Movements

The vast majority of starfish do not move quickly. Thus the leather star (Dermasterias imbricata), manages to move 15 centimeters per minute.

Other species, belonging to the Luidia and Astropecten genera, have, instead of suckers, some points along the entire length of the tubular feet. This makes it easier for them to move faster as they glide over the seabed. In dangerous situations, starfish could move bilaterally.

References

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