Starfish They are echinoderms that do not have mobility and yet are living beings. They are quite peculiar and live in the oceans. Accustomed to talking about various species de peces, this article is quite special and curious. These animals are similar and related to sea urchins and spongesThe scientific name is Asteroidea, and we can find various species that we will see throughout the post. Although they are often said to be immobile, yes they move on the background by hundreds of tube feet driven by an internal hydraulic system.
Do you want to learn everything about the starfish? Keep reading because this article is packed with valuable information
Key features
The starfish differs from many other species such as the fish that we are used to talking to in many ways. The first thing is that they do not need gills to breathe.They have an opening through which they exchange gases to introduce oxygen dissolved in the water into their bodies. In reality, the exchange occurs mainly through dermal papules (small extensions of the skin) and the tube feet themselves, and water enters the system through the madreporite.
Unlike many other animals, they are quite long-lived, being able to live up to 35 years if conditions are favorable. Depending on the conditions and the species, they can weigh up to 5 kg. Their skin is spiny and is made of a tough, hard covering of calcium carbonate with internal plates called ossicles, which constitute their endoskeleton. Thanks to this covering, they go unnoticed and can protect themselves from predators.
The starfish has 5 limbs around a central disc-shaped body.They are animals with pentaradial symmetrySome species that increase the number of limbs are capable of having up to 40 arms.
Although they cannot move, as their calcium carbonate coating prevents it, they can move from one place to another. Some species, while not particularly mobile, are capable of moving some limbs. To move, they crawl along the ground, as they cannot swim. Their arms are covered with pincer-like organs and suction cups, which they use to expel air for propulsion and move slowly across the ocean floor. In reality, their movement is not based on air: tube feet operate with water pressure of the water vascular system; when its internal blisters contract, the water pushes each foot, which adheres by means of sticky secretions and not by suction.
On the tips of the arms they have sensors that help them to locate, perceiving the amount of light and that's how they find the food they need to live. Those sensors are ocelli, simple eyes that detect light and shadows and, together with chemoreceptors in the feet, help locate prey or shelters.
In addition, many starfish present pedicellariae (small pincers) on its surface, useful for defending oneself and cleaning the body; and they have a hematological system (simple circulatory system) that accompanies the digestive system. Its body wall combines calcified dermis and mutable connective tissue: it can harden or soften quickly to facilitate autotomy (loss of arms) or grasping.
Types of starfish
Starfish comprise thousands of species with different characteristics each of them. They are distributed throughout the world. The best known for their abundance and for their spread in the media are those classic 5-armed starfish. As we have mentioned before, this is not always the case. It has been possible to find specimens of other species of echinoderms with up to 40 arms.
Within the class Asteroidea several are recognized orders main ones that group diversity: Brisingide, Forcipulatide, Notomyotide, Paxilloside, Spinuloside, Valvatide y veiledThere is also a peculiar group, Concentricycloidea, whose position is uncertain. Below, we review the most representative ones with their distinctive features.
Brisingide
It is starfish that They are composed of between 6 and 16 armsThis type of starfish comprises six families and 16 genera of sea stars, the same number of arms it is composed of. Many species live in deep waters and They feed by suspension, extending its slender arms into the flow of particles.
Forcipulatide
This type is composed of 400 species distributed in 6 families of 70 genera. Its main characteristic is that it has forcipulate pedicellariae visible on the surface of their body, similar to pincers. They tend to be robust and live from temperate waters even cold and abysmal.
Notomyotide
This type of star has about 70 species, which are contained in about 12 genera. These arms are more flexible than most starfish. This movement is due to the fact that they have some muscle bands along their internal surface that allow them to move and help in their displacement along with the aforementioned flatulence.
veiled
This starfish has a rather large body. strong which has a large disc in the center of the body and small depressions. There are more than 300 species of velatidas in 25 genera and 5 families, with a notable presence in cold and deep waters.
Valvatide
They are the best known in the world. They exist 700 species with a total of 170 genera and 14 families. They are the most famous for having 5 arms and for having evident marginal plates and pincer-shaped pedicellariae.
Paxilloside
This order groups species adapted to soft funds (sand or mud). They lack an anus and their tube feet are usually not having suction cups, which facilitates digging and moving quickly over sediments. During larval development, They do not present a brachiolar phase, a peculiarity of the group.
Spinuloside
The Spinulosida are characterized by a more delicate and abundant low thorns on the upper surface. They generally lack pedicellariae and show discrete plate patterns, with very varied species in shallows and deep.
Taxonomic note: the group Concentricycloidea (known by the so-called "sea daisies") has traditionally been assigned to Asteroidea but its position is uncertain and disputed, which is why it is considered separate within modern classifications.
Habitat and food
Starfish live in almost all marine habitatsThey are vulnerable to contamination because they take water directly into their bodies to filter out dissolved oxygen. Therefore, if the water is contaminated They die from poisoning and drowning.
In the seas and oceans, these animals make up a large part of the biomass present. They also play an important role in the ocean floor and the communities that inhabit it. The habitats where we can find them are oceans, rocky coasts, seagrass beds, coral reefs, seagrass beds, tidal pools and sandy bottoms; their depth ranges from intertidal zone to abyssal areas of great darkness.
As for food, starfish feed mainly on molluscs like some oysters, snails, and clams. They have different forms of feeding that are the result of their evolution and adaptation. Once the starfish has attached its body to the prey it wants to eat, extends his stomach outward, pushing it out through its mouth. The stomach generates enzymes capable of breaking down its prey until it is completely ingested. This helps the food pass directly to the stomach, allowing it to be digested completely and easily. Smaller organisms are easy prey for starfish.
In addition to preying on molluscs, many species are opportunists and supplement their diet with detritus, small crustaceans or suspended particles that capture with mucous membranes and cilia; others, such as some brisingids, specialize in filtration. On certain reefs, corallivorous species can cause population outbreaks that damage the coral if there are no natural controls.
Contrary to them, the main predators of these echinoderms are sharks like the White shark o the bull shark, manta rays, other larger starfish and some species de pecesThe hardness of the endoskeleton, the pedicellariae and the regeneration capacity Arms are key defenses against predation.
Lifestyle
To defend themselves from predators they use some defense mechanisms such as hard skin and spines, others have bright colors to appear poisonous and are able to camouflage themselves among plants and corals or lose an arm in order to stay alive.
These animals they are not social at all, but live solitary lives for most of their lives. Occasionally, they can be seen together with other birds during times when food is more available.
Locomotion is carried out thanks to the aquifer vascular system. Water enters through the madreporite, circulates through a stone channel to the central ring and from there to the radial channels in each arm. Each tube foot has a internal blister which, when contracted, pushes the water towards the external podium, extending it. Adhesion is achieved with sticky substances which they secrete and then neutralize to release. Some soft-bottom species, with feet without suckers, move faster on sand.
Regarding the senses, although they lack a centralized brain, they have a nerve ring and coordinated radial nerves, as well as ocelli with which they detect light and chemoreceptors that perceive dissolved substances (e.g., chemical signals from prey or conspecifics). They can respond to temperature, orientation and state of the water, which guides their feeding and shelter behavior.
About reproduction, most species are dioecious (separate males and females) and perform external fertilization, releasing sperm and eggs into the water synchronously, often in groups to increase the success of gamete encounters. Some are hermaphrodites (simultaneous or sequential) and various species practice the parental care, incubating the eggs under the disc or in body cavities. The planktonic larvae (bipinnaria and brachiolaria) swim using cilia before settling and metamorphosing into radially symmetrical juveniles.
They also reproduce asexually by fission of the disc or by autotomy of arms. Many species can regenerate lost limbs and even reconstruct the disk if a sufficient fragment remains. This process, although energy-consuming, provides resilience to attacks or accidents.
An important aspect for your well-being is that they do not regulate salinity Internally, like fish, they maintain the same salt concentration as seawater. Therefore, they don't live in freshwater and are sensitive to low salinity. This physiology explains why handling them out of water is dangerous for them: if they are taken out of the water They stop gas exchange, become stressed, and can die quickly. The best thing is do not extract them nor manipulate them for photographs or souvenirs.
The most common threats include contamination (oil, metals, biocides), the souvenir trafficking, capture for aquariums and habitat degradation (reefs, seagrass meadows). As key predators in many seabeds, their decline can disrupt entire communities; for example, when sea stars that feed on sea urchins are missing, they proliferate and overgraze kelp forests, impoverishing the ecosystem.
Knowing their anatomy, diversity and ecological role allows us to appreciate and protect them better: leaving each specimen where it is, avoiding pollutants and supporting marine protected areas are simple gestures that make a difference for these creatures. fascinating asteroids and for the health of the oceans.