Asteroidea - Sea stars


STARS OF THE SEA

Sea stars are certainly the best known representatives of the great echinodermata phylum.

CLASSIFICATION

Kingdom:Animalia
Phylum:Echinodermata
Subphylum:Eleutherozoa
Superclass: Asterozoa
Class:Asteroidea
Common name: sea ​​stars or starfish

HABITAT AND GEOGRAPHICAL DISTRIBUTION

In the classroom Asteroidea (Phylum echinoderms) we find that vast group of animals, very familiar to all, commonly known as starfish or sea stars.

There are about 1500 species that live mainly in sandy bottoms, rocky mudflats, especially along the coasts.

With the exception of a few specimens that live in brackish waters, sea stars are animals that live in marine waters around the world.

CHARACTER, BEHAVIOR AND SOCIAL LIFE

We cannot speak of social life in sea stars. An intraspecies form of interaction can be considered the emission of pheromones by adults to attract the larvae and make them establish in their immediate vicinity even if one cannot in any case speak of parental care.


Crossaster papposus (Note 1)


Ctenodiscus crispatus (Note 1)

PHYSICAL CHARACTERISTICS

Being such a large group, the morphology is also extremely vast. In fact, we find sea stars ranging from just under 2 cm to 1 m in length although most of the specimens are 12-24 cm.

The colors are very variable in fact we find sea stars of red, orange, green, blue, purple and combinations of different colors.

The characteristic of sea stars is that they have a central body from which they generally develop 5 arms even if they can reach up to 40, varying in length from very short to very long (in some examples they are so small as to look like bearings).

In the lower part of the central disc (the one in contact with the ground) it is found mouth from which depart a series of channels that reach the extreme parts of the arms. In the upper part of the disk there are theanusand the madreporite (which is an opening of theaquifer postain position opposite the mouth in the upper part of the disc also called platamadreporica which also corresponds to one of the genital plates) through which the liquid of the aquifer system communicates with the outside and radiates up to 5 or more extreme tips of the arms.

Along the arms of the starfish are located i ambulacral pedicels which are the adhesive, tactile and locomotional organs that protrude outward with a terminal sucker and vesicles that allow locomotion.

The internal skeleton is made up of calcareous ossicles.

Along the arms are the ramifications of the digestive system; the simple nervous system, which is not centralized; the system that distributes nutrients and the reproductive system.

In starfish, the senses of touch, smell and taste are well developed. In addition, most species are equipped with photosensitive spots on the extremities of the arms, thanks to which they respond to light stimuli.

The outer surface can be smooth, covered with spines, tubercles or ridges.


Pycnopodia helianthoides (Note 1)


Dipsacaster borealis (Note 1)

EATING HABITS

Sea stars are considered high-level predators, feeding on prey that move slowly in the water such as gastropods, bivalves, crustaceans, annelids and other invertebrates. In practice, they grab the prey and then overturn the stomach outside the body to make it adhere to the predated animals, secreting the enzymes necessary for digestion and then sucking up the decomposed substance.

They manage to open without major problems the valves of the bivalves to which they adhere with the suckers placed in the arms (on the pedicels), then feeding on the animal that is inside.

Many starfish feed on the plankton that adheres to the body which they bring to the mouth through a system of cilia placed throughout the body.


Ceramaster patagonicus (Note 1)


Leptasterias coei (Note 1)

REPRODUCTION AND GROWTH OF THE SMALL

Starfish are animals that have separate sexes (although there are few cases of hermaphroditism): both spermatozoa and eggs are deposited in water where they fertilize.

In some species the fertilized eggs are protected by the mother at the bottom of the sea.

One is born from the eggs larva call bipinnaria which has the particularity of having a bilateral symmetry (unlike that of the adult form which is pentaraggiate symmetry). The bipinnaria swims freely and feeds on plankton.

After a certain period of time, the bipinnaria larva is transformed into a second larval form called brachiolaria with five small arms (starting to resemble the adult form). This larval form of sea star also swims freely in the water until it stops on the seabed, loses its arms and undergoes the final metamorphosis, becoming an adult starfish with 5 (or more) definitive arms.

It appears that the larvae are attracted to pheromones emitted by adults which allow them not to disperse but to remain close to the parent also does not provide parental care.


Lethasterias nanimensis (Note 1)


Pteraster tesselatu (Note 1)

PREDATION

The larvae are considered plankton and therefore as such have numerous predators while the adult sea stars have practically no natural enemies as they are considered unattractive and not very nutritious with the exception of Hyperoodon ampullatusa cetacean that includes them in its diet.

STATE OF THE POPULATION

The different species of sea stars are not threatened with extinction.

SOCIAL, ECONOMIC AND ECOSYSTEM IMPORTANCE

The sea stars are real predators and in some cases they are considered harmful as for example they damage the oyster beds or they can be so numerous as to damage coral reefs (for example the species Acanthasterplanci, photo on the side) or the colonies of molluscs. bivalves in general.

CURIOSITY'

The members belonging to the classAsteroidea are closely related to theOphiuroidea, having found common ancestors with five arms attached to a central disc (synapomorphy).

Sea stars have the ability to regenerate missing body parts, such as arms, and in some species it also represents an asexual reproduction system: an entire individual regenerates from a single piece of the parent.

Note

(1) Original photograph courtesy of Alaska Fisheries Science Center (NOAA)


Video: Sea Stars


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