Starfish / Sea Star

Sea stars, also known as starfish, are echinoderms belonging to the class Asteroidea. The names "sea star" and "starfish" are sometimes differentiated,with "starfish" used in a broader sense to include the closely related brittle stars, which make up the class Ophiuroidea, as well as excluding sea stars which do not have five arms, such as the sun stars and cushion stars.

Sea stars exhibit a superficially radial symmetry. They typically have five "arms" which radiate from a central disk (pentaradial symmetry). However, the evolutionary ancestors of echinoderms are believed to have had bilateral symmetry. Sea stars do exhibit some superficial remnant of this body structure, evident in their larval pluteus forms.

Sea stars do not rely on a jointed, movable skeleton for support and locomotion (although they are protected by their skeleton), but instead possess a hydraulic water vascular system that aids in locomotion. The water vascular system has many projections called tube feet on the ventral face of the sea star's arms which function in locomotion and aid with feeding. Sea stars usually hunt for shelled animals such as oysters and clams. They have two stomachs. One stomach is used for digestion, and the other stomach can be extended outward to engulf and digest prey. This feature allows the sea star to hunt prey that is much larger than its mouth would otherwise allow. Sea stars are able to regenerate lost arms. A new sea star may be regenerated from a single arm attached to a portion of the central disk.

Diet

Most species are generalist predators, eating clams, oysters or any animal too slow to evade the attack (e.g. dying fish). Some species are detritivores, eating decomposed animal and plant material or organic films attached to substrate. Others may consume coral polyps (the best-known example for this is the infamous Acanthaster planci), sponges or even suspended particles and plankton (sea star from the Order Brisingida).The processes of feeding and capture may be aided by special parts; Pisaster brevispinus or short-spined pisaster from the West Coast of America may use a set of specialized tube feet to extend itself deep into the soft substrata to extract prey (usually clams). Grasping the shellfish, the sea star slowly pries open the shell by wearing out the adductor muscle and then inserts (also called evisceration) its stomach into an opening to devour the organism. Sea stars have also been known to attack animals such as marine snails. Protoreaster Nodotus, or horned sea stars, eat snails in an aquarium setting if the detritus in the tank is not sufficient to satisfy the sea star's hunger.

Reproduction

Sea stars are capable of both sexual and asexual reproduction. Individual sea stars are male or female. Fertilization takes place externally, both male and female releasing their gametes into the environment. Resulting fertilized embryos form part of the zooplankton.

Sea stars are developmentally (embryologically) known as deuterostomes. Their embryo initially develops bilateral symmetry, indicating that sea stars probably share a common ancestor with the chordates, which includes the fish. Later development takes a very different path however as the developing star fish settles out of the zooplankton and develops the characteristic radial symmetry. Some species reproduce cooperatively, using environmental signals to coordinate the timing of gamete release; in other species, one to one pairing is the norm.

Sea stars commonly reproduce by free-spawning: releasing their gametes into the water where they hopefully are fertilized by gametes from the opposite sex. To increase their chances of fertilization, sea stars probably gather in groups when they are ready to spawn, use environmental signals to coordinate timing (day length to indicate the correct time of the year, dawn or dusk to indicate the correct time of day), and may use chemical signals to indicate their readiness to each other.

Fertilized eggs grow into bipinnaria and later into brachiolaria larvae, which either grow using a yolk or by catching and eating other plankton. In either case, they live as plankton, suspended in the water and swimming by using beating cilia. The larvae are bilaterally symmetric — unlike adults, they have a distinct left and right side. Eventually, they undergo a complete metamorphosis, settle to the bottom, and grow into adults.Some species of sea star brood their young: the males spawn gametes which fertilize eggs held by the females. The females may hold the eggs on their surface, in the pyloric stomach (as in Leptasterias tenera), or even attach them to the ground (as in Asterina gibbosa). Brooding is especially common in polar and deep-sea species, environments less favourable for larvae.

Male and female sea stars are not distinguishable from the outside; one needs to see the gonads or be lucky enough to catch them spawning. The gonads are located in each arm, and release gametes through gonoducts located on the central body between the arms.

Some species of sea star also reproduce asexually by fragmentation, often with part of an arm becoming detached and eventually developing into an independent individual sea stars. This has led to some notoriety. Sea stars can be pests to fishermen who make their living on the capture of clams and other mollusks at sea as sea stars prey on these. The fishermen would presumably kill the sea stars by chopping them up and disposing of them at sea, ultimately leading to their increased numbers until the issue was better understood. A sea-star arm can only regenerate into a whole new organism if some of the central ring of the sea star is part of the chopped off arm.