Echinaster

Sea stars can live up to 35 years in the wild under the proper conditions,[5] but the lifespans of Echinaster can be variable and affected by environmental factors such as salinity, temperature of water, light availability, and pollution.[6] Furthermore, the lifespan of an Echinaster can also be affected by humans. Habitat destruction and overfishing are two factors which have adverse affects on populations of Echinaster.[6]

Echinaster are able to start reproduction by broadcast spawning gametes into the water, where externally fertilized eggs develop into planktonic larvae.[6] Most species only produce brachiolaria larvae which are lecithotropic, non-feeding larvae.[6] However, some Echinaster produce larvae which first go through a smaller, feeding bipinnaria stage, and then through the second brachiolaria larval stage.[6] Their eggs vary in size depending on parental investment, the amount of nutrients found in eggs, and the surrounding habitat.[6] Sea stars are deuterostomes and the first cleavage begins shortly after fertilization and is holoblastic.[6] Fourteen hours after fertilization, a wrinkled blastula is formed.[6] Twenty hours after fertilization, a blastula with an invaginated pore at the vegetal pole forms.[6] The blastula then rotates around an axis in circular motions, the embryos then undergo a longitudinal stretching.[6] Cilia begin to surround the entire body causing movement along the anterior-posterior axis.[6] Six days after fertilization occurs, the anterior body extends, while the posterior body flattens laterally.[6] Shortly after, tube feet and the central disk begin to appear on the body.[6] The mouth and spine begin to form on the body and after fifteen days, symmetry is more pronounced and the eyespot has fully developed.[6] 60 days after fertilization occurred the sea stars can evert their stomach, their mouths become active, and they begin to feed on algae.[6] The madreporite develops after 88 days and the hydropore develops on one of the primary plates.[6] The first 40 days of Echinaster's development are distinguished by pronounced growth, after 40 days this growth begins to slow down significantly.[6]

Echinaster sepositus regeneration

Echinaster can suffer frequent damage to their arms, so it is important they are capable of quick repair. When any part of the arm becomes damaged, the stump of the arm constricts, causing the coelom to seal.[13] The combination of coelomic fluid migrating to the wound site and arm constriction, clots formed and the stump begins to seal, closing any open sites.[13] Papullae appear to deflate and the dense connective tissue becomes more densely packed.[13] Circular muscle fibers contract in the arm causing the tube feet to pull towards the wound.[13] Twenty-four hours into arm regeneration, the body wall of the arm is still contracted and the aboral side of the arm has folded to the oral side.[13] A thin epithelial layer has begun to form and the stump has begun to reform due to dedifferentiated epidermal cells.[13] At this stage, the radial nerve cell has also begun to heal.[13] 72 hours into arm regeneration, the aboral arm wall is still covering the wound site, however, now the body wall has relaxed and the papullae began to inflate again.[13] A new epidermis has formed and is now thicker and stronger than the previous epidermis.[13] Directly beneath the epidermis of the wound, phagocytes can be observed ingesting any bacteria or extra cells which are not necessary for regeneration.[13] Morula cells are also present; these cells focus on wound healing and repairing the extracellular matrix.[13] When the arm is completely repaired, myocytes can be ingested by phagocytes, used a direct source for new cells, or used as a source of energy.[7] Stem cells may also contribute to regeneration in sea stars but little is known about their contributions in the Echinodermata.[7] Altogether, arm regeneration takes a couple of weeks for members of the Echinaster genus, however, the arm begins to repair itself in as few as 3 days.[13]

Echinaster brasiliensis: A-Spines of arm, B-Tube feet, C-Abactinal view of Arm.

Starfish generally move by using their tube feet.[14] Water enters through the madreporite and reaches the tube feet, causing an expansion and contraction of the tube feet, which moves the sea star.[14] When fully inverted, Echinaster and other sea stars are able to exhibit a behavior known as righting.[15] This response is the ability to adjust tube feet to their proper orientation after a full body inversion.[15] It can be useful if an organism gets caught in a storm surge or is dislodged by a predator, and also serves as a marker to assess their functional status when exposed to environmental changes.[15] As the temperature increases, tube feet can lose their ability to adhere to surfaces.[15] Studies show this is because the neuromuscular system does not adjust to thermal changes well.[15] This directly affects the Echinaster's ability to right, causing their response to be slower and less efficient.[15] When faced with thermal stress, Echinaster were found to right by somersaulting, this is where two adjacent arms twist, with the oral sides facing each other, and touch the substrate to become the sea star's lead arms.[15] Another arm opposite the lead arms then touches the substrate.[15] After this, the lead arms move towards the center of the organism and begin to move under the animal.[15] Finally, the final arm releases the substrate and the free arms flip over the sea star resulting in a somersault motion.[15] Furthermore, not only did a thermal increase result in a change in righting response, it also increased mortality rate in Echinaster and slowed metabolic response rates overall.[15]

Sea stars of the Echinaster genus are typically found in tropical and temperate waters at the bottom of the sea floor in shallow waters and in rocky shores across the globe.[16] Most of the genus Echinaster can be found within the Caribbean and Mediterranean Seas, Pacific Ocean, Atlantic Ocean, and Indian Ocean.[16] Evidence suggests that some species of these sea stars may linger around mangroves and reefs to prey upon the sponges which populate these areas.[16]

Echinaster feed mostly on biofilms, encrusting invertebrates, such as sponges, and microalgae.[2] One study performed showed that sea stars of the Echinaster have no problem eating the spicules of sponges, along with the sponge skeleton.[16] Studies also showed that the Echinaster prefer sponge species that lack chemical defenses.^[18] They are the least likely to eat sponges with a rubbery texture.[16]

This genera typically associates themselves with their prey in the same area, if not found in a feeding position, their stomachs are typically found partially-everted.[11] When Echinaster settle in feeding positions, their stomachs tend to evert into a button-like structure.[11] Echinaster are able to receive nutrients through external digestive activity or they can acquire nutrients through detritus.[11]