![]() ![]() Another chemical bond is secreted to release the tube feet. The end of the tube feet have suckers, which chemically adhere to the substrate. Cilia within the epithelium of the water vascular system moves the water. Internal muscle contractions squeeze fluid to the tube feet, which then elongate. ( Canada's Aquatic Environments, 2002) Behavior ( Brusca and Brusca, 2003 Canada's Aquatic Environments, 2002) Brooding species are usually found in environments that are harsh for the larval stage. gonochoric/gonochoristic/dioecious (sexes separate)Īlthough there is generally no parental investment beyond fertilization, a few hermaphroditic species brood their eggs.A few hermaphroditic species brood their young. Most asteroids are free spawners, releasing sperm and eggs into the water. Gonopores are usually at the base of each arm. Asteroids usually have two gonads in each arm and a gonopore opening to the oral surface. In sexual reproduction, asteroids are mainly gonochoristic (having separate sexes), but a few are hermaphroditic. ( Anonymous, 2005 Barnes, 1987 Brusca and Brusca, 2003 Canada's Aquatic Environments, 2002)Īsteroids can regenerate arms and some can reproduce asexually as the central disc divides. After settling, the larvae go through a sessile stage and metamorphose. Metamorphosis in some species is triggered by adult pheromones. ![]() Adult pheromones may attract larvae, which tend to settle near conspecific adults. Embryonic coelomic structures have specific fates as the bilaterally symmetrical larvae metamorphose into radially symmetric adults. Fertilized eggs develop into bilaterally symmetrical planktonic larvae, which have 3-part paired coeloms. ( Brusca and Brusca, 2003 Canada's Aquatic Environments, 2002)Īsteroids are deuterostomes. Larvae are bilaterally symmetrical and adults are radially symmetrical. The hemal system parallels the water vascular system and probably distributes nutrients from the digestive tract. Oral surface, under the central disc, is where the mouth is located. Tube foot is made of an ampulla, podium, and usually a sucker. Each radial canal ends in a terminal tube foot, which has a sensory function.Įach radial canal has a series of lateral canals that terminates at a tube foot. Tiedemann’s bodies and polian vescicles are pouches on the ring canal whose function may be osmoregulation or hydraulic regulation within the water vascular system. The stone canal is attached to a ring canal which leads to each of the five (or more) radial canals. Internally, the madreporite leads to a stone canal, made up of skeletal deposits. Water vascular system of the sea stars open up at the madreporite, a perforated opening in the central part of the animal. Calcareous ossicles make up the internal skeleton. A majority have 5 arms, although some can have up to 40. Arms extend from the body from a central disk and can be short or long. ( Brusca and Brusca, 2003 Canada's Aquatic Environments, 2002 Waggoner, 1994)Īsteroids can range from less than 2 cm to over one m in diameter, although the majority are 12 to 24 cm. ( Brusca and Brusca, 2003 Canada's Aquatic Environments, 2002)Įxcept for a few species which inhabit brackish waters, asteroids are benthic organisms found in marine environments. Mainly a marine group, asteroids are found in all the oceans. ( Brusca and Brusca, 2003 Canada's Aquatic Environments, 2002) Geographic Range There are approximately 1500 extant species in the Asteroidea. ![]()
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