MARB 435 1st Edition Lecture 20Current LecturePhylum Echinodermata Approximately 6,000 living species Good fossil record: 13,000 described fossil species Morphology:  endoskeleton of calcareous ossicles pentamerous radial symmetry in the adults: body can be divided into five similar parts arranged around a central axis (but bilateral symmetry in larvae) Tricoelomate Catch connective tissue Coelomicwater vascular system forms hydraulic tube feet for locomotion, feedingand gas exchangeCatch connective tissue A.k.a. mutable connective tissue Variable and reversible rigidity of connective tissue Nerves control the stiffness of extracellular matrix Increase in Ca2+ stiffens the ECM (probably by forming bridges between proteins) Allows automy (voluntary casting off) of arms in a sea star or viscera in a sea cucumber as escape mechanismsDevelopment Typical deuterostome development:These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute. radial cleavage Blastopore becomes anus, mouth forms secondarily Coelomic cavities develop by enterocoely Mesoderm is derived from archenteron Larva is always bilaterally symmetricalClass StelleroideaSubclass Asteroidea Approximately 1500 species Star-shaped body Occur worldwide, mostly in coastal waters Benthic Can be colorful Usually 5 arms, but sometimes up to 40Asteroidea morphology• Mouth lies in the middle of the oral surface• ambulacra (grooves) extend into the arms• Each ambulacrum contains 2-4 rows of tube feet for locomotion• Movable spines on each side can cover the ambulacrum• A small eyespot lies at the end of each ambulacrumEndoskeleton Consists of ossicles (small calcareous plates secreted by sclerocytes)  Can form spines, rods, plates or crosses pedicellariae: claw-shaped ossicles Defense against parasites or small settling organisms Sometimes also used for prey captureNervous system Central nervous system: Circumoral nerves Radial nerves Peripheral nervous system ectoneural system: lies in the epidermis and is mainly sensory hyponeural system: lies in the coelomic lining and is mainly motoric Sense organs: Eyes at tip of arms Sensory tube feetDigestive system and feeding• Many asteroids can evert their cardiac stomach for extra-oral digestion• Stomach releases digestive enzymes and engulfs the prey• Most asteroids are scavengers and carnivoresAsexual Reproduction Some species can automize and regenerate arms Some can reproduce by fission by softening themutable connective tissueSexual Reproduction Gonochoric 10 gonads: 2 in each arm Each gonad has its own gonopore between the bases of the arms Most sea stars are free spawners: a single female can release up to 2,500,000 eggs! Most have planktonic larvae but some brood their offspringSea Star Wasting Syndrome First discovered in the ochre star, Pisasterochraceus, in June 2013 in Olympic National Park Affects at least 20 species Has been observed on the north American west coast from Alaska to northern Mexico inintertidal and subtidal habitatsSubclass Ophiuroidea About 2000 species Mostly cryptic lifestyles Five jointed arms that are clearly set off from the central disc Arms lack ambulacral groove Tube feet have no suckersReproduction and development Asexual reproduction common (by autotomy of arms) Sexual Reproduction: Have small pockets at the bases of the arms called bursae Gonads open into the bursae Larvae are often brooded in the bursae Planktonic