Presentation on theme: "Phylum Echinodermata. Introduction n Echinodermata are all marine, triploblastic unsegmented coelomates n Phylum has 3 unique features: u pentagonal symmetry."— Presentation transcript:
Introduction n Echinodermata are all marine, triploblastic unsegmented coelomates n Phylum has 3 unique features: u pentagonal symmetry (bilateral in larvae) u calcite spicules embedded in the skin, often partly fused u Tube feet (podia)
Affinities n The only connected phylum is our own, the chordates - based on embryological evidence.
An unhurried phylum.. n No echinoderm moves fast, apart from a very few deep sea holothurids which swim actively n Crinoids are sessile, the others crawl at a rate of mm / minute n During one Antarctic marine survey a starfish was tagged. A year later the same animal was in the same exact spot, having apparently done nothing at all!
Anatomical basics: n There is no cephalization n There is a meaningful gradient in all echinoderm bodies: one surface has the mouth and tube feet (ORAL or AMBULACRAL), while one does not (ABORAL) n The anus is often, but not always, aboral.
Originally… n The ancestral echinoderm was a sessile filter-feeder, extending its oral surface upwards to capture food n This sedentary design has evolved into motile forms where the feeding surface faces downwards
Functional groups 1: nerves n Echinoderms have a diffuse nervous system with no brain n There is a 5-radial circum-oral nerve ring, and a superficial net running close to ectoderm
Hydraulics n These are far more complex than the nervous system! n Main hydraulic systems are derived from the coelom, although separate sections of the coelom also surround viscera n The podia are operated by a hydraulic system called the water- vascular system
5-radial layout n Many organ systems in the echinoderms follow the same basic structure as the water-vascular and nervous systems: a 5-radial circum- oral ring n These rings give rise to 5 radial branches (canals in the case of the WVS) n A few asteroids have 7, 10, 11 arms - in which case 7,10, 11 radial branches
Hydraulics, contd. n Each radial canal of the WVS supplies water to tube feet, each with its ampulla n There is one asymmetric element: a single tube (the stone canal) running from the oral WVS ring to the outside via the madreporite
Surface features n Echinoderm skin has several distinctive sets of organs protruding from their skin: u Tube feet (podia) u Spines u Pedicillaria
Tube feet.. n Podia are not scattered haphazardly over the body surface n They lie in 10 rows (5 pairs), the ambulacral grooves n Each tube foot + its ampulla is isolated from the WVS by a valve n Tube feet vary - starfish have muscular suction cups, other forms have sticky tips. n Crinoids are different - primitive
Tube feet.. n Originally began as outgrowths of the WVS. In crinoids and ophiuroids these remain essentially as tentacles. n In other radiations, notably asteroids, these have evolved a highly specialised suction cup used for locomotion and prey capture.
Tube feet.. n Have retractor muscles and can bend, but no extensors n To extend, muscles around the ampulla contract n Each podium has a nervous arc to its branch of the hyponeural system
Role of WVS n Hydraulics n Respiration - O2 is exchanged between ampulla and perivisceral coelomic fluid n Probably (?) this was the ancestral function of the WVS, with tubes + podia lining arms to exploit ciliary current already used in food collection
Pedicillaria n …Are defensive organs, assumed to protect against encrusting organisms n Are active, independent local effector units able to inject toxins on contact
Madreporite n Allows pressure equalization and top up water supply to the WVS n Is absent in crinoids
Gonads n Lie as 10 (2N) paired structures at the base of ambulacral grooves. n Sexes are separate, and discharge gametes into the sea water n Gonads can be large - echinoid gonads almost fill the test, and can be eaten as a delicacy.
Sadly... n Of the 13 classes of echinoderms known, 7 are extinct. n Echinoderms were dominant forms in Carboniferous seas, but have suffered a long-term decline in phyletic richness
Crinoidea n Feather stars & Sea lilies n Abyssal filter feeders n 5000 fossil spp, 620 living
Crinoidea n Body made of ossicles n 10 arms have podia (no ampullae) feeding particles to the mouth. n Arms can move n Mouth and anus are both on oral side (!)
Asteroidea n Starfish n Active predators u feed on bivalves u use suction cups to pull open the shells with forces of up to 5kg n The stomach is eversible, and can be partially inserted inside preys shell (enzymes but no toxins)
Echinoidea n Recipe: take a starfish and roll its 5 arms together into a ball, then fuse and calcify with an external armor n The armor is called the test n Very small aboral surface
Echinoidea n Herbivores, preferring macro-algae n They can be highly effective grazers, creating urchin barrens devoid of algae n The mouthparts are unique, known as Aristotles Lantern. u 5 continually growing chisel teeth u Each tooth with 8 supporting skeletal pieces
Irregulars n All are sand burrowing n Heart urchin Echinocardium has no lantern; n Sand-dollars (Clypeaster) are more flattened with a lantern
Noli tangere n Many echinoids have wickedly sharp spines, which break off in your skin. n Only a few fish, trigger fish attack long-spined species n Spines are under muscular control, and can be used to move
Noli tangere n Very few echinoids are lethal to touch - their pedicillaria inject a neurotoxin n Toxopneustes is feared by pearl divers
Ophiuridae - brittle stars n Have arms sharply demarcated from the body disc. n The internal structure of the arms involves interlocking internal ossicles, confusingly called vertebrae n Are primarily detrital or filter feeders, raising their arms in a current to capture particulates
Holothuridae- Sea Cucumbers n They have no calcitic skeleton, except for spicules embedded in a leathery skin n Most are immobile, and lie on the sea bed rolling back and forth with the swell. Some have limited mobility using their tube feet. n Despite retaining 5-radiate anatomy, they have re-evolved bilateral symmetry along their long axis (the oral-aboral)
Holothuridae n They mainly feed on detritus n Oxygen exchange is performed using gills inside their anus n They have 2 odd defensive strategies: u Squirting a sticky goo u Voiding their entire intestines