Phylum Echinodermata

Phylum Echinodermata

Introduction • Echinodermata are all marine, triploblastic unsegmented coelomates • Phylum has 3 unique features: • pentagonal symmetry (bilateral in larvae) • calcite spicules embedded in the skin, often partly fused • Tube feet (podia)

Affinities • The only connected phylum is our own, the chordates - based on embryological evidence.

An unhurried phylum.. • No echinoderm moves fast, apart from a very few deep sea holothurids which swim actively • Crinoids are sessile, the others crawl at a rate of mm / minute • 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: • There is no cephalization • 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) • The anus is often, but not always, aboral.

Originally… • The ancestral echinoderm was a sessile filter-feeder, extending its oral surface upwards to capture food • This sedentary design has evolved into motile forms where the feeding surface faces downwards

Functional groups 1: nerves • Echinoderms have a diffuse nervous system with no “brain” • There is a 5-radial circum-oral nerve ring, and a superficial net running close to ectoderm

Hydraulics • These are far more complex than the nervous system! • Main hydraulic systems are derived from the coelom, although separate sections of the coelom also surround viscera • The podia are operated by a hydraulic system called the water-vascular system

5-radial layout • Many organ systems in the echinoderms follow the same basic structure as the water-vascular and nervous systems: a 5-radial circum-oral ring • These rings give rise to 5 radial branches (canals in the case of the WVS) • A few asteroids have 7, 10, 11 arms - in which case 7,10, 11 radial branches

Hydraulics, contd. • Each radial canal of the WVS supplies water to tube feet, each with its ampulla • 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 • Echinoderm skin has several distinctive sets of organs protruding from their skin: • Tube feet (podia) • Spines • Pedicillaria

Tube feet.. • Podia are not scattered haphazardly over the body surface • They lie in 10 rows (5 pairs), the ambulacral grooves • Each tube foot + its ampulla is isolated from the WVS by a valve • Tube feet vary - starfish have muscular suction cups, other forms have sticky tips. • Crinoids are different - primitive

Tube feet.. • Originally began as outgrowths of the WVS. In crinoids and ophiuroids these remain essentially as tentacles. • In other radiations, notably asteroids, these have evolved a highly specialised suction cup used for locomotion and prey capture.

Tube feet.. • Have retractor muscles and can bend, but no extensors • To extend, muscles around the ampulla contract • Each podium has a nervous arc to its branch of the hyponeural system

Role of WVS • Hydraulics • Respiration - O2 is exchanged between ampulla and perivisceral coelomic fluid • Probably (?) this was the ancestral function of the WVS, with tubes + podia lining arms to exploit ciliary current already used in food collection

Pedicillaria • …Are defensive organs, assumed to protect against encrusting organisms • Are active, independent local effector units able to inject toxins on contact

Madreporite • Allows pressure equalization and top up water supply to the WVS • Is absent in crinoids

Gonads • Lie as 10 (2N) paired structures at the base of ambulacral grooves. • Sexes are separate, and discharge gametes into the sea water • Gonads can be large - echinoid gonads almost fill the test, and can be eaten as a delicacy.

Sadly... • Of the 13 classes of echinoderms known, 7 are extinct. • Echinoderms were dominant forms in Carboniferous seas, but have suffered a long-term decline in phyletic richness

Crinoidea • Feather stars & Sea lilies • Abyssal filter feeders • 5000 fossil spp, 620 living

Crinoidea • Body made of ossicles • 10 arms have podia (no ampullae) feeding particles to the mouth. • Arms can move • Mouth and anus are both on oral side (!)

Asteroidea • “Starfish” • Active predators • feed on bivalves • use suction cups to pull open the shells with forces of up to 5kg • The stomach is eversible, and can be partially inserted inside prey’s shell (enzymes but no toxins)

Echinoidea • Recipe: take a starfish and roll its 5 arms together into a ball, then fuse and calcify with an external armor • The armor is called the test • Very small aboral surface

Echinoidea • Herbivores, preferring macro-algae • They can be highly effective grazers, creating “urchin barrens” devoid of algae • The mouthparts are unique, known as Aristotle’s Lantern. • 5 continually growing chisel teeth • Each tooth with 8 supporting skeletal pieces

Irregulars • All are sand burrowing • Heart urchin Echinocardium has no lantern; • Sand-dollars (Clypeaster) are more flattened with a lantern

Noli tangere • Many echinoids have wickedly sharp spines, which break off in your skin. • Only a few fish, trigger fish attack long-spined species • Spines are under muscular control, and can be used to move

Noli tangere • Very few echinoids are lethal to touch - their pedicillaria inject a neurotoxin • Toxopneustes is feared by pearl divers

Ophiuridae - brittle stars • Have arms sharply demarcated from the body disc. • The internal structure of the arms involves interlocking internal ossicles, confusingly called vertebrae • Are primarily detrital or filter feeders, raising their arms in a current to capture particulates

Holothuridae- Sea Cucumbers • They have no calcitic skeleton, except for spicules embedded in a leathery skin • Most are immobile, and lie on the sea bed rolling back and forth with the swell. Some have limited mobility using their tube feet. • Despite retaining 5-radiate anatomy, they have re-evolved bilateral symmetry along their long axis (the oral-aboral)

Holothuridae • They mainly feed on detritus • Oxygen exchange is performed using gills inside their anus • They have 2 odd defensive strategies: • Squirting a sticky goo • Voiding their entire intestines

Phylum Echinodermata