Phylum Echinodermata

Phylum Echinodermata Peter Shaw Invertebrate Phyla: BIOH20.043

I hereby salute the echinoderms as a noble group especially designed to puzzle zoologists Libbie Henrietta Hyman 1955

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. • No chordate stores calcite skeletally - we use calcium phosphate • No common ancestor of echinoderms and chordates has been found; if the link is indeed valid the separation occurred early in metazoan evolution.

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 cephalisation, hence notions of anterior/posterior are inapplicable here • 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… • It seems that the ancestral echinoderm was a sessile filter-feeder, extending its oral surface upwards to capture food • Uniquely, 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 • In addition to the ectoneural net, there is an endoneural net (muscles between ossicles) and a hyponeural net (local motor ganglia for podia)

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

Mutable connective tissue(also called catch connective tissue). This is another echinoderm uniqueness! Their skeleton, consisting of calcite ossicles, is embedded in a collagenous connective tissue whose properties can be modified under nervous control. The degree of cross-linking of the collagen can be increased (making it tougher and stiffer) or decreased (making it soft, in extreme cases able to flow into new shapes). It can actively contract, like a slow muscle. Thus the body plan relies heavily on extra-cellular materials (calcite and collagen) which are low maintenance, allowing these animals to persist on a low nutrient/energy budget.

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 • The oral ring also has 4 (sometimes 9 if paired) water-storage sacs “Tiedman’s bodies” or “Polian vesicles”. Why not 5 or 10? 1 is the stone canal.

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 • Acts as a local effector unit working on a hydraulic skeleton • Each podium has a nervous arc to its branch of the hyponeural system - a an individual motor centre

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 • Is stated to allow pressure equalisation and top up water supply to the WVS • There is something of a mystery here - the madreporite shows a continual water influx, but animals in which it is experimentally blocked appear to function and move normally • Is absent in crinoids

Haemal system • This consists of spongy tissue enclosed in coelomic tissue, running up the body in 5 strands from a circum-oral ring. • Despite its name this is nothing to do with respiration (can be surgically removed without affecting metabolism), and seems to be involved in immune defences

Gonads • Lie as 10 (2N) paired structures at the base of ambulacral grooves. • Sexes are separate, and discharge gametes into the sea water in response to chemo-stimulus of other gametes. • There are 2N gonopores, ie 2 per arm in asteroidea. • Gonads can be large - echinoid gonads almost fill the test, and can be eaten as a delicacy.

Living forms

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

Concentricycloidea • A little known abyssal class, described in 1986 • These are circular but with 5-radiate symmetry, and live on wood remains at great depths • One genus: Xyloplax

Crinoidea • Crinoids or feather stars - almost certainly close to the ancestral form of the phylum • These are mainly abyssal filter feeders, though in previous geological periods were dominant in shallow waters • Some Carboniferous fossil beds are made of crinoid ossicles

Crinoidea • Feather stars / sea lilies - are poorly studied due to habitat + difficulty of keeping alive in aquaria • 5000 fossils spp, 620 living • of 620 living spp, 80 are sessile (4 subclasses) • The remainder are comatulids: free living spp of shallow waters • All have many commensals, inc annelids

Crinoidea • Body is mainly made of ossicles • 10 arms have podia (no ampullae) lining ciliated grooves feeding particles to the mouth. Podia seem to catch large particles • Arms can move, thanks to muscles between arm ossicles • Mouth and anus are both on oral side (!)

Comatulids • Free living crinoids - “feather stars” • Have >10 arms, often migrating vertically to filter feed in shallow waters at night, usually by crawling • Can be common: > 70 /m2 in Red Sea reefs • Typical genus: Antedon: A. bifida is found in UK waters. This can swim actively.

Asteroidea • “Starfish” - one of best known invert groups • Most are active predators, usually on sessile prey (bivalves), using suction cups on podia 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)

Odd asteroidea • Acanthaster plankii attacks coral polyps, causing widespread diebacks. • A few spp are ciliary filter feeders, using mucus. • The ciliation in stomach of Porania & Henricia resembles the gut-sorting mechanisms of ciliary-feeding molluscs • Giant sand star Oreaster is a facultative microphage

Echinoidea • Recipe: take a starfish and roll its 5 arms together into a ball, then fuse and calcify with an external armour • The armour is called the test, and is primitively roughly spherical • Note that this design greatly reduces the aboral surface • Also ponder how tissues outside the continuous test get food from inside the gut… (active uptake from seawater seems to be involved).

Echinoidea • Are all herbivores, preferring macro-algae so are mainly found in sunlit waters. • They can be highly effective grazers, creating “urchin barrens” devoid of algae • The mouthparts are unique, 5-radiate (of course!), known as Aristotle’s Lantern. This involves 5 continually growing chisel teeth, each with 8 supporting skeletal pieces. This gives the teeth remarkable versatility in their action.

Irregulars • At least twice echinoids have evolved a 2ndry bilateral symmetry - these are the irregular echinoids • All are sand burrowing, and have re-evolved an antero-posterior axis • The heart urchin Echinocardium has an anterior mouth and no lantern; sand-dollars (Clypeaster) are more flattened with a lantern inside a ventral mouth.

Noli tangere • Many echinoids have wickedly sharp spines, which break off in your skin. Beware Diadema! • Only a few fish, trigger fish, persist in attacking long-spined spp. • Jacques Cousteau was more scared of spiny urchins than of sharks • Spines are under muscular control, and can be used to locomote (metachronal rhythms in Diadema)

Noli tangere • There are a very few echinoids that are lethal to touch - they have pedicillaria that inject a neurotoxin which happens to paralyse mammalian muscles. • Toxopneustes is known and feared by pearl divers • Doubtless this is an unlucky coincidence, and the real target is some marine predator or fouling organism

Ophiuridae - brittle stars • These resemble bony starfish in general appearance, but have arms sharply demarcated from the body disc. • The internal structure of the arms involves interlocking internal ossicles, confusingly called vertebrae. (Thus Ophiuroids are invertebrates with vertebrae!)

Ophiuroids • .. Are primarily detrital or filter feeders, raising their arms in a current to capture particulates • Can be locally highly abundant, with densities of 100s per square metre recorded (in Atlantic off W coast Ireland). • Ophiocoma wendtii has been found to be covered in thousands of perfectly formed microscopic lenses (calcite of course) which focus light on a bed of nervous tissue 0.05mm below the surface: these animals change colour between day and night, and it seems that their whole body surface is a light-receptive organ.

Holothuridae- Sea Cucumbers • These animals have converted the basic echinoderm design into something akin to a sausage roll • 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, collected by oral tentacles which are derived from tube feet. • Oxygen exchange is performed using gills inside their anus • They have 2 odd defensive strategies: • Squirting a stick goo from cuverian glands. (It is hard to be sticky in seawater, but this is!) • Voiding their entire intestines - a drastic procedure from which they will recover.

Holothuridae - odd snippets • Are the basis of a commercial fishery: “Beches de mere”. This is intensively exploitative, and operates by stripping an area then moving on. • A few deep sea holothurids are pelagic, swimming constantly a few metres above the bottom - Pelagothuria.

Phylum Echinodermata