Echinodermata07.ppt

ECHINODERMATA

-

“

spiny-skin animals

”

• exclusively marine • 6000 living species

Five classes:

Asteroidea - sea stars

Ophiuroidea - brittle stars Echinoidea - sea urchins

Holothuroidea - sea cucumbers Crinoidea - feather stars

Typical characteristics:

Pentamerous radial symmetry

• adults, larvae with bilateral symmetry

Calcareous internal skeleton

• CaCO₃ ossicles embedded in collagenous connective tissue: dermis

• ossicles create internal skeleton

• ossicle projections give surface structure • internal attachment for muscles

• different rigidity (echinoids, holothurians)

• dermis can stiffen and relax (polymerization of macromolecules)

Water-vascular system

• internally closed, fluid-filled channel system • locomotion

• reflects pentamerous body symmetry (5 rays) • extends into fleshy body parts (e.g. tube feet)

Class Asteroidea - sea stars

• ca. 1500 species

• central disc with projecting rays / arms (5 or multiple of 5) • oral and aboral side - mouth facing substrate, anus on aboral side

oral side

• ambulacral grooves from mouth to arm tips • 2-4 rows of tube feet (podia)

aboral side

• madreporite = opening of water-vascular system

• surface structure from ossicle projections = paxillae

• dermal projections = papulae for gas exchange and excretion • sometimes specialized ossicles for defense: pedicellaria

Circulatory system

• pentamerous channel system within dermis • function in nutrient transportation

• ring system around mouth • channels reach into arms

Excretion and Gas Exchange

• no specialized excretion organ

⇒ _{unable to osmoregulate !!!}

• excretion and gas exchange through thin-walled body parts, e.g. papulae, tube feet

Nervous system

• little concentration of nerves, no cephalization • circumoral nerve ring, nerve extensions into arms • eye spots at arm tips, long sensory tube feet

• concentrated sensory cells at tube feet suckers

Reproduction and development

• usually separate sexes

• 1 pair of gonads per arm (= 10 gonads total)

• gamete release through gonopores

• external fertilization, sometimes brooding

A,B bipinnaria C brachiolaria

D start of metamorphosis (attached stage)

Class Ophiuroidea - brittle stars

• ca. 2000 species

• largest class of echinoderms • often cryptic lifestyle

• most small in size

• thin long arms distinctly set off from central disc • 5 arms (multiply branched in basket stars)

• tube feet present, but minor role in locomotion • disc round or pentagonal

Skeleton

• large shields embedded in dermis

Ophiuroidea - Internal Body Organization

Water-Vascular-System

• as in Asteroidea

• no ampullae at tube feet

Digestive system

• no digestive gland • digestion in stomach

Sensory system

• as in Asteroidea

Reproduction

• separate sexes

• gamete release into bursae

Feeding

• can be filter feeders, deposit feeders, predators and scavengers • filter feeding: arms stretched out in water

• particles adhere to mucus, spines and podia

• sweeping off with podia and transport to mouth

• deposit feeding: podia gather particles from bottom, transport to mouth

Class Echinoidea - sea urchins

• ca. 950 species

• regular sea urchins, heart urchins (irregular echinoids), sand dollars • no arms

• movable spines

• skeletal ossicles fused into solid test

Regular Echinoids

oral

aboral

e

Echinoids – external organization

• ten meridional sections connect oral and aboral pole: five ambularcal areas bearing tube feet (holes in test) five inter-ambulacral areas without tube feet

• both areas with two rows of plates and spines • ossicles fused to test, covered by epidermis • movable spines along ambulacral and

interambulacral areas

• ball-and-socket joint between spine and rounded tubercle on test

• spines used in locomotion

• pedicellaria between regular spines

CaCO₃ core

epidermis

nerve ring

muscle

Feeding apparatus

• specialized feeding apparatus: Aristotle’_{s lantern}

• five triangular calcareous plates (pyramids), tips pointed towards mouth

• pyramid tips with calcareous teeth • abraded teeth can be renewed

Class Holothuroidea - sea cucumbers

• ca. 900 species • no arms

• oral-aboral axis stretched • side of body faces substrate => ventral and dorsal sides

• ventral side with well developed podia

• dorsal side often with reduced or absent podia • skeletal ossicles microscopically small

• buccal podia modified into oral tentacles

• tentacles movable, retractable, can be branched

Holothuroidea - Internal

• typical

• many blind madreporites • large Polian vesicle

Digestive system

• no digestive gland

Circulatory System

• well developed, hemoglobin

Gas exchange/Excretion

• along respiratory tree

Reproduction

• separate sexes • single gonad

“Evisceration”

Class Crinoidea - feather stars and sea lilies

• ca. 600 species

• stalked sea lilies abundant in Paleozoic era, now few species in deep sea

• modern group is Order Comatulida, feather stars (free living)

Ecological significance

of echinoderms

• invasion of many habitat types (benthic, hard bottom, soft bottom, infaunal, intertidal, deep-sea, partially pelagic)

• BUT: not in freshwater or estuarine regions (no excretory system for osmoregulation!)

• all feeding types: herbivores, carnivores, scavengers, detritus feeders, suspension feeders

• radiation into all geographic areas: poles to tropics

Why so successful

• basic body plan allowed many adaptations to different life styles

=> combination of internal hard skeleton and a hydrostatic water-vascular system: being well armored but still flexible