college of william and mary - home | virginia institute of ... · phylum echinodermata: class...
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What are Echinoderms?
Jon Allen
College of William and Mary
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5 extant classes of Echinoderms
Echinoidea
Ophiuroidea
Asteroidea
Holothuroidea
Crinoidea
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Phylum Echinodermata
• General Characteristics
• Water Vascular System
• Pentameral (five part) Symmetry as adults
• Bilateral Symmetry as larvae
• Calcareous ossicles form internal skeleton
• Mutable connective tissue
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Water Vascular System
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WVS Used for Locomotion
• RED indicates muscular contraction
Ampulla
Tube foot
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Echinoderm Ossicles
• CaCO3 ossicles form internal skeleton
• Vary widely across classes (cf urchin test with sea cucmbers)
• Unique stereom structure: porous structure filled with living tissue
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Mutable Connective (Collagenous) Tissue
• MCT used for feeding in seastars
• MCT allows autotomy in Seastars and Bstars
• Changes in MCT are under nervous control
• Two nerve types present in extracellular matrix: one softens, one hardens
• Mechanism unknown: related to [Ca2+]
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Development and Symmetry Switch in Echinoderms
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5 extant classes of Echinoderms
Echinoidea
Ophiuroidea
Asteroidea
Holothuroidea
Crinoidea
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Crinoidea
• Sea lilies (deep sea) and feather stars (shallow tropical waters)
• 625 species • Filter feeders • Generally sessile but can
move when agitated • Generally considered
primitive among the classes
• Far more numerous in fossil record than today
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Crinoidea
• Amazing mobility for such delicate creatures
• Most echinoderms are light-averse
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Phylum Echinodermata: Class Ophiuroidea
• Defining Characteristics:
– Highly developed arm ossicles form ‘vertebrae’
– 5 pairs of bursal slits for gas exchange and brood chambers
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Ophiuroidea
• Brittlestars and basketstars
• Most speciose class (> 2000 species)
• Arms slender and distinct from central disc
• No suckers or ampullae on tube feet
• No intestine/anus
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Ophiuroidea
• Brittlestars and basketstars
• Most speciose class (> 2000 species)
• Arms slender and distinct from central disc
• No suckers or ampullae on tube feet
• No intestine/anus
• Basket Star
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Amazing Ophiuroids
• Ophiarachna feeding
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Asteroidea
• 1500 species • Common intertidal and
subtidal predators • Mouth located on
underside of body • Ambulacrum runs from
mouth to tip of each arm and bordered by tube feet
• Typically 5 arms (or multiples of 5; like Ophiuroidea)
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Class Asteroidea: External Anatomy
Oral Aboral
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Class Asteroidea: Top Predators
• Seastars scare bivalves
• Why the bivalves are scared
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Vision in Asteroids and Ophiuroids
• Asteroidea possess eyespots at arm tips
• Composed of 80-200 pigment-cup ocelli
• Act as a crude compound eye, can detect reef from three ft (but not 6 ft) away
• Ophiuroidea possess individual lenses acting (maybe) as a compound eye
• Covered by chromatophores that adjust intensity of light; focal depth corrsponds to nerve fibers below the lenses
Chromatophore cells move up and down within stereom during day/night transitions
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Phylum Echinodermata: Class Echinoidea
• ~1000 spp.
• Exclusively marine
• Ossicles joined to form a rigid test
• Unique set of ossicles and muscles for feeding: Aristotle’s lantern
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Class Echinoidea: External anatomy (aboral)
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Class Echinoidea: External Anatomy
• Close-up of body surface
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Don’t be scared of Diadema Taylor…but these can be trouble
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Class Echinoidea: Irregular vs Regular
• Regular Echinoids
– Sea urchins
– Nearly spherical symmetry
– Anus and mouth on opposite sides of body
• Irregular echinoids
– Sand dollars, sea biscuits, heart urchins
– Bilateral symmetry
– Anus posterior to mouth on the edge of the test
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Phylum Echinodermata: Class Holothuroidea
• 1200 spp.
• Soft bodied and vermiform
• Ossicles highly reduced
• Highly branched respiratory trees connected to anus (ugh)
• Oral-aboral axis elongated
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Class Holothuroidea
• Feeding in sea cucumbers
• Note maintenance of pentameral symmetry
• Note coordination of inner and outer tentacles
• How else do sea cucumbers feed?
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Sea Cucumbers as Habitat
• Pearlfish use sea cucumbers as a home to live in
• Recall animals living in sponges etc.
• But pearlfish are a bit unusual…
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Holothuroidea
• One last trick: evisceration and regeneration of internal anatomy
• Sea Cucumber's Bad Day