echinoderms Flashcards
phylum echinodermata characteristics
- Appeared in Cambrian Period (~540 MYA)
- Most show pentaradial symmetry
- Almost all benthic, very slow moving
- Spiny exterior - endoskeleton covered by epidermis
- Endoskeleton consists of calcareous plates (ossicles) - secreted by the dermis
- Larvae = bilateral -develop by radial cleavage - important clue to their phylogeny
- No obvious segmentation
- No centralised nervous system
- Regeneration of tissues, organs and limbs
- Asexual reproduction – regeneration from a single limb
5 classes in phylum echinodermata
-Asteroidea (starfish)
-Crinoidea (sea lilies or feather stars)
-Echinoidea (sea urchins)
-Holuthuria (sea cucumbers)
-Ophiuroidea (brittle stars)
class Asteroidea characteristics
- often brightly coloured
- 5 or more arms radiate from central ‘disc’
- Opportunistic feeders – predators of benthic invertebrates
- Reproduce sexually or asexually
explain the water vascular system in class Asteroidea
- A series of water-filled canals, which extend numerous muscular tube feet by hydraulic pressure
- Ampulla = a rounded sac above each tube foot, acts like a squeeze bulb
- Tube feet = used for feeding, locomotion and gaseous exchange
- Amblacrum = walking surface
- Ossicles with tube feet: ambulacral plates
- Papulae (gills) on upper (aboral) surface
what is a Pedicellariae in echinoderms
- Jaw-like appendages
- Some asteroids and echinoids
- Often in clusters
- Defence mechanism / cleaning
- Stalked and sessile types
explain feeding in Asteroidea
- Underside = oral surface
- Central mouth with radial digestive system
- Short oesophagus opens into large stomach (fills most of central disc)
- Short-armed species = swallow entire prey
- Long-armed species = evert stomach - partial digestion outside body
- Bivalve feeders = fatigue bivalve adductor muscles - slide stomach between valves
explain reproduction in Asteroidea
- most = gonochoristic - gonads in each arm
- Eggs and sperm = shed into the water column (broadcast spawning)
- Typically 1 breeding season per year
- A single female = 60,000,000 eggs per year
- Early development typically planktonic (some Arctic and Antarctic species brood)
- Bipinnaria and pluteus larvae with ciliated bands
- Metamorphosis into adult form after settlement
explain regeneration in Asteroidea
- Any part of an arm can be regenerated (common)
- Destroyed sections of disk can be replaced
- If 20% of disk is attached to arm – a new starfish is generated
- Many asteroids reproduce asexually: central disk divides, breaking animal in two
- Each half then regenerates the missing section
what did Paine (1966) show relating to Asteroidea being the top predators in the intertidal and subtidal
that the removal of Pisaster ochraceus caused a pronounced decrease in diversity, giving rise to the concept of “keystone species”
explain starfish wasting disease
- causing catastrophic mortality with many species extirpated from their ranges
- Largely attributed to a virus worsened by increasing water temperatures
- Loss of key predators has triggered trophic cascades
explain the Crown-of-thorns starfish
- Outbreaks on the Great Barrier Reef – can cause plagues
- Up to 1 metre across
- Up to 21 arms extending from centre
- Found on coral reefs (tropics)
- Body surface covered in long venomous spines
- Corallivore (coral predator) - climbs onto polyps and extrudes stomach
- Solitary nocturnal predator
- One can consume up to 6 m2 of living reef per year
- Can survive on energy reserves for 6 month
what are primary and secondary outbreaks
-Primary outbreak: A sudden increase (by orders of magnitude) of the population - Causes debated in the literature
-Secondary outbreak: Result of enormous number of gametes being produced by an increased population upstream
what is top-down and bottom-up control
predators (top) / resources (bottom) control the ecosystem and population growth
class Ophiuroidea characteristics
- Body with 5 slender, whip-like arms
- Arms delineated sharply from central disc
- All internal organs contained in the disc
- All depths (basket stars tend to be deep water)
- Generally scavengers or detritivores (some suspension feeders)
- Sexual reproduction (also by fission)
what are the 2 large clades within class Ophiuroidea
Ophiurida (brittle stars)
Euryalida (basket stars)
