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)
class Echinoidea characteristics
- Hard shell known as a ‘test’
- Covered in spines
- Use tube feet for locomotion
- Mainly feed on algae
- Pentaradial symmetry in urchins
- Regular and irregular (sand dollars / heart urchins) echinoid
- Aristole’s lantern = Centrally located jaw on oral surface, Surrounded by 5 rigid teeth
class Crinoidea characteristics
sea lilies / feather stars
- Much more prevalent in fossil record
- Cup shaped body bearing flexible arms, each with slender lateral branches (pinnules)
- Cup is attached to substrate by a jointed stalk (sea lily) or by flexible, unbranched cirri (feather star)
- Mouth on top surface surrounded by feeding arms
- Suspension feeders
- Found in deep water
class Holuthuria characteristics
sea cucumber
- Elongated body with tough outer skin
- Have many different modes of life
- Sub-dermal endoskeleton (except pelagic and some deep-sea spp.)
- Tube feet around mouth modified into tentacles
- Generally scavengers
- Extract O2 from water using respiratory trees, which branch from the cloaca just inside the anus
- Breathe by drawing water in and out of anus
explain sea cucumbers Evisceration
- Some sea cucumbers defend themselves by expelling part of their respiratory tree to entangle potential predators
- Can also expel non-essential organs (e.g. gonads)
- Replacement tubules grow back in a few weeks
- Toxic chemical: holothurin
economic importance of class Holuthuria
- Large species collected and boiled - the body wall dried and sold as ‘Trepang’ or ‘bêche-de-mer’
- Some have medicinal properties
what is the toxic chemical in sea cucumbers called
holothurin
