echinoderms

Question 1

phylum echinodermata characteristics

Answer 1

• 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

Question 2

5 classes in phylum echinodermata

Answer 2

-Asteroidea (starfish)
-Crinoidea (sea lilies or feather stars)
-Echinoidea (sea urchins)
-Holuthuria (sea cucumbers)
-Ophiuroidea (brittle stars)

Question 3

class Asteroidea characteristics

Answer 3

• often brightly coloured
• 5 or more arms radiate from central ‘disc’
• Opportunistic feeders – predators of benthic invertebrates
• Reproduce sexually or asexually

Question 4

explain the water vascular system in class Asteroidea

Answer 4

• 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

Question 5

what is a Pedicellariae in echinoderms

Answer 5

• Jaw-like appendages
• Some asteroids and echinoids
• Often in clusters
• Defence mechanism / cleaning
• Stalked and sessile types

Question 6

explain feeding in Asteroidea

Answer 6

• 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

Question 7

explain reproduction in Asteroidea

Answer 7

• 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

Question 8

explain regeneration in Asteroidea

Answer 8

• 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

Question 9

what did Paine (1966) show relating to Asteroidea being the top predators in the intertidal and subtidal

Answer 9

that the removal of Pisaster ochraceus caused a pronounced decrease in diversity, giving rise to the concept of “keystone species”

Question 10

explain starfish wasting disease

Answer 10

• 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

Question 11

explain the Crown-of-thorns starfish

Answer 11

• 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

Question 12

what are primary and secondary outbreaks

Answer 12

-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

Question 13

what is top-down and bottom-up control

Answer 13

predators (top) / resources (bottom) control the ecosystem and population growth

Question 14

class Ophiuroidea characteristics

Answer 14

• 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)

Question 15

what are the 2 large clades within class Ophiuroidea

Answer 15

Ophiurida (brittle stars)
Euryalida (basket stars)

Question 16

class Echinoidea characteristics

Answer 16

• 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

Question 17

class Crinoidea characteristics

Answer 17

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

Question 18

class Holuthuria characteristics

Answer 18

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

Question 19

explain sea cucumbers Evisceration

Answer 19

• 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

Question 20

economic importance of class Holuthuria

Answer 20

• Large species collected and boiled - the body wall dried and sold as ‘Trepang’ or ‘bêche-de-mer’
• Some have medicinal properties

Question 21

what is the toxic chemical in sea cucumbers called

Answer 21

holothurin