Echinoderms

Question 1

What animals are Echinodermata?

Answer 1

Sea stars
Sea urchins
Sea cucumbers

Question 2

What is the huge difference between Echinoderms and Molluscs?

Answer 2

Echinoderms are deuterostomes, whereas molluscs are protostomes

Question 3

Echinoderm anatomy

Answer 3

• Pentaradial symmetry (imposed on bilateral symmetry)
• Calcareous endoskeleton (inside their body) of separate plates called OSSICLES
• Water vascular system used to move around
• Tube feet called podia used to move around
• Complete gut
• Decentralised nervous system
• Upper surface is aboral surface with anus in the middle of it
• Mouth is underneath on the oral surface
• On oral surface are grooves in the arms known as ambulacral grooves or walking grooves. Tube feet protrude from these ambulacral grooves. They can be retracted or extended by use of the water vascular system

Question 4

Echinoderm reproduction

Answer 4

• Most echinoderms reproduce by spawning (releasing eggs)
• The fertilised eggs develop into planktonic free-swimming larvae
• The larvae are 1mm long
• The plankton have arm structures to stop them from sinking, and different echinoderm classes have different arrangements of these arms
• The larvae are bilaterally symmetrical
• They also have bands of cilia called ciliary bands which are used for motility and feeding
• They have complete digestive tracts with a mouth, oesophagus, stomach, intestine and anus

Question 5

Classes of Echinoderm

Answer 5

• Echinoidea - sea urchins, 1000 species
• Holothuroidea - sea cucumbers
• Concentricycloidea - sea daisies
• Asteroidea - star fish / sea star
• Ophiuroidea - brittle stars

Question 6

What is the difference between each class of Echinoderm?

Answer 6

The oral and aboral surfaces are oriented differently

Question 7

Echinoderm ossicles

Answer 7

• Ossicles form the endoskeleton which provides rigidity and protection
• Asteroids - ossicles are small and in the shape of crosses, rods or plates. They are each about 1mm in size. They are a single crystal of magnesium-rich calcite

Question 8

Ophiuroid ossicles

Answer 8

Ossicles similar in size to those of Asteroids but more developed. They can form vertebral ossicles which articulate against each other to form a ‘joint’, making Ophiuroids the most mobile Echinoderm. They are capable of rapid walking by pushing and pulling themselves with arm move meant. In brittle stars the podia are not used for locomotion as they are just sensory and do not have suction capabilities

Question 9

Holothurian ossicles

Answer 9

Very small ossicles, separates from each other. Well-developed muscle layers

Question 10

Echinoid ossicles

Answer 10

Ossicles are much better developed and form plates of variable sizes which are fused into a hard armoured covering called the test. Each plate is a single crystal of calcite. The ambulacral ossicle plates have holes in them, through which podia can protrude

Question 11

What is self-evisceration?

Answer 11

A holothurian (sea cucumber) defence mechanism - when they are threatened their internal organs shoot out

Question 12

Echinoderm spines

Answer 12

• The spines are related to ossicles because they are also made of calcite, and they protrude through the dermis rather than being hidden
• In Echinoids they are large and well developed
• Usually the spines are mobile and mounted on socket joints
• They are moved by muscles and are for protection
• They respond by moving towards the point of touch
• There is often poison at the tip of the spine, produced by a poison sac

Question 13

What sea urchin causes irritant wounds if you step on them?

Answer 13

Diadema

Question 14

Echinoderm pedicellaria

Answer 14

• Modified spines that keep the surface of an echinoderm free of settling organisms, which they need because they are slow moving
• Tiny, claw-like structures
• Best developed in Asteroids and Echinoids
• Cilia also help to remove settling organisms
• The simplest arrangement of pedicellaria is 3 simple, unstalked nipping jaws, most common in starfish
• Pedicellaria were once thought to be parasitic hydroids

Question 15

What type of pedicellaria does the most common UK starfish have?

Answer 15

Asterias
Two types of pedicellaria: the nipping and cutting type - these have different arrangements of opening and closing muscles

Question 16

What are the three types of pedicellaria of sea urchins?

Answer 16

1. Triphylla (3 leaf-like jaws)
2. Tridens (3 tooth-like jaws)
3. Globifera (poisonous and with round ends)
   There are short or long-jawed pedicellaria. They can be very complex with serrated jaws and complicated arrangements of muscles

Question 17

What can sea urchins emit when threatened?

Answer 17

A semi-autonomous cloud of poisonous pedicellaria, which can still bite and release venom when separated from the body

Question 18

Asteroid water vascular system

Answer 18

• A system of fluid-filled tubes used for locomotion
• Near the middle of the aboral surface, there is an opening known as the madrepodite, which is where water enters and leaves
• This madrepodite is connected via the stone canal (surrounded by ossicles)
• The stone canal connects to the ring canal, which makes a ring around the mouth of the echinoderm
• From the ring canal, the radial canals lead off down each arm. They run on the oral surface of the arms in the ambulacral grooves
• Off the radial canal are short lateral canals, which connect to the podia and a flask shaped reservoir called an ampulla
• The podia are external to the dermis and have muscles and sensory nerves
• The entire water vascular system is filled with fluid and operates during locomotion as a hydraulic system. Then the ampullae contract, a valve in the lateral canal shuts and fluid is pushed down into the podium, which extends outwards
• When the podium touches something, the centre of the terminal sucker is withdrawn, so that suction is created and the sucker holds on. This is helped by sticky secretions
• To retract, the podia muscles contract, shortening the podia and forcing fluid back into the ampulla
• During walking, each podium performs the same hydraulic mechanism in a stepping motion. They are strong, allowing most starfish to climb vertically

Question 19

Echinoid water vascular system

Answer 19

• The madrepodite is at the top of the organism
• The holes of the outer surface mark the places where the tube feet leave the test. They protrude though the ambulacral ossicles
• Internally, the radial canals of the water vascular system run up the centre line of the ambulacral plates and the ampullae and podia lead off these via the lateral canals just like in Asteroids

Question 20

Holothurian feeding

Answer 20

• Mostly suspension or deposit feeders
• They lie on the substrate and extend this buccal tentacles. These are mucus-covered so trap particles
• The tentacles are then pushed into the mouth and the food is stripped off
• The gut is modified. It is a looped tube from the mouth to the anus, but the anus has unusual structures
• The respiratory tree is a gas exchange structure, which works by having water pumped into and out of it via the anus. It branches off the rectum next to the anus
• The cuverian tubes are used in defence. They are sticky tubules that can be shot through the gut wall and out of the anus by contractions of the body wall, which entangles predators
• Some Holothurians ‘fly’ through the water using a web of modified tentacles eg Pelegothuria

Question 21

Asteroid feeding

Answer 21

• They are carnivores and feed on many invertebrates (bivalves, snails, crustaceans)
• In Asteroids, the mouth leads to the cardiac stomach, which in turn leads to the pyloric stomach
• The pyloric stomach has pyloric caecae branching off it running down each arm. These secrete enzymes which break down food
• Finally, the pyloric stomach leads to the anus on the aboral surface
• The cardiac stomach can be everted during feeding, which means it is pushed out of the mouth to directly surround its prey
• Digestion is competed internally, and water is voided through the anus. Any bivalve shells are voided through the mouth

Question 22

Echinoid feeding

Answer 22

• Most echinoids are herbivores, although some are suspension feeders
• Their intestine is coiled around the inside of the animal, and ends at the anus on the aboral surface
• The mouth has 5 teeth at its entrance. There is a structure known as Aristotle’s lantern. These teeth can be extended, retracted and rocked from side to side by the lantern and are used to scrape and chew algae. The 5 teeth grow continually from a growing point and are fed down the major ossicles of the lantern, which are known as pyramids

Question 23

Echinoderm regeneration

Answer 23

• When echinoderms self-eviscerate, this does not kill them. They are extremely good at regeneration, and can grow back lost body parts
• Brittle stars cast off their fragile arms as fragments as a decoy when they are attacked
• Most asteroids and ophiuroids can regenerate lost arms
• Echinoderms are able to shoot body parts because they have mutable connective tissue, which means the tissue can be made rigid or flexible (such as Stichopus horrens)
• The hardening of connective tissue allows sea urchins to wedge themselves into tiny crevices on rocky shores so they are not knocked around. It does not require much energy compared to muscles

Question 24

What starfish can regenerate an entire animal from one arm?

Answer 24

Linckia