Echinoderms

Question 1

What kind of organisms does the echinoderms include?

Answer 1

Starfish, sea urchins, sand dollars, sea cucumbers and sea lillies.

Question 2

What percentage of deep sea biomass is made up of echinoderms?

Answer 2

95%

Question 3

Why do we have a good fossil record of echinoderms?

Answer 3

Their calcareous (calcium carbonate) skeletons are well preserved.

Question 4

When do the earliest echinoderm fossils date back to?

Answer 4

The Cambrian.

Question 5

What is a) stereo and b) stereom mesh?

Answer 5

a) A calcareous mineral with a magnesium oxide component

b) The porous stereom structure that makes up echinoderm skeletons

Question 6

How is it that echinoderms can regrow their limbs?

Answer 6

The stereom mesh coalesces into ossicles that can grow in all directions.

Question 7

Define an ossicle.

Answer 7

A bony plate.

Question 8

Is the arrangement of ossicles the same in all the echinoderms?

Answer 8

No: in sea urchins they are very tightly packed, getting progressively looser from starfish to sea cucumbers whose ossicles are microscopic.

Question 9

Echinoderms are highly regenerative. What advantage does this give them?

Answer 9

They evade predators by shedding limbs and regrowing them.

Question 10

Echinoderm spines have birefringent optical properties. What does this mean?

Answer 10

They can refract light by 2 different indices.

Question 11

Echinoderms occupy a wide variety of feeding niches, from predator to detritivores to filter feeding. True or false?

Answer 11

True.

Question 12

Echinoderms are all marine yet they lack gills. How do they breathe?

Answer 12

Through the WVS

Question 13

Echinoderms are all marine yet they lack gills. How do they breathe?

Answer 13

Through the WVS or water vascular system.

Question 14

Explain how the WVS works.

Answer 14

A series of canals or ambulacra branch from a central ring canal around the mouth. Water is brought in through the madreporite. The madreporite is connected to the central ring by the stone canal. The ambulacra are connected to the podia (tube feet) via the ampulla.

Question 15

The seawater and water in the WVS are in constant equilibrium. Why has this limited echinoderms?

Answer 15

They are unable to leave the sea.

Question 16

What are the 3 functions of the WVS?

Answer 16

1. Locomotion: hydraulic pressure extends/retracts the tube feet
2. Respiration: gas exchange occurs across papillae (dermal gills)
3. Feeding: hydraulic pressure causes ejection/retraction of the gut

Question 17

The circulatory system runs parallel to the WVS. True or false?

Answer 17

True.

Question 18

Define a coelomocyte.

Answer 18

Immune cells in the circulatory fluid

Question 19

Is the nervous system central or decentralised?

Answer 19

Decentralised: There are radial nerves running down each limb that form networks. There is no head or ‘brain’, however each limb essentially has its own nervous system.

Question 20

There are no obvious sensory organs in echinoderms. Some starfish have an optic cushion - what is this?

Answer 20

A group of photoreceptor cells. Some echinoderms also have sensory receptors at the end of their podia.

Question 21

There are distinct sexes in echinoderms. Do they display dimorphism?

Answer 21

No

Question 22

Is fertilisation internal or external?

Answer 22

External: one sex releases a chemical that induces gamete release in the other.

Question 23

How have echinoderms been classified with the deuterostomes?

Answer 23

They have ~7000 deuterostome genes but only ~5000 protostome genes.

Question 24

What are the 5 classes of echinoderm?

Answer 24

1. Crinoidea: sea lillies
2. Asteroidea: starfish
3. Ophiuroidea: brittle stars
4. Holothuroidea: sea cucumbers
5. Echinoidea: sea urchins

Question 25

Describe a sea lilly.

Answer 25

The oral and aboral surfaces ace upwards, with all body areas contained in the aboral cup. They are often stalked to cannot swim. They can have between 5 and 200 radii.

Question 26

Describe a starfish.

Answer 26

There are 5 limbs radiating from a central disc. The organs are located in the limbs. Most species are predatory. They feed by external digestion. The optic cushion facilitates hunting.

Question 27

Describe a brittle star.

Answer 27

Similar to starfish but the central disc is not connected to the limbs so they can deliberately fall apart when threatened. The organs are not located in the limbs. They are scavengers/detritivores. They have vertebral ossicles, skeletal elements in the limbs that resemble vertebrae.

Question 28

Describe a sea cucumber.

Answer 28

The skeleton is reduced to tiny calcareous spines, thus they are v. fleshy. The podia are either reduced or lost. They swim via muscle contraction - muscles are connected to a calcareous ring around the throat. The limbs have been lost.

Question 29

Define the cuverian tubules.

Answer 29

When threatened the sea cucumber discharges clusters of tubules through the anus. These enlarge on contact with seawater and are adhesive/toxic.

Question 30

Describe a sea urchin.

Answer 30

They can be globular or flattened in shape. The skeleton is made of tight, interlocking ossicles with gaps where spines fit in. Echinoids are herbivorous and feed on algae, thus they have a large intestine.

Question 31

Define Aristotle’s lantern.

Answer 31

The 5-part mouth apparatus of echinoids. It contains a single, continually growing tooth for scraping off algae.

Question 32

Why is the placement of ophiuroids disputed?

Answer 32

Palaeontological evidence suggests that ophiuroids and holothuroids are a sistegroup. Molecular evidence suggests that ophiuroids and asteroids are a sistergroup.

Question 33

Echinoderms have pentaradial symmetry. How do we know they are bilaterian?

Answer 33

Their larvae or bilaterally symmetrical.

Question 34

Define the pedicellariae.

Answer 34

Pincer-like defensive structures that prevent other species settling on them.

Question 35

Echinoderms do not display senescence. True or false?

Answer 35

True: in fact their reproductive capacity appears to increase throughout their lifetime. They have better maintenance/repair mechanisms than other organisms.