Echinoderms Flashcards Preview

2nd Year: Animal Biodiversity > Echinoderms > Flashcards

Flashcards in Echinoderms Deck (35):

What kind of organisms does the echinoderms include?

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


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



Why do we have a good fossil record of echinoderms?

Their calcareous (calcium carbonate) skeletons are well preserved.


When do the earliest echinoderm fossils date back to?

The Cambrian.


What is a) stereo and b) stereom mesh?

a) A calcareous mineral with a magnesium oxide component

b) The porous stereom structure that makes up echinoderm skeletons


How is it that echinoderms can regrow their limbs?

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


Define an ossicle.

A bony plate.


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

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


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

They evade predators by shedding limbs and regrowing them.


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

They can refract light by 2 different indices.


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



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

Through the WVS


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

Through the WVS or water vascular system.


Explain how the WVS works.

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.


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

They are unable to leave the sea.


What are the 3 functions of the WVS?

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


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



Define a coelomocyte.

Immune cells in the circulatory fluid


Is the nervous system central or decentralised?

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.


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

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


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



Is fertilisation internal or external?

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


How have echinoderms been classified with the deuterostomes?

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


What are the 5 classes of echinoderm?

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


Describe a sea lilly.

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.


Describe a starfish.

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.


Describe a brittle star.

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.


Describe a sea cucumber.

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.


Define the cuverian tubules.

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


Describe a sea urchin.

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.


Define Aristotle's lantern.

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


Why is the placement of ophiuroids disputed?

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


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

Their larvae or bilaterally symmetrical.


Define the pedicellariae.

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


Echinoderms do not display senescence. True or false?

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