Asterias : A sea star or starfish

Classification of Asterias

Phylum : Echinodermata

Subphylum : Eleutherozoa

Class : Asteroidea

Subclass : Euasteroidea

Order : Forcipulata

Genus : Asterias

Species : rubens

Habit and Habitat (Ecology) of Asterias

• Sea-stars are free-living marine animals that occur on sandy or muddy bottoms or crawl about over rocks and shells.
• All sea stars are carnivorous, feeding mainly on crustaceons, polychaetes and mollusks. They also feed on detritus and plankton entrapped in mucus, secreted by the body.
• Sea stars, in general, exhibit remarkable powers of autotomy and regeneration.

External features of Asterias

Shape, Size and Colour

• Most sea stars possess a pentamerous radial symmetry.
  In Asterias, body consists of an indistinct central disc, from which radiate out five elongated and tapering rays and arms.
• Colouration is variable including shades of brown, yellow, orange, pink and purple.
• Body is strongly flattened with distinct oral and aboral surface.

Oral Surface

• Flat lower surface of the body which is directed to the substratum is the oral or ventral surface.
• Central disc on this surface bears a centrally located aperture, the actinostome or mouth. It is a pentagonal aperture with five angles, each directed towards the arms.
• The mouth is surrounded by a soft and delicate membrane, the perioral membrane or peristome, and is guarded by five groups of oral spines or mouth papillae.
• From each angle of mouth extends radially a narrow groove, called the ambulacral groove, which runs all along ventral surface of the corresponding arm.
• Each groove is guarded on each side by two or three rows of movable, calcareous, adambulacral spines.
• Each ambulacral groove contains two double rows of short, tubular retractile projections, called as podia or tube feet.
  Tube feet are characteristic organs of echinoderms, serving variously for locomotion, capturing food and respiration, e.t.c.
• Tip of each arm bears a small median, non-retractile and hollow projection, the terminal tentacle. It acts as tactile and olfactory organs.

Aboral Surface

• It is upper and slightly convex dorsal surface of the body.
• It bears a large number of short and blunt, immovable, calcareous spines or tubercles arranged in irregular rows parallel with long axes of arms.
• A minute circular aperture, the anus is situated close to the centre on the aboral surface on central disc.
• A distinct, flat, circular area, the madreporite, is present in an interradius between two arms (i.e., at the base of two arms). These two arms are called bivium and the remaining three, the trivium.
  Madreporite is a sieve-like plate, with numerous narrow and radiating grooves bearing pores which lead into the water-vascular system inside the body.

Water Vascular System or Ambulacral System

What is Water Vascular System in Echinoderms?

• It is a unique system of echinoderms which helps mainly in locomotion.
• It is in fact a modified part of coelom consisting of a system of canals containing sea water and amoeboid corpuscles.
• All the canals have muscular walls and are lined by ciliated epithelium.

Water vascular system of Sea star consists of the following parts:

1. Madreporite – It is a thick, rounded, sieve-like calcareous plate, situated on the aboral surface of the central disc.

• It lies on an interradius near the bases of two adjacent arms forming the bivium.
• Its surface bears numerous, fine radiating furrows permeated by minute pores. These pores lead to pore canals which further leads to collecting canals.
• Collecting canals finally lead to ampulla lying below the madreporite which continues into the stone canal.

2. Stone canal – Also known as madreporite canal, is a S-shaped tube opening on the oral side into a ring canal around the mouth.
Its walls are supported by a series of calcareous rings, hence the name stone canal.

3. Ring canal – It is a wide, somewhat pentagonal canal forming a ring around the oesophagus. Angles of pentagon lie in the radial positions.

4. Tiedemann’s bodies – These are also known as racemose glands.

• These are small, rounded, yellowish, glandular sacs opening into ring canal on its inner side.
• There is one Tiedemann’s body between each radius and interradius except in one such position where stone canal enters the ring canal.
  There are thus 9 Tiedemann’s bodies.
• Exact function of Tiedemann’s bodies is still unresolved. However, some workers consider them as filtering devices, some look upon them as enzyme-forming bodies and others as lymphatic glands, probably manufacturing phagocytotic amoebocytes which are released into the water-vascular system.

5. Radial canals – Along each radius, the ring canal gives out a radial canal which extends up to the tip of the corresponding arm.
Radial canal lies below the ambulacral ossicles and terminates as lumen of the terminal tentacle.

6. Lateral canals – Each radial canal, in its corresponding arm, gives out two series of narrow lateral or podial canals, along its entire length.

• The lateral canals of two series are alternately long and short.
• Each lateral canal opens into a tube foot, the opening being provided by a valve to prevent the back flow of fluid into radial canal.

7. Tube feet – There are two double rows of tube feet in each arm, one double row in relation to each series of alternately placed long and short lateral canals.

Each tube foot can be distinguished into three regions:-
(a) A rounded sac-like ampulla situated above ambulacral ossicles.
(b) A middle tubular podium extending through the ambulacral groove.
(c) A cup-like sucker at the lower end of the podium.

Function of Water Vascular System

• Most peculiar and interesting role of the water-vascular system is in bringing about locomotion by providing a hydraulic pressure mechanism.
• Thin walls of tube feet may serve for respiratory exchange of gases.
• Tube feet help in anchoring the body to the substratum and in capturing and handling the food.