Star fish, Asterias: A Type study (P-1)

Star fish, Asterias: A Type study

In this Part-1 we will study about 

1. Taxonomic position

2. Habitat

3. Habits

4. External characters    

     a) Shape      

     b) Size      

     c) Colour 

5. Oral or lower or actinal surface      

     a) Mouth     

     b) Ambulacral Grooves     

     c) Podia or Tube feet    

     d) Eyes and Tentacles

6. Aboral surface     

     a) Tubercles or Spines     

     b) Dermal branchiae or Papulae     

     c) Madreporite     

     d) Pedicellariae

1. Taxonomic position 

Phylum         :          Echinodermata

          Sub phylum      :     Asterozo

                      Class              :       Asteroidea

                               Order             :        Forcipulata

                                         Genus           :       Asterias

                                                      Species        :    rubiens

2. Habitat

• Sea-stars are exclusively free living marine animals.
• It is found in all oceans at all depths and more abundant in the rocky patches of the shores.
• They can be seen partially embedded in the sand immediately often a high tides. They live at the bottom. 

 3.Habits 

·  Sea star are nocturnal and carnivorous feed mainly on crustaceans, polychaetes and molluscs.

    

·    The common food of star fish are bivalves.

·    They creep slowly along the sea bottom with the help of tube feet or podia.

·    Sea star exhibit remarkable power of autonomy (cells or a tissue which is independent on neighbour cells during development) and regeneration.

·      They are dioecious and fertilization is mainly external.

·     Development is always indirect and include two larvas i.e.  Bipinnaria and Brachiolaria

 4. External characters

a)    Shape

·        The body of star fish is oral-aborally compressed.

·        It forms a central disc from where five arms will project.

·        The arms are broad from the proximally while free at the distal end.

·        The arms occupy the radial axis.

b)  Size

·        The smallest star fish is one cm in diameter, whereas the largest one is 200 cm in diameter.

·        The average size varies from 10 to 20 cm.

c) Colour

·        Star fish show variety of colours.

·  This may show combination of colours and common colours are yellow, orange and brown.

 5. Oral or lower or actinal surface

Flat lower surface of the body which is kept towards the substratum.  The oral or ventral surface bears following structures

      a)     Mouth

• Central disc of oral surface bears a centrally located opening called as the actinostome or mouth. 
• It is a pentagonal aperture (two ; 1 and 2 shown in figure below) with five angles, each directed towards an arm. 
• The mouth is surrounded by a soft and delicate membrane called peristomial membrane or peristome, and is guarded by five groups of oral spines or mouth papillae.

       b)  Ambulacral Grooves

• From each angle of mouth extends out five narrow grooves called ambulacral grooves. 
• Each groove shows two rows of short tubular retractile projections called tube feet or podia. 
• Each groove guarded on each side by two or three rows of movable calcareous spines.

   c)  Podia or Tube feet

Each ambulacral groove contain two rows of short, soft,  thin walled,  tubular structure called tube feet or podia that end in sucker. Tube feet consist of three parts, ampulla, podium and sucker. Sucker work as suction for the firm attachment on the surface to which it is applied. Tube feet are characteristic organs of echinoderms.

Function: 

They are mainly used for locomotion and capturing of food. They are also help in respiration.

      
    d)  Eyes and Tentacles

 Tip of each arm bears a small median, non-retractile and hollow projection, called terminal tentacle. 

It acts as a tactile and olfactory organ.  At its base occurs a bright red light sensitive eye spot contains several ocelli.

D.      6.       Aboral surface

 It is the upper convex dorsal surface is called abactinal surface bears following   structures

 a) Tubercles or Spines

The entire aboral surface bears by short stout immovable blunt calcareous irregular arranged spines.

b) Dermal branchiae or Papulae

• It bears a large number of pores are present between the ossicles of body wall. 
• Through each pore projects out a very small soft, minute, finger like, hallow retractile process called   dermal branchiae or papulae. 
• Each Dermal branchiae or Papulae is a hallow process of the body wall containing extension of the coelom. 
• It is respiratory and excretory in function

c) Madreporite

• The aboral surface of starfish bears distinct, flat, circular area and conspicuous button like structure called madreporite. 
• It is located eccentrically on the aboral surface. 
• It is present in an interradius between two arms, these two arms are called the bivium and the remaining three arms are called trivium. 

Anus

A minute circular aperture, the anus is situated close to the centre on the aboral surface of central disc. Echinoderms are the only animals that have an anus in a dorsal position.

d) Pedicellariae

• The pedicellariae of starfish are minute, whitish like structures, found on both oral and aboral surfaces of the body, in association with spines. 
• Pedicellariae are modified spines and microscopic jaw like bodies. 
• Each consists of a short fleshy and a movable stalk, bearing two  articulating calcareous blades or valves, resting upon and articulating with a third calcareous plate, the basal or basilar piece. 
• According to disposition of valves, two types of pedicellariae occur in Asterias.

1.    Straight type

• In this type of pedicellariae the two valves are straight and when closed, these meet together along their entire length like a forceps. 
• Valves open and close with the help of two pair of adductor muscles and one pairs of abductor muscle, respectively.

2.  Crossed type

• In this type of pedicellariae the two valves cross each other like a pair of scissors. 
• Straight type occurs largely among the dermal branchiae, whereas the crossed type occurs in cluster at the bases of the spines. 
• Valves open and close with the help of three pair of adductor muscles and one pairs of abductor muscles, respectively.

Function

• The pedicellariae help in the capture and removal of debris and minute organisms, which may settle on the body surface and interfere with respiration by covering the dermal branchiae and tube feet. 
• In some starfishes, the pedicellariae may help in capturing of small prey.

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