Habit Habitat and external feature of Asterias.

Habit and Habitat of Asterias:

The sea stars are free- living marine animals, that occur on sandy or muddy bottoms or crawl about over rocks and shells.They are well represented in almost all seas from tide-marks to deep waters. They move slowly on hard substratum or adhere firmly to it with the help of their characteristic locomotory podia or tube feet. All sea stars are carnivorous feeding mainly on crustaceans, polychaetes, and molluscs. They also feed on detritus and plankton entrapping in mucus, secreted by the body and carried to the mouth by cilia. Sea stars, in general, exhibit remarkable powers of autonomy and regeneration.

The most species of Asterias are generally solitary but under certain ecological conditions, such as to avoid direct sunlight or excessive drying, many individual may gather at some place for the purpose of protection. Most of them are nocturnal, remain quite in a daytime and becomes active during the night. They move by crawling on the bottom, mostly at a rather slow rate.

All sea stars are carnivorous feeding mainly on crustaceans, polychaetes, and molluscs. They also feed on detritus and plankton entrapping in mucus, secreted by the body and carried to the mouth by cilia. Sea stars, in general, exhibit remarkable powers of autonomy and regeneration.

External Features of Asterias.

1. Shape, size and colour:

Asterias has a radially symmetrical and pentamerous body. The body consists of a central, pentagonal central disc from which radiate out five elongated, tapering, symmetrically spaced projections, the rays or arms. In some genera, the number of arms may be more than five, for example, there are 7-14 arms in Solaster and more than 40 arms in Heliaster.

The size varies from 10-20 cm in diameter though some forms may be much smaller or longer. The colour is variable having shades of yellow, orange, brown and purple. There may be a combination of colour.The body has two surfaces, the upper convex and much darker side is called the aboral or abactinal surface.

The lower surface is flat, less pigmented and is called the oral or actinal surface. The oral and aboral surfaces are not the ventral and dorsal surfaces but correspond to the left and right sides of the bilaterally symmetrical larva. The axes occupied by the arms are known as radii and the regions of the central disc between the arms are inter-radii. A well-defined head is entirely absent.

2. Oral surface:

The side of the body, which in natural condition remains towards the substratum and contains the mouth or oral opening, is flat and of dark orange to purplish colour, is called oral or actinal surface.

The oral surface bears the following structures.

1. Mouth.
2. Ambulacral Groove.
3. Tube feet or podia.
4. Ambulacral spines.
5. Sense organ.

1. Mouth.

On the oral surface, in the center of the pentagonal central disc is an aperture, the actinosome or mouth. It is a pentagonal aperture with five angles, each directed towards an arm. The mouth is surrounded by a soft and delicate membrane, the peristomial 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 ambulacral.

2. Ambulacral Groove.

From each angle of the mouth radiates a narrow groove called the ambulacral groove which runs all along the middle of the oral surface of each arm.

3. Tube feet or podia.

Each ambulacral groove contains double rows of locomotory, food capturing, respiratory and sensory organs called tube feet or podia. The tube feet are soft, thin- walled, tubular, retractile structures provided with terminal discs or suckers. The suckers function as suction cups to afford a firm attachment on the surface to which they are applied.

4. Ambulacral Spines:

Each ambulacral groove is bordered and guarded laterally by 2 or 3 rows of movable calcareous ambulacral spines which are capable of closing over the groove. Near the mouth, these spines often become larger, stouter, assemble in five groups, one at each interradius of discs and are called mouth papilla.

Outside the ambulacral spines are three rows of stout immovable spines, beyond which occurs another series of marginal spines along the borders of the arms demarcating the oral from the aboral surface.

5. Sense Organs:

Sense organs include five unpaired terminal tentacles and five unpaired eye spots. The tip of each arm bears a small median, non-retractile and hollow projection, the terminal tentacle. It acts as a tactile and olfactory organ. At the base of each tentacle occurs a bright red photo-sensitive eye spot made up of several ocelli.

Aboral Surfaces:

The side of the body, which remains directed upward or towards the upper surface, is convex and light orange to purplish colour, is called aboral or abactinal surface.

The aboral surface bears following structures:

1. Anus.
2. Madreporite.
3. Spines.
4. Papulae or gills.
5. Pedicellariae.

Anus:

A minute circular aperture, called the anus, is situated close to the center of the central disc of aboral surface.

Madreporite.

At the aboral surfaces of the central disc occurs a flat, sub-circular, asymmetrical and grooved plate called madreporite plate or madreporite between the bases of madreporite is marked by a number or radiating, narrow, straight or slightly wavy grooves with pores in them. The madreporite is, thus, a sieve-like porous plate and it leads to the stone canal of water vascular system.The number of madreporite to an individual through remains one, but the presence of more than one madreporite in some species is due to the increase in the number of arms beyond the normal number of five.

The two arms having madreporite between their bases are called as a bivium and the other three arms called as trivium.The symmetrical position of madreporite, thus, converts the radial symmetry of Asterias into bilateral symmetry.

Spines.

The entire body of the aboral surface is extremely covered by a numerous short, stout, blunt, calcareous spines or tubercles. The spines are variable in size and are arranged in irregular rows running parallel to the axis of the arms.The spines of supported by the irregular -shaped calcareous plate or ossicle which buried in the integument and form the endoskeleton.

Papulae or gills.

Between the ossicles of integument are present a large number of minute dermal pores. Through each dermal pore project out a very small, delicate, tubular, or conical finger-like or thin walled, membranous and retractile projection called the dermal branchial or gills or papule.

The body wall of papulae are hollow evagination and their lumen remains in continuation with the coelom. They have respiratory,as well as excretory functions and are also internally lined by coelom.

Pedicellariae

Besides the spines and gills, the entire aboral surface is covered by many whitish modified spine like tiny particle or jaws called pedicellariae. The oral body surface also bears pedicellariae. Each pedicellaria of sea stars consists of a long or short, stout ,flexible stalk heaving no internal calcareous support. The stalk bears three calcareous ossicles or plates a basilar piece or plate at the extremity of the stalk and jaws or valves which remain movably articulated with the basilar piece and serrated along their opposed edges.Pedicellariae heaving three calcareous pieces and a stalk are also known as called forcipulate pedunculate pedicellariae.

According to the deposition of the valve, two types of pedicellariae occur in Asterias.

1. Straight type pedunculate pedicellariae.

In these type, the valve is straight,simple . When the valve closed they meet together along their entire length.Their two jaws are more or less straight and attached basally to the basal piece.When closed they remain parallel and they meet through their length. The two jaws work against one another like the blade of a force with the help of three pairs of muscles.

2. Closed type Pedunculate pedicellariae.

In this type of pedicellariae, the basal ends of the two jaws cross each other like the mandibles of a crossbill, so that the basal piece is enclosed between their crossed portions. In this type of pedicellariae, the jaws are also operated by two pairs of adductor muscles. Certain another type of pedicellariae heaving no stalk and thus, called sessile pedicellariae are also found on the body of Asterias. They serve as defence and offensive organs and provide the protection to gills and general body surface by keeping the body surface free from debris and organism like sponges and coelenterates setting on the body.

Reference:

Aggarwal Sarita. A Text Book of Biology,New Delhi.: Madhuban Educational Books, 2011.

Bhamrah, H.S., and Kavita, Juneja. A Text Book of Invertebrates, New Delhi: Anmol Publications Pvt Ltd, 2011.

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L., Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.

http://www.biologydiscussion.com/

https://en.wikipedia.org/wiki/Starfish