Economic Importance of Molluscs

 

Economic Importance of Molluscs

The term mollusca was first applied by Aristotle to the cattle fish of the Aegean Sea. It is derived from Latin word Mollis or molluscs which mean soft bodies.This includes the claim, snails, slugs, octopods, squids, and nautili which are bilaterally symmetrical triploblastic, animals with anus and coelom and without segmentation. There are about 80,000 living species are known in this phylum.They usually have the shell and a characteristics ventral muscular foot.In Latin, molluscs are the name of the soft nut with a thin shell, referring to the bivalve shell and the soft-bodied animal within the shell. The term mollusca are the most abduent of all animals. In numbers of species, the mollusca is the second largest phylum after arthropods. Mollusca has no uniform plan as well as no specific shape. Most of them are slow moving and confined to rather special habitats. In the evoluntary sense, they are plastic materials as the outlines of the body are freely altered as new habitats are acquired and new structure are needed.

Economic importance of Mollusca:

Molluscs are of major interest to man .About 10,000 species are of economic importance. Mostly they are beneficial to man and some are indirectly harmful. Some of the beneficial mollusca are used for-

1. Beneficial Mollusca;Most of the mollusca are beneficial for man which are describe below.

1.       As food.

2.        As bait.

3.        As ornament.

4.        As money.

5.         To form dye and ink.

6.         Buttons and pears.

7.         In art and medicine.

8.        In literature.

9.        Animal inventions.

A Little description about why this point is important is described below.

1. As food:
Most of the Mollusca species are eaten as food by people. Fore,g Chitons are eaten as main food of Red Indians. The gastropods are consumed by the numerous predictor chiefly fish, birds, and mammals. Mytilus edulis is used in chowder, and adductor muscle of pectin are served in flour and dried. Pelecypods also furnish food for starfish, boring sponges, drilling snails, some marine leeches, fish and shore birds.Other bivalves, octopus and cuttlefishes furnish the large quantity of a roof in Europe. Million of mounds are regarded as a delicacy some of them are clams,oysters, scallops and mussels are eaten in China, Japan, Malaya, Europe, and America.

2. As bait:

Many gastropods are very useful to man, as bait for catching fish. Squids make an excellent bait for marine fishes . Small Octopus are used as bait by the line fisherman of Palk Bay.

3. Money:
Red Indian tribes of America used the common Dentalium indianorum,as money or sawampum for various native races.Some of the Mollusca are commercially cultivated and harvested from which a large amount of money is earned by selling them. Squids, cuttlefish and devil fish earn money as they are sold in the market for food in China, Japan, India and Italy.

4. Ornament:

Nautilus shell is much used for decoration , art and for many other useful purposes. Dentalium tooth is used as an ornament. Gastropods shells are used as an ornament. some Mollusca and their parts are used as ornaments by a human. Gastropods shell are also used in forming tools. Calcareous shells of some marine snails are sought as curious.

5. To form dye and ink:

Some of the Mollusca secrets the coloured substance acting as a dye. Ink sac of cattle fish has the rich brown pigment called 'sepia' which is used by artists. Nowadays a certain brown fish of photograph is termed as a sepia finish.

6. Button and pearls:
Gastropoda shells are also used to manufacture buttons and their articles. Shells of certain bivalves have been used for mother of pearl layer also for buttons, knife handles.Pearls are made by the clam and pearl oysters themselves and are among the most beautiful and valuable of our jewels.

7. In art and medicine:

Some Mollusca shells are used as art and medicine, for eg, internal calcareous shell of Sepia " the cattle bone" is used as medicine as well as for the other purposes.

8. In literature:

Giant Squids and octopus are used as the major character in various stories ; so that they have literature valve also.

9. Animal inventions:
These methods used by Mollusca as formed millions of years ago later invented by human and are used for mankind. Cephalopods get the credit for two inventions. One is the "Jet propulsion" and another is the "smoke screen".

II Harmful Mollusca

In spite of some beneficial valve of the Mollusca, some of the Mollusca are harmful too.Some of the destructive activities of some mollusca are of great important to the global economy. Some of them are:

1. Herbivores.

Some of the herbivorous gastropods like slug and snails damage garden plants, vegetables, mushroom bed and other wooden structures.

2. Carnivores.

Some carnivorous Mollusca causes destructive by preying. Giant squid engages in battle with whale causing their tongue injured. Some carnivorous Mollusca can cause destructive by preying.

3. Parasites.

Some gastropods also bear a parasites life. Gastropoda acts as ectoparasitic life in echinoderms. Some act as the endoparasites of a stomach.

4. Intermediate hosts:

Some snails act as intermediate hosts for various harmful parasitic flatworms, such as Fasciola, Schistosoma.

Nutrition in Protozoa

Nutrition in Protozoa- 

By Vidhin Kamble.  
Department of Zoology.  Sangola College, Sangola 

The of intake of food and used  for growth, metabolism are repair into the body is called nutrition. It involves ingestion, digestion, assimilation absorption, and excretion. Nutrition is necessary for survival of animals.

There are various kinds of modes of nutrition in protozoa, which are as follows.

1. Holozoic or Zoo-Trophic Nutrition

2. Pinocytosis

3. Autotrophic or Holophytic Nutrition

4. Saprozoic Nutrition

5. Parasitic Nutrition

6. Coprozoic Nutrition

7. Mixotrophic Nutrition.

1. Holozoic or Zoo-Trophic Nutrition:

Majority of Protozoa are holozoic in feeding habit like animals feeding upon on solid food. In this type, the food consists of microorganisms like bacteria, diatoms, rotifers, crustacean larvae, algae, small trashes of large animals, other protozoans and plants, etc.  This type of nutrition involves ingestion, digestion, assimilation, absorption and elimination or egestion of undigested food.

2. Pinocytosis: 

Pinocytosis is also called as  cell-drinking.  Pinocytosis found in protozoan such as  Amoeba proteus and certain flagellate and ciliates.

This type of mode of nutrition involves the ingestion of liquid food.

Some these protozoa pinocytic channels are developed from their outer surface due to invagination deep into the body. Toward the inner end of pinocytic channel pinocytic vesicles or pinosomes are present. these pinosome engulf liquid through the channel and get separate from the base of channel and becomes the food vacuoles.

3. Autotrophic or Holo-phytic Nutrition:

Some protozoan contains chlorophyll or some similar pigment in their cytoplasm. Because of this these protozoans are capable to synthesis of their own food as complex organic food like green plants, e.g., Euglena, Noctiluca.

4. Saprozoic Nutrition:

These protozoans are feeding upon dead or decaying organic matter. Some protozoa absorb liquid organic and inorganic substances by secreting enzymes, act upon decaying organic matter and break down into simple forms that are absorbed by organism.

Some protozoa absorb liquid organic and inorganic substance from surrounding through body wall by the process of osmosis called as osmotrophy. The saprozoic organisms require amino acid, ammonium salts, as nutrient.

Ex. Some protozoa ,Bacteria and fungi.

5. Parasitic Nutrition:

These are organism nothing but the parasitic forms feed either holozoically or saprozoically.

1.     Food-robbers:

These are the parasitic protozoan feeding upon digested and undigested food of their host. These parasitic forms are known as food robbers. Eg. Some ciliate such as Nyctotherus and balantidium 

2.     Pathogenic:

These are the  protozoan act as parasite feeding upon the living tissue of host causing harm to their host.

  

 6. CoprozoicNutrition:

These are the protozoans feeding upon faecal matter of the other organisms. Eg. Clamydophrys

7. Mixotrophic Nutrition:

Some protozoans are capable to feed themselves by more than one method. The best example of the mixotrophic nutrition are flagellates. They are autotrphic as well as zootrophic in feeding habit. e.g., Euglena

Locomotion in Protozoa -Theory by Dr. Vidhin Kamble

LOCOMOTION IN PROTOZOANS

“Movement of the animal from place to place is called locomotion.”

Animals are capable to move place to place in search of food, shelter, and reproduction. Locomotion in animals is influenced by so internal and external stimuli. For locomotion specials kinds of organs are developed in animals are called locomotory organs. In higher animals, limbs, wings, fins, etc. are present.

The chief locomotory organelles found in  protozoa are - Pseudopodia, Flagella, Cilia and Myonemes.

Kind of locomotory organelle	Type of locomotion	Examples
Pseudopodia	Amoeboid locomotion	Amoeba
Cilia	Swimming locomotion	Paramecium
Flagella                      :	Swimming locomotion	Euglena
Myonemes	Gliding movement	Sporozoans

Pseudopodia-

Pseudopodia are nothing but the temporary outgrowths of the cell formed on the surface of the body.  In amoeba, many pseudopodia are present at a time called Polypodial  organisms. In Entamoeba only one pseudopodium is presently called as monopodial organism.

On the basis of their form and structure, four types of pseudopodia  are as  follows.   

1.                                                                   1.      Lobopodia, 

2.      Filopodia, 

3.      Reticulopodia 

                                 4.Axopodia or Actinopodia

Lobopodia – these types of pseudopodia are blunt and finger-like projection with a rounded tip arise from body containing ectoplasm and endoplasm Eg: Amoeba, Entamoeba.

Filopoda – These types of pseudopodia are filamentous and slender with pointed tips. Eg : Euglypha, Lecithium.

Reticulopodia  –These type of pseudopodia are also called myxopodia filamentous branched and net like meant for food collection found in foraminifers. 

Axopodia or Actinopodia – These types of pseudopodia are meant for food collection.  These pseudopodia are found in Helozoans (Actinosphaerium) and Radiolarians (Collozoum).

 

Formation of Pseudopodia in Amoeba. (Amoeboid Movement in Amoeba.)

Amoeba is a microscopic unicellular animal that belongs to phylum protozoa. The shape of amoeba is irregular in which locomotion is carried out by pseudopodia. The movement of amoeba is called amoeboid movement.

The pseudopodia are nothing but the formation of temporary outgrowth of the body of amoeba. The number of pseudopodia is varying with species to species of amoeba. The individual with many pseudopodia is called Polypodial organisms,eg. Amoeba proteus.  Organism with only one pseudopod is called Monopodial organisms, eg. Entamoeba hystolytica.

Sole-gel  Theory or Change in Viscosity Theory

During the formation of pseudopodia, cytoplasm in Amoeba undergoes a series of biochemical changes by altering viscosity or thickness at different regions of the body.

Pseudopodia in amoeba develop at any point in the body. The mechanism of the formation of pseudopodia is taken place by the Solation and Gelation process of endoplasm.  This theory is called “Sol-Gel Theory”.  This theory was the first time was proposed by Miss L.H. Hyman (1917).   This theory also called as “Change in Viscosity Theory”.

The body of amoeba encloses the protoplasm or cytoplasm surrounded by a membrane.   The protoplasm is of two types.

1.      Outer ectoplasm.

2.      Inner endoplasm.

Ectoplasm is thick, viscous, transparent and non-granular whereas, endoplasm is watery, less-viscous, opaque, and granular. In these to protoplasm physic-chemical changes take place during pseudopodia formation.

The membrane of  amoeba enclose protoplasm which is of two types i.e. outer thick, transparent, non-granular and contractile called Ectoplasm. The inner protoplasm is more in quantity, opaque, less viscous, granular and watery fluid called Endoplasm.

In an endoplasm, physic-chemical changes take place in which, Sole change into Gel and Gel into Sol. Conversion of plasma gel into plasma sol by taking water is called Solation. Conversion of plasma sol into plasma get by losing water is called Gelation.

During pseudopodia formation, the first stage is the formation of a Hyaline cap at the advancing end. The hyaline cap is clear ectoplasmic region between the plasma membrane and plasma gel called hyaline fluid.  At this region elasticity of plasma gel becomes weak below the hyaline cap. 

The conversion of Sol into Gel is taken place at the advancing end (Smooth rounded end) called the Gelation zone and the conversion of Gel into Sol is taking place near the uroid end called as Solation zone. The conversion of a sol into gel and gel into sol is taken place at the same rate.

During pseudopodia formation, a plasma gel tube develops at the advancing end which exerts hydraulic pressure resulted in the flow of plasma sol into the plasma gel tube leads to formation of pseudopodia. It helps Amoeba to move towards the chosen direction.  

During pseudopodia formation actin and myosin protein present in cytoplasm involved in amoeboid locomotion.  When protein molecules are in the relaxed condition the endoplasm remains in Sol state.

When protein molecules are in contracted state the endoplasm is in Gel state. For the conversion of Sol to Gel and Gel to Sol require energy is available from ATP by the action ATPase enzyme.

FLAGELLA.

“Flagellae are nothing but threadlike or whip-like extremely fine, vibratile structure are locomotory organelles.

The presence of flagellum is the characteristic of mastigophore protozoans. The flagellum is a long stiff axial filament or axoneme.  It arises from basal granules also known as kinetosomes.  Structurally, the flagellum shows the presence of 11 microtubules of micro-filaments lies in two groups, i.e. 9 –peripheral and 2 are central filaments.  Peripheral filaments are a doublet. The doublets are held in position by radial spokes. The central filaments are a singlet. The peripheral filaments are surrounded by a protoplasmic sheath. The microfilament at the base shows the presence of basal granules. Minute hair-like structures running along the length of some flagella are called  Mastigonemes  or  Flimmers.

On the basis of mastigonemes following are the kinds of flagellum

On the basis of the presence and absence of mastogonemes, there are various kinds of flagellum found in flagellates. The number of flagella varies with species to species.

For e.g.,

In Ceratium                    -        02  flagella,

In Trichomonas    -        04 .

In Giardia             -        08

In Trichonympa   -        Many.

1.      Anematic:

If Flagella are without mastigonemes are called as animatic flagellum

e.g., Noctiluca.

 

1.     Stichonematic:

If there is the presence of a single row of Flagella with a single row of mastigonemes on one side of the flagellum are called stichonematic flagellum. e.g., Euglena.

3. Pantonematic:

If there is the presence of two or more rows of mastogonemes on flagellum is called pantonematic flagellum. e.g., Peranema, Monas socialis.

4. Acronematic:

In this type, flagellum does not bear any arrangement of mastigonemes but a terminal filament is present, e.g., Polytoma.

Pentacronematic:

The flagellum bears two rows of mastigonemeson the sides and the flagellum ends in a terminal filament without mastigonemes, e.g., Urcoclus.

 Locomotion in flagellate protozoan.

In flagellates, movement is occurred by two methods as follows.

1.     Flagellar movement

2.     Undulation movement

1.     Flagellar movement:  The flagellar movement is caused by the bending of the long whip-like flagellum. The doublets are responsible for bending as they are held in position to the center with the help of spokes.

The sidewise movement of the flagellum is consisting of two strokes.

a. Effective stroke

b. Recovery stroke

2.     Undulation Movement:

In flagellates, flagellum shows undulation movement and sidewise beating of the flagellum. If undulation of the body from the tip of the body toward the base, causing forward movement.  If undulation of the body from the base of body toward the tip of the body causing backward movement. Due to spiral undulation, a body of animal rotates itself.

Ciliary movement:

Cilia are nothing but the hair-like structure present all over the body surface. The cilia act as locomotory organs in ciliate protozoan and also helpful for the collection of food.
Ultrastructure of cilium shows the presence of 11 microscopic axial filaments called as axoneme or microfibrils.  The axoneme are surrounded by a protoplasmic sheath. The microfibrils arranged into two groups. Nine are peripheral and two are central fibrils. Central fibrils are singlet whereas peripheral nine are a doublet. Central filaments are surrounded by central sheath and peripheral filaments are surrounded by peripheral sheath.

Types of cilia.

On the basis of location following are the types of cilia.

a.     In some ciliate cilia are present all over the body, such cilia are called as holotrichs Eg: Paramecium.

b.     In some ciliate cilia are present only in a peristomal region called Peritricha, Eg. Vorticella

c.      In this type, cilia are present only in young stage which are further replaced by sucking tentacles in adult called suctorians,  Eg. Acineta.

Ciliary   movement :

The cilia are chief locomotory organelle found in ciliata. 

The movement of each cilium is studied into two strokes as follows.

1.     Effective stroke

2.     Recovery stroke.

1.     Effective stroke: During locomotion each cilium become straight, stiffen rod like structure and moves backward, become parallel to the body surface and beats water.   During this stroke, the body of paramecium push forward.

2.     Recovery stroke: During recovery stroke, each cilium bends itself to reduce resistance of water and again brought to its original position.

By repeating an effective stroke and recovery stroke locomotion or movement is achieved in paramecium.

During forward movement,  the body of paramecium rotates itself. It is because of two reasons.

1.     The cilia do not beat directly backward but beat obliquely backward.

2.     The vestibular cilia beat faster than body cilia leads to central force.

Metachronous movement of cilia:

During movement, not all the cilia beats at once but beats alternately one after another. It  leads to characteristic wave-like movement of cilia, this movement of cilia is called metachronous movement.

MYONEMES (PELLICULAR CONTRACTILE EXTENSIONS)

Many protozoans have contractile structures in the pellicle or ectoplasm called as myonemes. These may be in the form of,

1.     Ridges or grooves                    -        Eg: Euglena

2.     Contractile myofibrils    -        Eg: Larger ciliates

3.      Microtubules                 -        Eg: Trypanosoma