Structure and Functions of the Water Net with Diagram
Water net, which is also called genus Hydrodictyon, is the genus of filamentous green algae (the Hydrodictyaceae family) sometimes found on the surface of quiet freshwater bodies. Due to its reproductive efficiency, Hydrodictyon proliferates rapidly and may be a problem in recreational waters, ponds, and irrigation canals.
About Waternet
Waternet forms a free-floating network of the multinucleated cells, which are arranged either in hexagons or pentagons and up to 20 cm (i.e., 7.9 inches) in total length. Sexual reproduction can be by fusion of the same gametes (or isogamy). Asexual reproduction is by the motile zoospores, where hundreds of them are contained in every cell, becoming arranged in a small netlike structure. When a mature Hydrodictyon net dissociates, every cell liberates a miniature net.
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Occurrence of Hydrodictyon
Hydrodictyon is a nonmotile coenobium of macroscopic and beautiful alga. Because of its netlike plant body, it is commonly called ‘water net’ and is represented by 5 species. Only 2 species of Hydrodictyon, viz. H. Indicum and H. reticulatum, are reported from India. At the same time, H. reticulatum is cosmopolitan in the distribution.
Commonly, the species are found between the rainy and spring season in slow running water or still water of pools, ponds, and lakes. In general, it floats on the surface of the water but can also lie on the bottom. Often, because of profuse growth, the nets assume a big size and cover the entire pond.
Thallus Structure of Hydrodictyon
A mature coenobium has a hollow cylindrical network that is closed at both ends (shown in Fig. 1). It is saucer-shaped and flat, and its maximum size is normally 20-30 cm. Rarely it can reach up to the length of 60 cm. The mature net of coenobium is formed of some hundred to several thousand cells.
These cells are joined at the end and form either hexagonal or pentagonal structures. These structures are referred to as meshes. Generally, the bach mesh interspace is bounded by 5-6 or rarely 3 cells. At every angle of the mesh or net meet three cells.
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Cell Structure of Hydrodictyon
Every cell is long, ovoid, or cylindrical in shape. Its internal structure may be varied into two parts: protoplasm and cell wall. The cell wall is two-layered and is made of cellulose. It encloses the protoplasm. When it is young, the cells are uninucleated, but at maturity level, they become multinucleate (or coenocytic).
Cells also contain reticulate chloroplast with several pyrenoids. All of the classic green algal structures, such as mitochondria, ribosomes, and dictyosomes, are present. As the cell gets matured, a central vacuole appears, and the protoplasm will become peripheral.
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Reproduction in Hydrodictyon
It is given as three types: Vegetative, asexual, and sexual.
Vegetative Reproduction
It occurs by fragmentation. Coenobium breaks up into small pieces known as fragments, which have the capability to grow into new colonies. It can be because of the movement of aquatic animals or water currents.
Asexual Reproduction
It occurs by the formation of daughter colonies or the auto colonies, as shown in the below figure. These colonies can be formed by the uninucleate, biflagellate zoospores. Under favourable conditions, every coenocytic cell behaves as a zoosporangium. Its nuclei undergo mitotic divisions to produce numerous nuclei count (7000-20000).
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Protoplasm gets segmented into a possible number of segments as there are nuclei. Every segment gets surrounded by a small amount of cytoplasm, which is a limiting membrane and develops 2 whiplash-type equal flagella and also represents biflagellate zoospore (as shown in Fig. 3 A-C). In Hydrodictyon, a peculiar phenomenon can be noticed. Thus, the zoospores formed are never liberated outside of the parent cell.
They remain motile within the restricted region. It means within the cell. They ultimately withdraw their flagella after swimming inside the cell and get themselves arranged into the characteristic pentagonal or hexagonal fashion to form a new net (which is shown in Fig. 3 D, E). This new net is known as daughter colony or auto colony (as shown in Fig. 3 F, G).
The auto colonies can be liberated by the parent cell wall disintegration. The cell count in the daughter colony is fixed. Further coenobium growth is entire because of an increase in the cell size, but not the cell count.
Sexual Reproduction
It is defined as isogamous. Any coenobium’s vegetative cell can function as gametangium. The biflagellate gametes can be produced by the cleavage of the gametangia-like protoplasm that of zoospores (which is shown in Fig. 4A, B). They are produced in large numbers, and they are smaller in size compared to the zoospores. They are individually liberated via a hole in the parent cell wall and swim in the water freely.
The gametes are biflagellate and uninucleate. Where hydrodictyon is given as monoecious. The gametes, either from similar or different coenobia after liberation, fuse to form Quadri flagellate zygotes (as shown in Fig. 4C).
Soon they will lose their flagella and then settle down. The immobilized zygote type enlarges in size, becomes spherical, and develops a thick wall to form a zygospore. First, it is green in colour, but it becomes red colour due to the development of a red pigment haematochrome.
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FAQs on Water Net or Water Vascular System in Echinoderms
1. What is a water net in biology?
A water net in biology refers to the water vascular system of echinoderms, a fluid-filled network used for movement, feeding, and respiration. It is a unique hydraulic system found in animals like starfish and sea urchins.
- Composed of canals filled with seawater
- Operates using hydraulic pressure
- Controls small projections called tube feet
2. Which animals have a water vascular system?
The water vascular system is found exclusively in members of the phylum Echinodermata. These marine animals include:
- Starfish (sea stars)
- Sea urchins
- Sea cucumbers
- Brittle stars
- Crinoids (sea lilies)
3. What is the function of the water vascular system?
The primary function of the water vascular system is locomotion through hydraulic control of tube feet. It also performs several other vital functions:
- Movement by extending and retracting tube feet
- Feeding by helping capture and manipulate food
- Gas exchange through thin-walled tube feet
- Excretion of metabolic wastes by diffusion
4. How does the water vascular system work?
The water vascular system works by using hydraulic pressure to extend and retract tube feet. The process occurs in steps:
- Seawater enters through the madreporite
- Water passes into the stone canal
- It moves to the ring canal around the mouth
- Water flows through radial canals into tube feet
- Muscle contraction creates pressure that extends the tube feet
5. What are the main parts of the water vascular system?
The main parts of the water vascular system include a series of interconnected canals and tube feet. The key components are:
- Madreporite – sieve-like opening for water entry
- Stone canal – connects madreporite to ring canal
- Ring canal – circular canal around the mouth
- Radial canals – extend into each arm
- Tube feet – external projections for movement and feeding
6. What is the role of tube feet in the water net?
The tube feet are small, flexible projections that enable movement, feeding, and respiration in echinoderms. They function by:
- Extending through hydraulic pressure
- Attaching to surfaces using suction
- Pulling the body forward during locomotion
- Assisting in capturing prey
7. What is the difference between the water vascular system and the circulatory system?
The water vascular system is a hydraulic system for movement and feeding, while the circulatory system transports blood for nutrient and oxygen distribution. Key differences include:
- Water vascular system uses seawater, not blood
- It mainly controls locomotion
- Circulatory system transports oxygen, nutrients, and hormones
- Found only in echinoderms, unlike circulatory systems in most animals
8. Why is the water vascular system important in starfish?
The water vascular system is essential in starfish because it allows them to move, feed, and breathe efficiently. In starfish:
- Tube feet help open bivalve shells during feeding
- Hydraulic pressure enables crawling on surfaces
- Gas exchange occurs through thin tube foot walls
9. How does water enter the water vascular system?
Water enters the water vascular system through a porous structure called the madreporite. The pathway is:
- Seawater passes through the madreporite
- Flows into the stone canal
- Reaches the ring canal
- Distributes to radial canals and tube feet
10. Is the water vascular system found in freshwater animals?
No, the water vascular system is found only in marine echinoderms and not in freshwater animals. Echinoderms are exclusively marine organisms because:
- The system depends on seawater pressure balance
- Their body structure is adapted to saltwater environments
- No freshwater echinoderms exist
