Water Net

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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