Hydrodictyon is a genus of green algae that forms large floating colonies composed of elongate, coenocytic cells linked together in a reticulated net-like pattern. Each cell is connected to two other cells, forming meshes of 5-6 cells. The colonies reproduce vegetatively through fragmentation, and asexually through the formation of daughter colonies inside parent cells. Sexual reproduction involves the production of biflagellate gametes that fuse to form quadriflagellate zygotes, which develop into thick-walled zygospores. Zygospores remain dormant over winter and germinate in spring, undergoing meiosis to produce zoospores that develop into new daughter colonies.

1. Hydrodictyon is usually found as floating masses
Dr. Saurabh Kumar, HOD, Botany, KCMT

2. Hydrodictyon, known as the "water net", has large
colonies composed of elongate cells linked in a
reticulated, net-like pattern in fresh water.
Each cell is connected at its end walls to two
other cells, forming meshes of five or six cells.
The colonies can be as large 4-6 cm wide and 1 m
long.
The cells are coenocytic and multinucleate.
Young cells have a single parietal chloroplast with
one pyrenoid, while in older cells the chloroplasts
become net-like with multiple pyrenoids.
Large vacuoles take up most of the cellular space
and push the cytoplasm around the periphery of
the cell.
A nonmotile coenobium
Thallus Structure of Hydrodictyon

3. 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 uninucleate, 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.

4. 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.

5. Asexual Reproduction
It occurs by the formation of daughter colonies or the autocolonies.
These colonies can be formed by the uninucleate, biflagellate zoospores.
Under favorable conditions, every coenocytic cell behaves as a zoosporangium.
Its nuclei undergo mitotic divisions to produce numerous nuclei count (7000-20000).
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
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.
This new net is known as daughter colony or autocolony.
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.

6. 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
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 Quadriflagellate zygotes.
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 color, but it becomes red
color due to the development of a red pigment haematochrome.
Stages in Sexual Reproduction in Hydrodictyon

7. Germination of Zygospore
During winter season the zygospore remains in resting
period, and germinate at the onset of spring season.
The diploid nucleus divides meiotically to form four
haploid nuclei.
Each uninucleate segment develops into a biflagellate
zoospore.
After a short period of motility the zoospores come to rest.
Each zoospore or swarmer secretes a thick spiny wall
around itself and makes polyhedron.
The polyhedron increase in size and its content divide to
make 200-300 zoospores.
The zoospores arrange themselves in the form of a new
daughter colony.
After the rupture of polyhedron the young coenobium is
released. Life cycle is Haplontic