by Jan Parmentier
|One of the most beautiful and interesting algae that we can find in the plankton of fresh water is the water net or Hydrodictyon. It has the form of a netlike hollow sack and in extreme cases it can grow to a length of several tens of centimeters. The mesh of the net is formed by five or six cylindrical cells lying against each other. Hydrodictyon likes clean, eutrophic water and can sometimes reproduce so very fast that it behaves like a pest. The alga has recently been introduced in New Zealand and has become a plague there. Fish farms, lakes, irrigation ditches and rice fields are ravaged by these algae and research up till now did not have much results.|
Hydrodictyon, the water net
|From experience we know that
in time the problem of such an introduced species becomes
less serious, by development of natural enemies,
competition and other, unknown factors. In The
Netherlands it can sometimes be difficult to find
Hydrodictyon, but if you find it, it can be present in
large amounts. Hydrodictyon is easy to grow, so there is
much known about its life cycle. And that life cycle is
truly special. Hydrodictyon can propagate in two ways,
asexually and sexually. The asexual propagation takes
place by zoospores, formed simultaneously in masses
(thousands of them) in a cell, a process also happening
simultaneously in all other cells of the net. The
zoospores have two flagellae but hardly move, quite
understandable if you look at the dense packing. The
zoospores form a cell wall, become cylindrical and
arrange themselves in such a way that a miniature issue
of the water net is formed.
The mother cell disappears and the daughter net comes free, so small that you need a low power microscope to see it. If under favourable conditions several hundreds of these nets start with the formation of daughter nets in each cell, there will be formed hundreds of thousands of new nets in a very short time, a real explosion of algae. In favourable conditions therefore this alga can become a real pest in eutrophic water.
The sexual propagation takes place by iso- gametes, smaller even than the zoospores, and also formed in enormous amounts. They escape through a hole in the cell wall of the mother cell. If two gametes fuse, a zygote is formed that develops a thick cell wall and becomes somewhat angular. After a rest period 2-5 rather big zoospores are formed from this cell and these enlarge into polygonal cells. The protoplasm of these cells divides into new zoospores which loose their flagellae and by lying against each other form a new net.
Detail of the water net
1. B. Fott, Algenkunde, Fischer Verlag. Stuttgart 1971
2. J. Tilden. The Algae and their Life Relations. Hafner Publishing Co, New York 1968 (facsimile from the edition 1937)
3. H.Carter-Lund. Fresh Water Algae. Biopress Ltd Bristol 1995
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© Jan Parmentier 1998
Published in December 1998 Micscape Magazine.
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