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Hydrodictyon Green Algae
In the green algae division Chlorophyta, members of the genus Hydrodictyon (the water nets) are sometimes considered weed or pest organisms because they are so prolific that they can overwhelm aquaculture facilities, lakes, irrigation ditches, and even rice fields, especially where introduced as alien water plants. Sexual and asexual reproduction are possible among these colonial green aquatic plants.
Hydrodictyon algal cells are typically arranged in pentagons or hexagons. Rapid reproduction occurs when each cell in the cylindrical net produces a new cylindrical net of small cells, contained within its own cellular structure. After escaping from the parent cell, the miniature net grows tremendously, and once again, each cell produces another miniature net. Provided the optimal growing environment, some colonies of Hydrodictyon are capable of reaching lengths up to 8 inches (or 20 centimeters). Although sporadic in distribution throughout water bodies, once established, this alga type can become quite abundant and is considered, in many cases, a nuisance plant species. The extensive nets or mats of Hydrodictyon serve as important refugia for aquatic invertebrates such as cladocerans, leeches, snails, and some beetle larvae.
The life cycle of this green alga is well studied, with asexual propagation occurring via zoospores formed simultaneously en masse. Although the zoospores are outfitted with two flagella, they barely move, perhaps because of the extremely dense packing. A cell wall is formed by the zoospores, the mass becomes cylindrical, and they arrange themselves into a miniature version of the water net. At this point, the mother cell disintegrates and the daughter net is freed, although it is microscopic. Under favorable conditions for growth, several hundred new nets are formed as each of their cells starts forming daughter nets. Subsequently, there is an algal bloom or explosion, particularly in enriched (eutrophic) waters, and hundreds of thousands of new nets are liberated.
Isogametes that are smaller than the zoospores, but also generated in incredible numbers, are responsible for propagating Hydrodictyon sexually. A hole occurs in the cell wall of the mother cell and the gametes escape into the water column. A zygote results from the fusion of two gametes and subsequently produces a thick cell wall and becomes angular. After resting, two to five large zoospores form from this sexual offspring and then enlarge into polygonal cells. Division of the protoplasm of these cells creates new zoospores that drop their flagella and construct a new net by lying side by side with each other.
Cynthia D. Kelly, Thomas J. Fellers and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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