You will not find many Coelenterata in fresh water. To this phylum belong jellyfishes, corals, sea anemones and polyps, characterized by stinging organelles, called cnidae or nematocysts. Only a few genera, all belonging to the Class Hydrozoa, occur in freshwater, due to the fact that the phylum of the Coelenterata evolved in the sea and not many species succeeded in conquering fresh water.
Hydras managed to overcome this barrier and are found almost everywhere in clean water where they can attach themselves to a substrate (plants, stones). Hydras had already been studied by Van Leeuwenhoek in 1703 and by Trembley in 1744, who did a lot of ingenious experiments and wrote a fine monograph about hydra. I was very happy to buy a modern reprint of this monograph. They were followed by many other investigators.
The best way to find them is to take home small waterplants (duckweed, pondweed) and to let these stay in a small vessel. After a while you often see hydras attached to the glass. They are coloured green, grey and brown. It is difficult to determine the grey and brown ones; you have to look at the nematocysts to decide the name of the species. However, there is in Europe only one green species, Hydra (or Chlorohydra) viridissima. Symbiotic green, unicellular algae (Chlorella vulgaris) are responsible for its green colour. This hydra is typical of small ponds and ditches and is less tolerant for water pollution than other species. I did not have to go far to find them, they are abundant in my own small garden pond (1.5 square meters).
Reproduction in hydras occurs most of the time by budding; in the photograph shown right, you will see a brown hydra with two other budding hydras already completely formed but still attached. Hydras can also reproduce sexually. The green hydra is hermaphroditic, meaning that male and female organs (gonads) are located on the same animal.
In the image shown right you will see the male organs just behind the arms; the female organ, much larger, is situated a bit lower on the animal.
In the image shown right the sperm is oozing out of the male organ, in the image shown below a ripe egg is visible. The egg is fertilized by the free swimming sperm (I could detect only one flagella in each sperm cell), becomes surrounded by a tough covering and is then released from the hydra. It sinks to the bottom.
The green symbiotic algae are already present in the egg, coming from the parent. The fertilized eggs develop into ciliated planula larvae, which can develop very quickly into new polyps. Due to the presence of these symbiotic algae, this hydra is attracted by light, contrary to other species and can survive long periods without food. A lot of other things can be studied on a hydra. The stinging cells (you need an oil immersion objective for that), the catching and digestion of prey, movement, specialization of skin cells and many other interesting features. Comments to the author Jan Parmentier welcomed.
Photographs by the author.
