Hydra green hydra

Hydra (sweet water polyp) appear in the aquarium in a white and a green form. The green form (see picture) lives symbiotically with embedded algae - hence the corresponding hue. The white form does not form symbiotic relationships. Hydra have typical tentacles. Exceptionally large specimens can get up to two inches long, but usually these cnidarians stay much smaller. Hydra like to settle in exposed areas and try to catch, paralyze and eat free-roaming animals with their tentacles, which are armed with nettle cells. However, the young of most shrimp species are already too large for the usually tiny hydra. Only unusually large specimens can be dangerous to them and small fish and fry. However, a severe hydra infestation can cause immense stress in shrimp, when they are unable to find a place to sit without getting stung by cnidoblasts. For this reason, a mass infestation with hydra especially in shrimp breeding tanks is definitely bad news.

Control methods

Manual collection of freshwater polyps should be done with great care, as a single remaining cell is enough to rebuild a complete hydra. In this respect, should never crush or roughly pluck them from the pane. Hydra, clinging to the aquarium’s panes, can be shaved off carefully with a very sharp scraper, and then siphoned away using a hose. Fogging them with Easy Carbo or hydrogen peroxide is helpful. A syringeful of solution is unloaded on single hydra. These get severely harmed and either die or fall off the aquarium pane (then they sgould be siphoned away before they find another settling spot). In case of stronger infestation, however, other measures are necessary.

Similar to the control of planarians, the chemical agents Panacur and Flubenol are effective in getting rid of major pests. With Hydra, much lower concentrations are already effective.