The term “plant hydra” is a common misnomer; Hydra is not a plant, but a freshwater invertebrate. It is often mistaken for a plant due to its stationary nature and tentacled appearance. This article clarifies what Hydra is, explores its potential impact on aquatic ecosystems, and provides strategies for managing its presence.
What is Hydra?
Hydra are tiny, translucent, tube-shaped organisms belonging to the phylum Cnidaria, making them relatives of jellyfish, sea anemones, and corals. They typically measure a few millimeters when fully extended. They possess a tubular body with a sticky basal disc, which they use to attach themselves to various surfaces like rocks, submerged wood, or aquatic plants.
At the opposite end of their body is a mouth opening surrounded by a crown of several slender tentacles. These tentacles are equipped with specialized stinging cells called nematocysts, which contain tiny, harpoon-like structures. Hydra are predatory, using these stinging cells to paralyze and capture small aquatic organisms like microworms, insect larvae, and tiny crustaceans. Once ensnared, the tentacles draw the prey into their single body cavity for ingestion and waste expulsion.
While generally sessile, Hydra can move by gliding on their basal disc or by a somersaulting motion. They are commonly found in freshwater environments such as ponds, lakes, and slow-flowing rivers, and frequently appear in home aquariums. The green coloration seen in some Hydra species is due to a symbiotic relationship with photosynthetic algae living within their tissues, contributing to their plant-like appearance.
Are Hydra Harmful?
For many established aquariums with adult fish, Hydra are generally considered harmless. Their stinging cells are typically too weak to affect larger fish. Adult fish are often too large to be captured by Hydra, and many can avoid stinging tentacles.
However, Hydra can pose a significant threat in specific aquarium scenarios, particularly in breeding tanks. Their stinging tentacles are capable of paralyzing and consuming fish fry and small invertebrates like shrimp larvae. Even if not consumed, numerous Hydra can stress these delicate young creatures. Dense populations might also impact very small aquatic plants, such as duckweed, by physically overwhelming them.
How to Identify and Control Hydra
Identifying Hydra in an aquarium involves careful observation. They typically appear as translucent, white, green, or light brown polyps attached to surfaces like aquarium glass, plants, or decorations. They have a distinct tubular body with a crown of waving tentacles. When disturbed, they retract into a small, jelly-like knob. A magnifying glass can help spot these tiny creatures, which might otherwise be mistaken for strands of algae.
Controlling Hydra involves several approaches, beginning with physical removal. For small infestations, manual removal by siphoning them out can be effective. Take care not to break them into pieces, as each fragment can regenerate into a new Hydra. Reducing light for a period can also discourage their growth.
Introducing certain biological controls can help manage Hydra populations. Some fish species, such as Gouramis, Mollies, and Paradise fish, are known to prey on Hydra. Certain snails, like Spixi, Ramshorn, and Pond snails, have also been reported to consume Hydra, though their effectiveness can vary. When introducing new species, ensure their compatibility with existing tank inhabitants.
Chemical treatments are another option, typically considered a last resort due to potential impacts on other aquatic life. Medications containing fenbendazole can effectively target Hydra. Potassium permanganate is also used. It is crucial to follow dosage instructions precisely, as these treatments can harm sensitive invertebrates like snails and shrimp, and some fish. Copper treatments can also eliminate Hydra but are generally toxic to most invertebrates.
Environmental adjustments play a significant role in preventing and controlling Hydra outbreaks. Reducing overfeeding is important, as Hydra thrive on excess nutrients from uneaten food. Maintaining good water quality through regular water changes and proper tank hygiene helps to limit their food sources and overall population. Quarantining and thoroughly cleaning new plants or decorations before adding them to the aquarium can prevent Hydra from being introduced.