Green hydra, a small freshwater invertebrate, belongs to the phylum Cnidaria, which also includes jellyfish and corals. This creature typically measures around 10 millimeters in length and possesses a simple, tube-like body. A crown of delicate tentacles extends from one end, giving it a distinctive appearance.
The Symbiotic Relationship
The green hydra’s unique coloration comes from a symbiotic relationship with Chlorella vulgaris algae. These microscopic algae reside within the hydra’s endodermal cells, protected within vacuolar membranes. The algae receive shelter, carbon dioxide, and nitrogenous waste from the hydra. In return, the hydra gains oxygen and sugars produced by the algae through photosynthesis. This partnership allows the hydra to survive for weeks without external food sources in adequate light. Some Chlorella strains are unable to grow independently outside their hydra host, indicating a loss of autonomy.
Anatomy and Hunting
The green hydra has a simple body plan: a basal disc for attachment, a tubular body column, and a mouth encircled by tentacles. The basal disc secretes a sticky fluid, allowing the hydra to adhere to submerged surfaces. Its tentacles are specialized hunting tools.
Each tentacle is covered with stinging cells called cnidocytes, containing nematocysts. When stimulated by prey contact, nematocysts explosively discharge, firing a harpoon-like thread that injects neurotoxins. This venom quickly paralyzes small aquatic organisms like daphnia, cyclops, and other zooplankton. Once immobilized, the tentacles guide the prey to the hydra’s mouth.
Reproduction and Regeneration
Green hydra primarily reproduce asexually through budding. A new hydra grows as an outgrowth from the parent’s body, gradually enlarging and developing its own mouth and tentacles before detaching. This process can take about three days.
Sexual reproduction also occurs, often initiated by environmental stressors like low nutrient availability or temperatures above 20°C. Green hydra can be hermaphroditic, with both male and female organs, or exist as separate sexes. Beyond reproduction, the hydra is known for its remarkable regeneration. Even a fragment as small as 1/6 of a millimeter can regrow into a complete individual. This ability is attributed to its totipotent interstitial stem cells, which can differentiate into any cell type for tissue repair. Its regeneration has led scientists to study the hydra as a model for biological immortality, as it does not appear to exhibit typical signs of aging.
Habitat and Aquarium Presence
Green hydra naturally inhabit calm, freshwater environments across temperate regions, including ponds, streams, and lakes. They attach to submerged aquatic vegetation like leaves and stems. In home freshwater aquariums, green hydra can sometimes appear as unwelcome guests.
They are often introduced inadvertently on live plants, decorations, or rocks from an infected tank. A hydra population can rapidly increase, especially in tanks fed small live foods like baby brine shrimp. While generally not a threat to adult fish, these tiny predators can risk very small fish fry or shrimplets due to their stinging capabilities. Their rapid reproduction and potential impact on vulnerable aquarium inhabitants lead many hobbyists to consider them a pest.