The Green Hydra is a small, fascinating freshwater invertebrate classified as a cnidarian polyp, sharing its phylum with jellyfish and sea anemones. Typically measuring up to ten millimeters when fully extended, the hydra resembles a tiny, translucent tube topped with a crown of delicate tentacles. It is a common subject for observation in stagnant or slow-moving freshwater habitats across the northern temperate zone.
Defining Characteristics and Lifestyle
The body of a Green Hydra is a hollow cylinder, or body column, anchored to a submerged surface by a sticky structure called the basal disc. At the opposite end, the hypostome is a raised mound containing the mouth opening, encircled by thread-like tentacles. These appendages are equipped with specialized stinging cells called nematocysts, which paralyze small prey. The hydra is strictly carnivorous, primarily feeding on tiny aquatic invertebrates such as water fleas (Daphnia) and copepods.
Although often considered stationary, the hydra is capable of distinct forms of movement when seeking new hunting grounds. It can detach its basal disc and move by slowly gliding across the substrate using amoeboid movements. A more common form of travel involves “somersaulting,” where the organism bends its body column, attaches its tentacles, and flips its foot over to a new location. These movements allow the hydra to reposition itself, often in response to changing light levels or food chemicals.
The Symbiotic Secret: Why It’s Green
The organism’s distinctive color results from a mutualistic relationship with unicellular green algae, referred to as Zoochlorellae. These algae, typically Chlorella species, live in an endosymbiotic arrangement within the hydra’s endodermal epithelial cells. The host hydra actively suppresses its innate immune system to prevent the digestion or rejection of the algae. This cellular tolerance allows the algae to persist and be passed down to the hydra’s offspring through its eggs.
The symbiotic algae perform photosynthesis, converting light energy into chemical energy, primarily maltose sugar. This carbohydrate is translocated to the host hydra’s tissues, providing a metabolic supplement, especially when the animal cannot capture prey. This nutritional subsidy allows the Green Hydra to survive for weeks without external food, an advantage not shared by non-symbiotic hydra species. In return, the hydra provides the algae with a protected, stable environment and a consistent supply of carbon dioxide and nitrogenous compounds.
Molecular analysis shows that the hydra upregulates genes for nutrient transporters to regulate this internal exchange. The hydra provides the algae with essential inorganic nutrients, such as glutamine, indicating a profound metabolic co-dependence. This partnership illustrates a biological trade-off where both organisms rely on the exchange of resources: the algae benefit from the hydra’s waste products, while the hydra gains a continuous, internal food source powered by sunlight.
The Science of Perpetual Renewal
The Green Hydra is studied for its exceptional capacity for regeneration, a property rooted in its unusual life history. Unlike most animals, which exhibit increasing mortality with age, certain Hydra species demonstrate negligible senescence, showing no measurable signs of biological aging. This near-immortality is sustained by a continuously active population of self-renewing stem cells. The hydra maintains three distinct lineages of stem cells—ectodermal, endodermal, and interstitial—that reside along its body column.
These stem cells constantly proliferate and differentiate, replacing older cells in an ongoing process of tissue turnover. This continuous cellular renewal prevents the accumulation of cellular damage and dysfunction characteristic of aging in other organisms. If a hydra is cut into several pieces, each fragment can completely regenerate a full, functional organism within a few days. This rapid regeneration is possible because the stem cells are distributed throughout the entire body column. The transcription factor FOXO, associated with longevity and stem cell maintenance, is highly active in the hydra’s stem cell population.
This robust regeneration capability suggests that the hydra’s body is more like a constant flow of new tissue rather than a fixed, aging structure. The continuous self-renewal of all three stem cell populations ensures the animal is perpetually rebuilt, maintaining its youthful state indefinitely under favorable laboratory conditions. This trait makes the Green Hydra an organism of interest to researchers studying the fundamental mechanisms of aging and tissue regeneration.