The Green Anomaly
In the still, sunlit freshwater ponds of Hesse, a tiny creature is hard to categorize. Hydra viridissima, the green hydra, is an animal that has effectively become part plant. Typically measuring just 10 millimeters long, this small polyp gets its distinct green color from thousands of single-celled algae of the genus Chlorella living inside its body. This is not a casual arrangement; it is an integrated and evolutionarily ancient symbiosis.
Each endodermal cell—the cells lining the hydra's digestive cavity—can house between 20 and 40 individual algae. The algae are safely contained within vacuoles, which protect them from the hydra's digestive enzymes. Here, they perform photosynthesis, converting sunlight into sugars like maltose. This energy is transferred directly to the hydra, providing a substantial food source. In return, the hydra offers the algae protection from predators and essential nutrients like nitrogen, which the algae can no longer acquire on its own. The partnership is so efficient that symbiotic hydra show higher growth rates and better survival under starvation conditions compared to hydra without their algal partners.
A Co-Evolved Existence
The relationship between Hydra viridissima and its Chlorella symbionts is the result of long co-evolution. Genetic studies show that the hydra's immune system is modified to recognize the algae as self rather than as a foreign invader. The algae, in turn, have become dependent on the hydra for survival. Some strains of the symbiotic Chlorella have lost the genes for nitrate assimilation and cannot live independently outside of their host's cells.
This partnership is maintained across generations through a process called vertical transmission. When the hydra reproduces, either asexually through budding or sexually, the algae are passed directly to the offspring within the eggs. This ensures every new hydra is born with its own internal power source. If the hydra is kept in prolonged darkness, the algae population will diminish, and can be repopulated within about two days once light returns.
The green hydra is a model organism for regeneration research. It can regrow its entire body from a small tissue fragment, a process that relies on a complex interaction of genetic pathways. This regenerative capacity, combined with its unique symbiosis, makes Hydra viridissima a subject of continuous scientific study.