1. Shelter: Algae take shelter in hydra’s endodermal muscle cells.
2. CO2 Acquisition: Algae use the CO2 produced in hydra respiration for photosynthesis.
3. Nitrogen acquisition: The nitrogenous waste products of hydra metabolism are used by algae to produce carnivorous food.
4. Protection from environmental stress: Algae take shelter in hydra’s body. So it is protected from heat, stress, dryness etc. Dr. Siddiq Publications