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Zoochlorellae or zooxanthellae of coloured Hydra occur in
A. Epithetio-muscular cells
B. Nutritive cells
C. Mesoglea
D. Interstitial cells

Last updated date: 20th Jun 2024
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Hint: Symbiotic association refers to an association between two organisms in which at least one is benefitted. Mutualism is a type of symbiotic relationship between a coloured hydra and zoochlorellae or zooxanthellae. The green colour of hydra is also due to zoochlorellae.

Complete answer:
Zoochlorellae or zooxanthellae are groups of small-sized algae that live inside freshwater protozoans and invertebrates. Zoochlorellae live inside nutritive cells of Hydra and impart green colour to the Hydra. Zoochlorellae have a symbiotic association with the host. They use carbon dioxide, nitrogenous and phosphorous waste, and provide oxygen and essential nutrients to the host. Zoochlorellae can also be inherited from one generation to another through germ cells. Zoochlorellae and zooxanthellae both are found in sea anemones of Pacific coast namely Anthopleura elegantissima and Anthopleura xanthogrammica. Both of these algae are responsible for the green colour of sea anemone tentacles. A symbiotic relationship is a type of relationship between two organisms in which at least one of the organisms is benefitted and the other may be benefitted, not benefitted or is harmed. There are mainly four types of symbiosis :
i. Mutualism: In this type of symbiosis, both the species are benefitted. Example; clownfish and sea anemone. The fish protects the anemone from predators and sea anemone protects the fish using its stinging tentacles. Another example is mycorrhiza, which is a mutualistic relationship between fungus and plants. Plants fix carbon dioxide and mycorrhiza provide water and minerals.
ii. Commensalism: In this type, one organism is benefited and the other is neither benefited nor harmed. An example is a hermit crab, as they use gastropods for their shell to protect themselves.
iii. Parasitism: In this relationship, the parasite is benefitted and the host is harmed. Example: Cuscuta
iv. Ammensalism: In this type, one is unaffected and other is harmed or killed.
Hence, the correct answer is option (B).

Note: Zoochlorella is from a genus of green algae assigned to Chlorella. Zooxanthellae is a colloquial term for single-celled dinoflagellates that are able to live in symbiosis with diverse marine invertebrates including demosponges, corals, jellyfish, and nudibranchs.