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What is Lichen ? Give some other examples for symbiotic relationship too.
Lichens have been described as "dual organisms" because they are symbiotic associations between two (or sometimes more) entirely different types of microorganism -

    a fungus (termed the mycobiont)
    a green alga or a cyanobacterium (termed the photobiont).

An example of symbiosis is a cat and a mouse. Another example is a Venus Flytrap and a fly.
Sea anemones (Calliactus spp) hitchhike on the back of hermit crabs, scoring a ride across the seabed and extending their tentacles to eat the crab’s leftovers. Crabs actively recruit these passengers. After poking an anemone with its pincers – causing it to release its grip from its current home – the crab holds it in place so the anemone can reattach to the crab’s own shell.

In return, the anemones fend off hungry octopuses and other predators using their barbed tentacles. The crabs return the favour by driving away creatures that eat anemones, such as starfish and fireworms.
Ants have been farming fungi for around 50 million years – weeding, mulching and fertilising their crops. Fungus-farming ants originated in South America, spreading throughout the New World tropics, from Argentina to southern USA. One well-known example is the leafcutter ant (species in the Acromyrmex and Atta genera).

They build their fungus farms in sheltered underground nests, feeding them on chewed-up leaves. The fungi is the ant’s only food. Although benefiting from free food and protection, these species of fungi occasionally escape enslavement and become free-living.

CORAL AND ALGAE

Corals have a fairly easy life: they just sit back and let specially adapted algae make much of their energy for them. The algae live in the coral, feasting on waste products. Crucially, the algae can photosynthesise, using sunlight to turn carbon dioxide into sugars. After taking what it needs, the algae produces a little extra sugar for the coral. Completing the cycle, carbon dioxide from the coral is used by the algae. The partnership may be the coral’s undoing however: worldwide, many species are losing their algae due to climate change. In a process called ‘coral bleaching’, algae are ejected when the temperature gets too warm, leaving ghostly-white reefs of dead coral in its wake.
BEES AND ORCHIDS

Male orchid bees (also known as Euglossine bees) collect perfume from a wide variety of South- and Central American neotropical orchids, and turn this into chemical signals called pheromones. It’s a messy procedure, which involves scraping brush-like foreleg tips all over orchid flowers before transferring the heady scent to storage sacs on the back legs.

In the process, orchid pollen is conveniently attached to the bee’s back, where it can subsequently rub off onto female parts of other flowers. The process is vital for orchid reproduction. Scientists aren’t yet sure what orchid bees do with their perfume-based pheromones.


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