Question

food chain between shark,seal,fish and algae

Answer 1

The food chain begins from the producer (the organism that produces its own food) and proceeds through the primary, secondary and tertiary consumers to the decomposers. 

Here, algae is the producer (being an autotroph); the fish is a primary consumer since it eats the algae; the seal is the secondary consumer since it feeds upon the fish, and lastly, the shark is a tertiary consumer since it eats the seal. 

Hence, the food chain is:

ALGAE⇒ FISH ⇒ SEAL ⇒ SHARK

Answer 2

In a food chain the first and basic elements are producers the produce food and energy from sunlight and soil. Producers are mainly plants. After producers there come primary consumers that feed on producers (plants) then secondary producers then tertiary consumers and in the end decomposers in a food chain.
In this example algae is the producer, fish is primary consumer because it feed on algae and other tiny organisms, seal is secondary consumer because it feed on fish, shark is tertiary consumer because it feed on fish and seal etc. The food chain will be like this:
Algae----->fish---->seal---->shark