Prawn is not fish. That is what the Madras High Court has said in a very recent judgement. The context was whether export cess was applicable for prawn as it was applicable for fish under the Agricultural Produce Cess Act, 1940. The High Court said that prawn is not fish and so the cess is not leviable.
Sukumar Mukhopadhyay takes exception. Although biologically prawns aren't fish, he argues that in South Indian culinary parlance, they are... based on reasoning that I don't quite get:
There are famous stories in Bengali of how a widowed lady was made to eat prawn without her knowing which resulted in her getting hugely embarrassed. From all these it doubtless that in common parlance prawn is regarded as fish in India .
The argument that prawns are sold in fish shops is a bit stronger, I suppose