Fat, tender crustaceans lay in their shells on a bed of fluffy egg-fried rice, smothered in a sauce that was at once refreshing, delicate and tasty; flavoured with lemon, chilli and crisp veggies that actually looked like they had until very recently been growing somewhere. You start to salivate. 'Where should I start from?' You notice a prawn hiding beneath a small lump of rice. You use your fork to tease the prawn out of its hermitage. You take a morsel of the prawn. You pause to admire the body of the decapitated prawn. On inspection, you spot a thick brown disgusting line on the back of the prawn. Out of curiosity, you ask your friends/family about the mysterious line.

'Anybody has any idea wad this shit is?'
Someone answers 'Prawn shit la'
Another replies, 'Vein la. Nothing one. Just eat, wont die'

You oscillate the limp prawn on the spoon as you vacillate over the fate of the prawn. With much dubiety and timidity, you shove the prawn aside and cover it with the uneaten parsley, trying your best to make it look as natural and untouched.

I have been plagued by this question about prawn shit or vein for a long time. As long as philosophers have been metagrobolized by the egg and chicken conundrum. I decided to end it all. Here's what I found from a cookbook.
"The easiest way is not to worry about it. In general, small and medium shrimp do not need de-veining except for cosmetic purposes. In larger shrimp, sometimes the intestinal vein contains grit; then you should prefer to remove it"

In short, it is neither the shit or main vein. Its their shit vein.