I saw this Facebook post floating on the internet and believe it is good knowledge for all Singaporeans. They say that unscrupulous PRC sellers have been injecting the poisonous chemical Formulin into crabs in order to maintain their freshness. So watch out for the injection marks whenever you buying crabs. Maybe Singapore not affected but good to watch out for it!
The tip is as follow: "When buying crabs in the market look for "black" injection marks such as these when buying crabs imported from China. Chinese inject the crabs with formulin to keep them fresh but it is a highly toxic substance. I heard this today at a Thai market."