How could you differentiate between "they talk about a shark" vs "they talk about the shark" vs "they talk about sharks" vs "they talk about the sharks"?
Eles falam sobre um tubarão, Eles falam sobre o tubarão, Eles falam sobre tubarão/tubarões, Eles falam sobre os tubarões
Does shark have secondary meanings like it does in the US? -- a person who unscrupulously exploits or swindles others (that shark took advantage of those rich widows) -- an expert in a specified field (a pool shark).
The uses of shark in your examples are coloquialisms, ie. western slang. Brazil would have its own, for instance "gatinha" - what you would call a very attractive woman