An "s" followed by a consonant takes the article "LO". There are a lot more rules for "LO" but I can't remember them all at the moment.
I am a native speaker and I had to look it up :P they are synonyms, I didn't know that! I think I would use only squalo, but maybe that's just me.
Could it be that a pescecane is shark that fetches a ball if you throw one at it? ;-)
The weird thing is the translation I just did before this said that sharks do NOT eat fish!
Is it just me, or is there somewhere to find the names of things before starting a new lesson e.g. animals, or does everyone just type in any old thing just to get the right answer? I feel I may have missed something only having started this course about 2 weeks ago.