Here an exemple :
Ayatollah Sistani :
Q : What type of fish is halal to eat.
A : Fish that have scales are halal, other types of fish are haram.
http://www.sistani.org/english/qa/01184/
Ayatollah Fadlallah :
Q : It is known that it is permissible to eat fish that have scales, but what about other kinds of sea animals? What is the general rule for the sea animals to be lawful or unlawful to be eaten? For example, is it permissible to eat snails or the like?
A : According to our research, we came to the conclusion that it is permissible to eat all kinds of sea animals, whether they have scales or not. However, it is a recommended precaution to avoid eating the sea food, other than fish, that have no scales for the lack of evidence on its unlawfulness. Allah knows best.
http://english.bayynat.org.lb/QA/qa.aspx?id=103