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Hake
There are two Dorsal fins on a perch. The anterior and posterior Dorsal fins.
Porpoises have triangular dorsal fins (as opposed to the curved dorsal fins of dolphins and whales). Some species of porpoise have small bumps on their fins, but the purpose of these bumps is unknown.
Sharks have straight dorsal fins, dolphins have curved dorsal fins and whales don't have dorsal fins :)
The opposite term is 'ventral' - For example in fish (and dolphins given the category) the dorsal fins are on the top of the body, while the ventral fins are on the underside.
Yes, giant cat fish do have dorsal fins.
Yes, shark dorsal fins are triangular and straight while dolphin dorsal fins are very curved and streamlined. A Google Image Search will clarify this even better.
dorsal fins
The tail fins are 'flukes'. Otherwise the fin on any marine animal is "... fin" where "..,." is the adjective denoting its position on the body.
Ichthyosaur
ichthyosaurs
Is it John Dory?