Complex sentence: The barracuda, however, is silvery rather than brown and has two similarly-sized dorsal fins with a wide space between them, whereas the lizardfish only has a single dorsal fin with a tiny fleshy fin behind it.
Split into simpler sentences: The barracuda, however, is silvery rather than brown and has two dorsal fins of approximately equal size with a wide space between them. The lizardfish has only a single dorsal fin with a tiny fleshy fin behind it.