He has gills so why not? Kisame has his like a shark look and abilities (gills for example) before he got samaheda. We don't why he looked like that (maybe his mother like sharks who knows). In narutoverse there are many characters with odd look (especially with animals features) like Kisame or Orochimaru and there is no reason of that. Propably this is just Kishmoto's vision, his whim.