mummichog vs killifish

mummichog and killifish both are nouns.

Word	Noun	Adjective	Verb	Adverb
mummichog	Yes	No	No	No
killifish	Yes	No	No	No

As nouns, killifish is a hypernym of mummichog; that is, killifish is a word with a broader meaning than mummichog:

mummichog: silver-and-black killifish of saltwater marshes along the Atlantic coast of the United States
killifish: small mostly marine warm-water carp-like schooling fishes; used as bait or aquarium fishes or in mosquito control

mummichog (noun)	killifish (noun)
silver-and-black killifish of saltwater marshes along the Atlantic coast of the United States	small mostly marine warm-water carp-like schooling fishes; used as bait or aquarium fishes or in mosquito control