The Collaborative International Dictionary of English v.0.48:
Mayfish \May"fish`\, n. (Zool.) A common American minnow (Fundulus majalis). See Minnow. [1913 Webster]WordNet (r) 3.0 (2006):
Fundulus majalis n 1: black-barred fish of bays and coastal marshes of the Atlantic and Gulf Coast of the United States [syn: striped killifish, mayfish, may fish, Fundulus majalis]