The Collaborative International Dictionary of English v.0.48:

mosquitofish \mosquitofish\ n.
   A silvery topminnow (Gambusia affinis) with rows of black
   spots of tropical North America and West Indies; important in
   mosquito control.

   Syn: Gambusia affinis.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Gambusia affinis
    n 1: silvery topminnow with rows of black spots of tropical
         North America and West Indies; important in mosquito
         control [syn: mosquitofish, Gambusia affinis]