The Collaborative International Dictionary of English v.0.48:

Megalops \Meg"a*lops\ (m[e^]g"[.a]*l[o^]ps), n. [NL., fr. Gr.
   me`gas, -a`loy, large + 'w`ps eye.] (Zool.)
   1. A larva, in a stage following the zoea, in the development
      of most crabs. In this stage the legs and abdominal
      appendages have appeared, the abdomen is relatively long,
      and the eyes are large. Also used adjectively.
      [1913 Webster]

   2. A large fish; the tarpum.
      [1913 Webster]