Let F N g be the moduli space of polarized Nikulin surfaces (Y, H) of genus g and let P N g be the moduli of triples (Y, H, C), with C ∈ |H| a smooth curve. We study the natural map χg : P N g → Rg, where Rg is the moduli space of Prym curves of genus g. We prove that it is generically injective on every irreducible component, with a few exceptions in low genus. This gives a complete picture of the map χg and confirms some striking analogies between it and the Mukai map mg : Pg → Mg for moduli of triples (Y, H, C), where (Y, H) is any genus g polarized K3 surface. The proof is by degeneration to boundary points of a partial compactification of F N g . These represent the union of two surfaces with four even nodes and effective anticanonical class, which we call half Nikulin surfaces. The use of this degeneration is new with respect to previous techniques.