A residual nucleus found in the plastids of certain single-celled algae. Nucleomorphs are the much-reduced nuclei of algal cells that were originally ingested by heterotrophic eukaryote cells as food; the algae retained their own plastid and the ability to photosynthesize inside the host cell. The ingested algae are thus secondary endosymbionts (see endosymbiont theory). Nucleomorphs occur in chlorarachniophyte algae, whose plastids are derived from endosymbiotic green algae, and in cryptophytes, whose plastids are from red algae. The nucleomorph is located in the space between the second and third plastid membranes, corresponding to the cytosol of the engulfed endosymbiont. It typically contains some 300–500 genes, which are transcribed.