Normally folded prion protein (PrP) and its functions in healthy brains remain underappreciated compared with the intense study of its misfolded forms ("prions," PrP) during the pathobiology of prion diseases. This impedes the development of therapeutic strategies in Alzheimer's and prion diseases. Disrupting the zebrafish homologs of PrP has provided novel insights; however, mutagenesis of the zebrafish paralog did not recapitulate previous dramatic developmental phenotypes, suggesting redundancy with the paralog. Here, we generated zebrafish loss-of-function mutant alleles and dual mutants. Zebrafish and dual mutants resemble mammalian knockouts insofar as they lack overt phenotypes, which surprisingly contrasts with reports of severe developmental phenotypes when either or is knocked down acutely. Previous studies suggest that PrP participates in neural cell development/adhesion, including in zebrafish where loss of affects adhesion and deposition patterns of lateral line neuromasts. In contrast with the expectation that 's functions would be redundant to , they appear to have opposing functions in lateral line neurodevelopment. Similarly, loss of blunted the seizure susceptibility phenotypes observed in mutants, contrasting the expected exacerbation of phenotypes if these prion gene paralogs were serving redundant roles. In summary, prion mutant fish lack the overt phenotypes previously predicted, and instead they have subtle phenotypes similar to mammals. No evidence was found for functional redundancy in the zebrafish prion gene paralogs, and the phenotypes observed when each gene is disrupted individually are consistent with ancient functions of prion proteins in neurodevelopment and modulation of neural activity.