Ciliopathies are a genetically and phenotypically heterogeneous group of human 
developmental disorders whose root cause is the absence or dysfunction of 
primary cilia. Joubert syndrome is characterized by a distinctive hindbrain 
malformation variably associated with retinal dystrophy and cystic kidney 
disease. Mutations in CC2D2A are found in ∼10% of patients with Joubert 
syndrome. Here we describe the retinal phenotype of cc2d2a mutant zebrafish 
consisting of disorganized rod and cone photoreceptor outer segments resulting 
in abnormal visual function as measured by electroretinogram. Our analysis 
reveals trafficking defects in mutant photoreceptors affecting transmembrane 
outer segment proteins (opsins) and striking accumulation of vesicles, 
suggesting a role for Cc2d2a in vesicle trafficking and fusion. This is further 
supported by mislocalization of Rab8, a key regulator of opsin carrier vesicle 
trafficking, in cc2d2a mutant photoreceptors and by enhancement of the cc2d2a 
retinal and kidney phenotypes with partial knockdown of rab8. We demonstrate 
that Cc2d2a localizes to the connecting cilium in photoreceptors and to the 
transition zone in other ciliated cell types and that cilia are present in these 
cells in cc2d2a mutants, arguing against a primary function for Cc2d2a in 
ciliogenesis. Our data support a model where Cc2d2a, localized at the 
photoreceptor connecting cilium/transition zone, facilitates protein transport 
through a role in Rab8-dependent vesicle trafficking and fusion.