omega-Crystallin of the octopus lens is related to aldehyde dehydrogenases (ALDH) of vertebrates (Tomarev, S. I., Zinovieva, R. D., and Piatigorsky, J. (1991) J. Biol. Chem. 266, 24226-24231) and ALDH1/eta-crystallin of elephant shrews (Wistow, G., and Kim, H. (1991) J. Mol. Evol. 32, 262-269). Only very low amounts of omega-crystallin are present in the squid lens. Here, we have cloned omega-crystallin cDNAs of the octopus (Octopus dofleini) and squid (Ommastrephes sloani pacificus) lenses. The deduced amino acid sequences of omega-crystallin from these species are 78% identical to each other, 56-58% identical to cytoplasmic ALDH1 and mitochondrial ALDH2 of vertebrates (which are 66-68% identical to each other), and 40% identical to Escherichia coli and spinach ALDHs. These data are consistent with the idea that the ALDH1/ALDH2 gene duplication in vertebrates occurred after divergence of cephalopods from the line giving rise to vertebrates, but before the separation of squid and octopus. Southern blot hybridization indicated that omega-crystallin is encoded by few genes (possibly just one) in octopus and squid. Northern blot hybridization revealed two bands (2.7 and 9.0 kilobases) of omega-crystallin RNA in the octopus lens and one band (4.2 kilobases) in the squid lens; omega-crystallin RNAs were undetectable in numerous non-lens tissues of octopus and squid, suggesting lens-specific expression of this gene(s). Finally, extracts of the octopus lens had no detectable ALDH activity using different substrates, consistent with omega-crystallin having no enzymatic activity. Taken together, our results suggest that omega-crystallin evolved by duplication of an ancestral gene encoding ALDH and subsequently specialized for refraction in the transparent lens while losing ALDH activity and expression in other tissues.