The sensory biology of the living jawless fishes: a phylogenetic assessment.

Despite the ancient origins and aberrant appearance of living jawless fishes, none of their features may be assumed to be primitive without comparisons among hagfishes, lampreys, and gnathostomes, and with the nearest relatives of all craniates, the cephalochordates. In this review, the sensory capabilities of lampreys and hagfishes will be compared, and the major features of early craniate sensory evolution will be infered using cladistic methodology and an accepted phylogeny of the hagfishes, lampreys and gnathostomes. Lampreys have well developed olfactory, visual and octavolateralis systems, each of which is known to play a role in lamprey life and behavior. Hagfishes have poorly developed visual and octavolateralis systems, but elaborate olfactory and chemosensory systems. Unfortunately, the natural behavior and lifestyle of hagfishes are poorly known, limiting our understanding of hagfish sensory biology. Both groups of living jawless fishes show mixtures of primitive and secondarily derived sensory features and have few shared derived sensory features that would indicate close relations between the two groups.