Structure and innervation of the inner ear of the bowfin, Amia calva

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Abstract
The hair cell orientation patterns and the number of innervating nerve cells was determined for the ear of the holostean fish Amia calva. Scanning electron microscopy was used to analyze the sensory cells of the three otolith organs: the saccule, the lagena, and the utricle. The utricular hair cell orientation pattern is similar to that found in most vertebrates, while the lagenar hair cell pattern is similar to that in ostariophysine fishes. The saccule has both horizontally and vertically oriented hair cells, although in the living animal many of the horizontally oriented cells are really oriented lateromedially. The transition region between cells oriented horizontally and vertically suggests that the horizontal cells are ontogenetically derived from vertically oriented cells. The lagenar macula has the largest sensory region, with over 216,000 cells in its main sensory part. The utricular macula contains over 56,400 cells in the main sensory region, and the saccular macula over 8,600 cells. The otic nerve divides into anterior and posterior rami, and ganglion cell counts totaled 2,021 in the anterior ramus and 1,619 in the posterior ramus. There are three populations of ganglion cells in each ramus, on the basis of differences in cell diameter. By a rough estimate, the ratio of sensory to ganglion cells is at least 90:1. By comparing SEM data from Amia, other nonteleosts, and various teleost species, we suggest that the ontogenetic derivations of the horizontal cells in the teleosts and nonteleosts are substantially different and apparently unrelated to each other and that the most primitive ray‐finned fishes had only vertically oriented sensory cells while the horizontally oriented cells subsequently evolved at least twice, once in Amia and other nonteleosts, and once in the ancestors of the teleosts.