Fig. 5

Skull structure and visual system of T. delaisi (a), T. melanurus (b), P. zvonimiri (c), and A. ocellaris (d; scale bars 1 cm). a1-d1 Close-up of the dorsal skull and dermis. Triplefins have delicate but solid bones (a1-b1, open arrowheads; te telencephalon) compared to the thicker, chambered bones (d1, black arrowhead) of the clownfish. The skull is only covered by a thin dermis in triplefins, while the blenny has muscles running between the dermis and the skull (c1, white star), and the clownfish has fatty tissue (d1, black stars) throughout its head. a2-d3 MRI segmentations of inner head anatomy in T. delaisi, T. melanurus, and A. ocellaris: retinae (blue), lenses (purple), ONs and tracts (green), optic tecta (red), telencephalon, olfactory nerves and bulbs, and diencephalon (yellow). T. delaisi (a2, a3) and T. melanurus (b2, b3) are practically identical, except for differences in absolute size. a2-d2 Dorsal view. The dashed triangle frames the exposed part of the ON, which coincides with the area of most efficient ONT eyeshine induction (see Fig. 4a). In A. ocellaris (d2, d3), the telencephalon covers a smaller part of the nerves, but these lie deeper in the head. a3-d3 Frontolateral view onto the left optic disc. While narrow and elongated in triplefins, it is shorter and elliptical in the clownfish (open arrowheads). The ridges along the nerve surface in b3 (solid arrowheads) result from the triplefins’ loose ON pleats. The clownfish’s ON is smooth in comparison, due to the denser pleating