Specialization in the diencephalon of advanced teleosts

Cytoarchitectonic studies of the pretectum and diencephalon of five teleosts (Gaidropsarus mediterraneus, Syngnathus acus, Gasterosteus aculeatus, Pleuronectes platessa, and Coris julis) have shown the hypothalamus to be the most highly developed region in all five. The nucleus praeopticus magnocellularis is well developed in Coris and the euryhalines Gasterosteus and Pleuronectes; in Coris and Pleuronectes the nucleus lateralis tuberis is also prominent. Except in Gaidropsarus, however, the most striking area in the hypothalamus is the glomerulosus complex, with its voluminous nucleus glomerulosus. In Coris and Pleuronectes a glomerular offshoot of this nucleus in the dorsal thalamus is evidence of its being homologous with the nucleus anterior thalami of primitive teleosts. The nucleus diffusus is also very large in all except Gaidropsarus. In Coris and Syngnathus the saccus vasculosus exhibits a peduncle, and in Pleuronectes it invades the hypophysis. The descriptive analysis is complemented by measuring the relative size and cell density of the cell groups studied. A comparison among the five species studied shows that nuclei probably related to the olfactory system are more developed in Gaidropsarus and Pleuronectes, whereas the supposed visual nuclei are prominent in Coris, Gasterosteus, and Syngnathus but poorly developed in Gaidropsarus. In general, the findings of the present study, together with published results concerning Lizza (Gómez‐Segade and Anadón, Trab. Inst. Cajal Invest. Biol. 72:187–214, 1981), show that Coris has the most complex diencephalon among these species. Moreover, Gaidropsarus presents an organization very different from that of the other five species and probably represents a parallel evolutionary lineage.