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G. Palmieri et al.

the rhinencephalon (Figure 7 a), anti cGnRH and tus) (Peter et al., 1975), the goldfish (Carassius
anti sGnRH sera immunoreacted with some auratus) (Peter and Gill, 1975), the rainbow trout
perikarya and nerve fibres in the olfactory bulb (Salmo gairdneri) (Billard and Peter, 1982) and
(Figure 7 b) and olfactory nerve (Figure 7 c). Anti the Atlantic salmon (Salmo salar) (Peter et al.,
cGnRH immunoreactive fibres were observed in 1991).
the junction between the olfactory bulb and the The nucleus preopticus-periventricularis corre-
olfactory lobe. sponds to the periventricular preoptic nucleus
described by Crosby and Woodburne (1940) and
Crosby and Showers (1969). The neurosecretory
Discussion activity of the nucleus preopticus in fish species has
been shown both anatomically and functionally
This paper provides a morphological description (Perks, 1969). In the goldfish, the nucleus preopti-
of the bluefin tuna brain and the diencephalic nuclei cus periventricularis and the nucleus preopticus are
thought to be involved in reproductive functions. continuous, although the latter can readily be dis-
Moreover, data on the immunolocalization of anti tinguished as it stains with neurosecretory stains
GnRH-positive neurons in the forebrain and mid- such as paraldehyde fuchsin (Peter and Gill, 1975).
brain of the species in question are also given. In the present study, GnRH-like neurons were
The QE, calculated on the basis of the brain mass immunolocalized both in the nucleus preopticus and
to body mass ratio reported by Lisney and Collin in the nucleus periventricularis of the bluefin tuna,
(2006) indicates that the brain mass of the bluefin thus suggesting a potential neurosecretory role of
tuna is less developed than expected compared with the entire nucleus preopticus periventricularis.
other fish. This finding is not surprising since the In different teleost species, the preopticus compo-
above-mentioned ratio has been calculated for 14 nent of the nucleus preopticus-periventricularis
fish species, 8 of which are teleosts and 6 sharks, consists of two parts: the pars parvicellularis and
the latter representative of fish with the largest and the pars magnocellularis (Charlton, 1932; Crosby
heaviest brain. When the comparison of the QE is and Showers 1969). The pars parvicellularis con-
limited to teleost fish,then our findings show a close sists of small cells with a lateroventral location,
similarity between bluefin tuna brain mass and the while the pars magnocellularis consists of large
brains of other large pelagic species, including cells located posterodorsally. A characteristic neu-
Coryphaena hippurus (L.), Katsuwonus pelamis ronal organization has been highlighted in bluefin
(L.) and Thunnus albacares (Bonn.) (Lisney and tuna: neurons of different size are present in the
Collin, 2006). nucleus periventricularis proper (although the
The observation of the gross morphology of the largest neurons are pre-eminent), while in the pre-
bluefin tuna brain shows the presence of a well optic component of the nucleus preopticus-periven-
developed optic tecta and a remarkably developed tricularis only the smaller cell type can be found.
integration area, the corpus cerebelli. The optic In the present study, the widely recognized termi-
tecta account for the dominant sensory brain area nology of “nucleus lateralis tuberis” was used to
of large pelagic teleosts, due to the importance of indicate the nucleus located in the ventral-lateral
vision for predation in pelagic environments (Lisney area of the diencephalon whose nervous fibres con-
and Collin, 2006). The great development of the stitute the pituitary peduncle. Bradford and
corpus cerebelli in the bluefin tuna is presumably Northcutt (1983) recommended renaming large
linked to the extraordinary locomotor performance portions of this nucleus on a functional basis. In the
of this high-speed and highly migratory fish goldfish (Peter and Gill, 1975) and in the killifish
(Altringham and Block, 1997; Safina, 1993). (Peter et al., 1975), the nucleus lateralis tuberis
The neuronal cell bodies of the diencephalon of was divided, on a topographical basis, into pars
the bluefin tuna involved in reproductive functions anterior, inferior, posterior and lateralis. In the
are grouped in two main nuclei, the nucleus preop- bluefin tuna, no morphological differences were
ticus-periventricularis and the nucleus lateralis observed in the neurons of the different putative
tuberis. These two nuclei have already been parts of the nucleus lateralis tuberis, and no prefer-
described in the diencephalons of different teleost ential distribution of the anti GnRH-positive neu-
species, including the killifish (Fundulus heterocli- rons could be found within this nucleus.

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