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Cephalic central nervous system


In acanthomorphs, the main central nervous system corresponds to the encephalon, the "brain". It is located in the neurocranium. The encephalon is mainly composed of a hollow and evaginated tube divided in 5 regions. These regions are more or less developped according to the species.

système nerveux d'un merlan Dorsal view of a dissection of the cephalic nervous system of a whiting(Merlangius merlangius, Gadidae) preserved in alcohol.

Specimen UNSCIBA.Z. 000673 - collection pédagogique, Université de Nantes, France.
Image: J. Baudet.


From the most forward part towards the back, these regions are:


 
The myelencephalon extends posteriorly out of the neurocranium with the spinal chord, included in the medullar canal of each vertebra.


Nerves exit from each of these cephalic regions [they are labelled from the front to the back in Roman numerals]. Cranial nerves start at the neurocranium, like the optic nerves (II) and vagus  nerves (X), while rachidian nerves are attached to the spinal chord.


Generally, acanthomorphs are weakly encephalised : the brain has a moderate size and does not fill the skull cavity completely  (up to only 1.5% in some species). Intraencephalic cavities (ventricles) can be huge (up to 50% of the total volume of the encephalon). Weakly active, mobile and solitary species like flatfishes (pleuronectiformes) have a small encephalon, while active, fast swimming and species able of complex behaviours possess a larger "brain": scombrids (tunas, mackerels) and dolphinfishes (Coryphaenidae) have a 3 to 5 times larger encephalon than benthic species of the same size.

 
Image: J. Baudet.

The same organisation isoccurs in other acanthomorphs, with, according to species, some variations in the volumes of the different regions.

 


 
Encephalon of a sea bass (Dicentrarchus labrax, Dicentrarchidae), dorsal view.
Image: B. Chanet.

A common mackerelMaquereau(Scomber scombrus, Scombridae), a sea bassbar(Dicentrarchus labrax, Dicentrarchidae), carangids, dolphinfishes Coryphène (Coryphaena equiselis, Coryphaenidae), are active predators swimming in the water column. In each of them, the mesencephalon (receiving and analysing  visual information) and the metencephalon (playing an important role in movements coordination  and equilibration) are well developped. At the opposite, in ambush and active by night predators, as a scorpion fish rascasse (Scorpaena porcus, Scorpaenidae) or a common sole  Sole (Solea solea) (Solea solea, Soleidae), these two encephalic regions are small while olfactory bulbs (receiving and analysing  olfactive information) are more developped.

These adaptations (like a developped mesencephalon) are found in an archerfish Poissons archers (Toxotes jaculatrix, Toxotidae) as well. The following image shows the , still in the skull,encephalon of an archerfish:

 
Image: B. Chanet.

Archerfishes use their sight from inside the water to spot preys above the water, like flying insects and gasteropods.

If you want to learn more about: History Life and hunting technique of archerfishes.