Brain evolution in decapod crustacea

Abstract

A survey of the morphology of the brains (cerebral ganglia) of 13 species of decapods shows that all have common areas of neuropil that are developed to differing degrees in the different groups. The neuropils of the paired accessory lobes, however, appear to have evolved de novo in the Reptantia. Phylogenetic relationships within the Reptantia suggest that the accessory lobes were initially large but became reduced in size during the evolution of the brachyurans and anomalans. The cerebral ganglia of many arthropods are subdivided into recognisable areas of neuropil linked to one another by axon tracts. Some of these neuropil areas are geometrically structured, with serially arranged fascicles of axons or synaptic areas, such as in the first three ganglia lying behind the retina. Other areas are "glomerular" and contain either columns or spheres of fine-fibered synaptic fields, such as the olfactory lobes. Still other areas have no recognisable substructure, but like the structured neuropils, are always found in the same place with the same axon tracts linking them to their neighbours or via nerve roots to specific receptor organs or muscles.Such a fixed and orderly organisation of the substructures in vertebrate brains has been long recognised and accepted, but that the same rigid organisation of neuropil areas in the arthropod brains can also be found has not received the same attention, nor has it been fully exploited in any comparative study of arthropod brains.In the arthropods, the primary function of many of the brain neuropils can be deduced from the sensory organ which projects to them and the motor neurons that extend from them. In crustaceans, for example, the olfactory lobe receives its total input from the chemoreceptors on the first antenna. The mechanoreceptors of the second antenna and the motor neurons that control the movements of the second antenna are confined to the antenna neuropil. This is not true for all neuropils, however, for there are several that apparently do not receive the endings of primary afferent fibers, nor do they contain the dendrites of motor neurons. There is little or no physiological information about many of these higher order neuropils that could provide a clue to their inputs, outputs, or function. Virtually the only information about them is that they do not occur in all species and that, ifthey are present, they may vary considerably in their relative sizes in different species.Neuropils that can be identified with particular receptor organs often reflect the numbers of primary afferent inputs, and therefore the importance to the animal, of the sense organ that projects to it. By the same token the relative sizes of the higher order neuropils may be reflected in some unique aspect of the behavioural habits of the different animals, thus providing a clue to their function. The problem with this approach is that species belonging to a monophyletic group may exhibit a great uniformity in the relative size of a given higher order neuropil. Their behavioural hab...

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