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Influence of dark-rearing on the ontogenetic development of Sarotherodon mossambicus (Cichlidae, Teleostei): II. Effects on allometrical growth relations and differentiation of the optic tectum.

The effect of dark-rearing (DR) on the allometrical growth relations of defined brain structures, on the lamination and the synaptogenesis of the optic tectum of the cichlid teleost Sarotherodon mossambicus was studied. Dark-rearing did not alter allometric growth relations of the optic tectum. Both in dark-reared (DR) and in normal-reared animals (NR) the predominant outgrowth of the nerve fibers in the optic tectum took place within the first 20 days after hatching. Dark-rearing changed the laminar structure of the optic tectum. In 100-day-old DR the optic layer was significantly reduced as compared to NR. The morphometric differentiation of synapses was affected by dark-rearing. In DR no significant reduction in the length of synaptic contact zones occurred between 30 and 100 after hatching, as it did in NR. Moreover, specific layers (stratum marginale, stratum griseum centrale) in 100-day-old DR showed significant increase in the number of synaptic vesicles per nerve terminal.