Sombke, A. | Search bioRxiv

bioRxiv2019

Visual pathways of the secondary eyes in the brain of a jumping spider

Some animals have evolved task differentiation among their eyes. A particular example is spiders, where most species have eight eyes, of which two (the principal eyes) are used for object discrimination, whereas the other three pairs (secondary eyes) detect movement. In the spider species Cupiennius salei these two eye types correspond to two visual pathways in the brain. Each eye is associated with its own first and second order visual neuropil. The second order neuropils of the principal eyes are connected to the arcuate body, whereas the second order neuropils of the secondary eyes are linked to the mushroom body. However, eye size and visual fields are considerably different in jumping spiders. We explored the principal- and secondary eye visual pathways of the jumping spider Marpissa muscosa. We found that the connectivity of the principal eye pathway is the same as in C. salei, while there are differences in the secondary eye pathways. In M. muscosa, all secondary eyes are connected to their own first order visual neuropils. The first order visual neuropils of the anterior lateral and posterior lateral eyes are further connected with two second order visual neuropils, whereas the posterior median eyes lack second order visual neuropils and their axons project only to the arcuate body. This suggests that the posterior median eyes probably do not serve movement detection in M. muscosa. Furthermore, the second order visual neuropil (L2) in Marpissa muscosa potentially integrates information from the secondary eyes and might thus enable faster movement decisions.

neuroscience↗

bioRxiv2019

The \"amphi\"-brains of amphipods: New insights from the neuroanatomy of Parhyale hawaiensis (Dana, 1853)

BackgroundOver the last years, the amphipod crustacean Parhyale hawaiensis has developed into an attractive marine animal model for evolutionary developmental studies that offers several advantages over existing experimental organisms. It is easy to rear in laboratory conditions with embryos available year-round and amenable to numerous kinds of embryological and functional genetic manipulations. However, beyond these developmental and genetic analyses, research on the architecture of its nervous system is fragmentary. In order to provide a first neuroanatomical atlas of the brain, we investigated P. hawaiensis using immunohistochemical labelings combined with laser-scanning microscopy, X-ray microcomputed tomography, histological sectioning and 3D reconstructions.\n\nResultsAs in most amphipod crustaceans, the brain is dorsally bent out of the body axis with downward oriented lateral hemispheres of the protocerebrum. It comprises almost all prominent neuropils that are part of the suggested ground pattern of malacostracan crustaceans (except the lobula plate and projection neuron tract neuropil). Beyond a general uniformity of these neuropils, the brain of P. hawaiensis is characterized by a modified lamina (first order visual neuropil) and, compared to other Amphipoda, an elaborated central complex. The lamina displays a chambered appearance that, in the light of a recent analysis on photoreceptor projections in P. hawaiensis, corresponds to specialized photoreceptor terminals. The presence of a poorly differentiated hemiellipsoid body is indicated and critically discussed.\n\nConclusionsAlthough amphipod brains show a general uniformity, when compared with each other, there is also a certain degree of variability in architecture and size of different neuropils. In contrast to other amphipods, the brain of P. hawaiensis does not display any striking modifications or bias towards one particular sensory modality. Thus, we conclude that its brain may represent a common type of an amphipod brain.

neuroscience↗