A new method for mapping spatial resolution in compound eyes suggests two visual streaks in fiddler crabs

Abstract

Visual systems play a vital role in guiding the behaviour of animals. Understanding the visual information animals are able to acquire is therefore key to understanding their visually-mediated decision making. Compound eyes, the dominant eye type in arthropods, are inherently low-resolution structures. Their ability to resolve spatial detail depends on sampling resolution (interommatidial angle) and the quality of ommatidial optics. Current techniques for estimating interommatidial angles are difficult, and generally require in vivo measurements. Here, we present a new method for estimating interommatidial angles based on the detailed analysis of 3D Micro-CT images of fixed samples. Using custom-made MATLAB software we determine the optical axes of individual ommatidia and project these axes into the three-dimensional space around the animal. The combined viewing directions of all ommatidia, estimated from geometrical optics, allow us to estimate interommatidial angles and map the animal's sampling resolution across its entire visual field. The resulting topographic representations of visual acuity match very closely the previously published data obtained from both fiddler and grapsid crabs. However, the new method provides additional detail that was not previously detectable and reveals that fiddler crabs, rather than having a single horizontal visual streak as is common in flat world inhabitants, likely have two parallel streaks located just above and below the visual horizon. A key advantage of our approach is that it can be used on appropriately preserved specimens allowing the technique to be applied to animals such as deep-sea crustaceans that are inaccessible or unsuitable for in vivo approaches.

A sexual dimorphism in the spatial vision of band-winged grasshoppers

“…Most band-winged grasshoppers have non-spherical eyes featuring an elongated vertical axis that gives them a jellybean-like shape. Similar to other invertebrates with nonspherical eyes (Bagheri et al, 2020;Kelber and Somanathan, 2019) this leads to better visual acuity in the axis of elongation: in band-winged grasshoppers visual acuity in the vertical axis (VAy) is around twice as fine as that in the horizontal axis (VAx; Figure 3). VAy appears to be enhanced by a particularly flat vertical region near the center of the eye.…”

InSegtCone: Interactive Segmentation of crystalline Cones in compound eyes

“…In this case, a sub-region dedicated to these challenging regions may be needed to improve segmentation. (3) Auto-segmentation can be complicated if neighbouring crystalline cones are in contact with each other, such as in the case of crab compound eyes (Alkaladi and Zeil, 2014; Bagheri et al, 2020). In this case, the texture-based segmentation algorithm either fails, or the cones are segmented together in a bundle of connected voxels and cannot be isolated without additional post-processing, e.g.…”

“…Ultimately, the fast auto-segmentation method makes it possible to compare theses accurate resolution maps across large numbers of specimens with different sizes, life-stages, sex, species, etc. This is likely to advance our understanding about the visual ecology and evolution of vision in arthropods (Bagheri et al, 2020;Scales and Butler, 2016;Somanathan et al, 2009;Taylor et al, 2019a). The auto-segmentation method not only extracts the elongation axis of the crystalline cones but also their full shape.…”

“…Imaging methods such as Transmission Electron Microscopy (TEM), a technique that is typically used to measure rhabdom properties, are slow and produce coarse topologies from only a few slices in each eye sample. The pseudopupil technique (Berry et al, 2007), which measures the topology of IO angle, is relatively fast but requires live animals and typically generates coarse resolution maps that do not include the full extent of the FOV (Taylor et al, 2019a), sometimes filtering out precious fine-scale information (Bagheri et al, 2020).…”

“…To address this problem, manual segmentation (or labelling) of each crystalline cone ‘by hand’ in volumetric analysis programs is an option. Unfortunately, unless it is possible to infer the elongation axes of the cones from their specific geometry, as is the case in fiddler crabs (Bagheri et al, 2020), manual segmentation across more than a few samples becomes nearly unfeasible for eyes that frequently consist of several thousands of ommatidia. More generally, the current time-limiting factor of micro-CT is often not the scanning itself but the volumetric analysis of the images it produces, particularly when segmentation is required.…”