The visual system of the genetically tractable crustacean Parhyale hawaiensis: diversification of eyes and visual circuits associated with low-resolution vision

Background

Eyes are the sensory organ of the visual system by which animals sense and interact with their environment. 96% of the animals species (representing 6 of the ~35 main phyla) possess a complex optical system, with image forming eyes being present in arthropodes, molluscs and chordates. These different phyla have evolved an incredible variety of forms and declinations in order to detect, focus, discriminate or analyse light, depending on the environment of their ecological niches and their life history traits. For example, while human and vertebrates eyes produce sharp images and can distinguish a few colors, the coumpound eyes of arthropodes, and insects in particular, can see at a very large angle, detect fast movement and polarization of light in some cases, or some crustaceans can resolve up to 10 different wavelengths.

The neural processing centers that integrate the light stimulus are also shown to be extensively diverse and are not always well understood outside model species. In malacostracan crustaceans and insects, visual stimuli are processed in the optic lobes by four neuropils (i.e. part of the brain, synaptically dense regions of unmyelinated axons, dendrites and glial cells containing a relatively low number of cell bodiess). Each neuropil is thought to process a different visual processing task such as contrast enhancement, color detection, polarised light or motion. Changes in neuropils have been documented in crustaceans groups but the functional aspect of these changes is not well understood nor is the diversification of these neural circuits. Indeed, while the mapping of the visual circuits is well established in Drosophila, it remains unclear for other groups, particularly for crustaceans, given the lack of genomic, genetic and expression tools.

To fill this gap, the authors investigated the basis of the visual system of Parhyale hawaiensis, an amphipod crustacean living in shallow marine habitats in the tropics. From eye anatomy to neural connectivity, they established genetic tools to explore and manipulate the relatively unexplored visual system of this group, opening new routes to study and understand the evolution of visual systems of genetically tractable species.

Key findings

Anatomical structure of the eyes and consequences on P. vision

All arthropod compound eyes share a common basal organisation: they are typically made up of repeated units called ommatidia that function as individual light sensors. Each ommatidium possesses a cornea, a lens and photoreceptors cells that detect light and are apposed to form a light-sensing rhabdom. P. hawaiensis adults have a pair of dark-coloured oval or kidney shaped compound eyes placed lateraly on the head, arranged in hexagonal packing. The authors found that these ommatidia are added gradually during development and lifetime after hatching, reaching up to 50 ommatidia per eye and the eye shape evolves from rounded to elongated. As a result, P. hawaiensis eyes keep growing due to both a parallel increase in ommatidial number and ommatidial size. Because the eye size increase during life, the authors proposed supposed that the acuity and the resolution is changing and improving during the life of P. hawaiensis, raising interesting questions related to the importance of vision at the different life stages of P. hawaiensis.

The authors then studied P. hawaiensis eye organization and anatomy. Sections and scanning electron microscopy within ommatidia reveal that P. hawaiensis eyes function as apposition eyes in which each ommatidium is optically isolated from the other, producing a mosaic image. The cornea covering the eye is smooth with constant thickness and not divided into facets, suggesting that the cornea does not focus light. Because of this morphology, the P. hawaiensis eye is supposed to perform equally well both in terrestrial and marine environments. This observation allowed the authors to determine the field of view as well as the ultimate resolution limit of this animal: large adults have a wide field of view of 120 x 150° but a low spatial resolution and a sampling angle of 15-30° per ommatidium. By equivalence to a photo pixel camera, P. hawaiensis have a very low 50 px resolution which is two orders of magnitude lower than other malascostracan crustaceans.

Structure of ommatidia, photoreceptors and color vision

The structure of ommatidia is a key anatomical point to investigate since it is relataed to what eyes can see in terms of the quantity of light, light polarization and color vision. To study this, the authors used TEM – transmission electron microscopy – which revealed that each ommatidium possesses four photoreceptor cells with large rhabdomeres (R1-4), and one photoreceoptor cell with a small rhabdomere (R5), apposed to form the fused rhabdom. The arrangement of the rhabdom suggests that photoreceptors groups are sensitive to different directions of light polarisation. This property is known to be important in aquatic animals for orientation, in particular celestial orientation, suggesting that P. hawaiensis can detect and use light polarization to orientate itself. Regarding color vision, the authors combined scans of the embryo and adult head transcriptome, ISH and opsin reporter constructs using opsin regulatory regions and showed that two opsins are expressed in P. hawaiensis: PhOpsin1 and PhOpsin2. By analysing the opsins sequences, the authors suggested that PhOpsin1 and 2 function as long wavelength-sensitive (LWS) in the blue and medium wavelength-sensitive (MWS) opsins in the green, respectively. While PhOpsin1 is expressed widely in the retina, likely in R1-4, PhOpsin2 is expressed only as a spot, likely in the R5. Together, these results show a regionalisation of opsin expression and suggest that P. hawaiensis probably has dichromatic vision, since R1-4 and R5 rhabdomeres have different spectral sensitivity.

Structure of the neural connexion and neuropils reveal interesting differences with other crustaceans

Studying the network linking neuropils and the retina is crucial to understand the processing of visual tasks. Indeed, axons from photoreceptors project to different neuropils depending on their input on the different visual tasks. Not surprisingly, these networks are known to vary between species depending on visual capabilities. For example, in other crustacens the first step in processing color vision arises in the meddula (which constitutes the second neuropil) that takes input from photoreceptors. By using immunostaining, thick brain sections and the opsin reporter constructs, the authors were able to identify three neuropils. Interestingly, the first optic neuropils are not apposed to the retina in P. hawaiensis, but are linked to it through neurons emerging from the back of the retina exhibiting a different organisation. All photoreceptor axons project toward the first neuropil, unlike what has been reported in insects and others malacostracan crustaceans in which photoreceptors project either in the lamina or the medulla. In the adult however, the axon of PhOpsin1 and 2 photoreceptors project in two distincts region of the first neuropil, suggesting a regionalisation of this neuropil that might act both as the lamina and the medulla of other athropods. As the homology between the neuropils and their equivalent (lamina, medulla and others) in other crustaceans is unclear, these results raise questions about the homology of neuropils and the evolution of sensory lobes in crustaceans, and more generally in arthropods.

Behavioral responses

In general, compound eyes adjust to light intensity by controlling the amount of light that touches the rhabdom, for example in response to circadian rhythms. This can be done by moving some structures in the ommatidium, the granules, or by changing the size of the rhabdom. To see if such changes happen in P. hawaiensis, the authors sectioned eyes of light adapted and non-light adapted P. hawaiensis and showed that rhabdom size or morphology is not affected whereas the granule repartition is, giving insight into how these animals adapt to light. Then, they performed behavioral experiments in a T maze and showed that P. hawaiensis is attracted to light at low intensity but not at high intensity. Together with previous results, these behavioral tests suggest that P. hawaiensis has a low resolution vision, which is likely not used for locating their mates or find food, but rather to find habitats or orientate themselves, especially since they are sensitive to light polarization and light intensity.

Why this work is important

This preprint sets the basis for a new model system to study vision and the evolution of sensory systems, not only in model arthropods but beyond. P. hawaiensis has been progressively established as a non-model system, with a growing set of tools avaible. By describing, testing and analyzing vision in P. hawaiensis, this preprint makes these resources available not only for this species, but also potentially for other organisms for which these tools are also available.

Second, this work is a great advance to understand functionally the evolution of vision in crustaceans in particular and arthopods in general. Indeed, while vision is well analyzed from an anatomical and genomic point of view in many arthopods, only few studies are able to manipulate and test functionally the characteristics of different visual systems that are known to be really diverse.

Finally, because of its position in the phylogeny, P. hawaiensis is at a key place, between hexapodes (including insects) and malacastraceans, to study the evolution of sensory systems. This study reveals important differences in the brain organisation in comparison with other crustaceans, raising questions about the homology of the different regions of the brain in relationship with the ecology and the life history traits of the different species. While the conclusions regarding homology remain unclear, this work suggest that the brain could evolve by fusing, loosing neuropils or by altering photoreceptor projections, maybe because of the development of a new light-driven response (here, based on polarized light). More broadly, this preprint highlights the importance of studying more non-model organisms from different ecological niches to fully understand the evolution of sensory systems.

 

Future directions and questions for the authors

• Do you expect trade-off with other sensory adaptations for a better performance in realizing other tasks? Could that explain why this organization might have been maintained for 100 Myr?
• Why could there be an evolutionary interest to improve vision during life after reaching the adult stage? Is there a threshold until which the vision does not increase?
• Do you expect some regionalization in the first neuropil? How often do you think this brain reorganization could happen in other species?

 

Additional references

1. Wern et MF, Perry MW, Desplan C. The evolutionary diversity of insect retinal mosaics: common de sign principles and emerging molecular logic. Trends In Genetics. 2015;31 :316 – 28.
2. Marshall J, Kent J, Cronin T. Visual adaptations in crustaceans: Spectral sensitivity in diverse habitats. Adaptive Mechanisms in the Ecology of Vision; 1 999. pp. 285–32 7.
3. Strausfeld NJ. The evolution of crustacean and insect optic lobes and the origins of chiasmata. Arthropod Structure & Development . 20 05 ;34:235– 56.
4. Ramm T, Scholtz G. No sight, no smell? – Brain anatomy of two amphipod crustaceans with different life styles. Arthropod Structure & Development. 2 017;46:53 7 –51.

Tags: crustacean, parhyale hawaiensis, vision, vision evolution

doi: https://doi.org/10.1242/prelights.10234

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