Biology subjects

Nilsson, D.-E.

Publications and source records attributed to Nilsson, D.-E..

6 recordsLinked to original sources

bioRxiv2025

A median eye origin of the vertebrate retina explains its unique circuitry

The vertebrate retina is a uniquely complex and evolutionarily conserved structure among bilaterians, combining ciliary (rods and cones) and rhabdomeric (ganglion, amacrine, and horizontal) photoreceptor lineages within a multilayered circuit. This arrangement contrasts with the ancestral bilaterian cephalic pattern, where rhabdomeric photoreceptors dominate lateral eyes and ciliary photoreceptors are limited to unpigmented, non-visual median positions. We propose that the vertebrate retina evolved through the lateralization of a complex median photoreceptive organ already containing both photoreceptor types. This shift likely followed the loss of lateral rhabdomeric eyes in a burrowing, suspension-feeding deuterostome ancestor and the retention of a median eye. In the early chordates leading to vertebrates, this structure diversified into the pineal/parapineal complex and lateral retinas. Central to this transformation was the emergence of a bipolar cellular identity, linking ciliary and rhabdomeric circuits--an unusual feature in animal nervous systems. We suggest bipolar cells have dual evolutionary origins: Off bipolar cells from a ciliary effector lineage and rod- On bipolar cells from a chimeric sensory cell. This model explains key similarities between retina and pineal and supports a scenario in which vertebrate vision emerged by integrating and repurposing preexisting circuits. It reframes the retina not as a de novo innovation, but as a modified and lateralized, solution to sensory challenges faced by early chordates.

neuroscience↗

bioRxiv2024

The sea urchin Paracentrotus lividus orients to visual stimuli

Though lacking eyes, some sea urchins can see: Several species exhibit resolving vision, as distinct from mere light detection. How and where light is captured in the eyeless sea urchins, and how this information is integrated to elicit visual behaviour, remains a fascinating enigma. We assessed the spatial resolution of the sea urchin Paracentrotus lividus in laboratory experiments using fifty adults from the Bay of Naples. This keystone species is an important grazer of the NE Atlantic and Mediterranean and a model system to study development. We carried out behavioural trials in which individuals were placed in a submerged cylindrical arena to determine if they orient towards a visual stimulus on the arena wall, under diffuse, downwelling light. We adopted a novel isoluminant stimulus, necessitating vision of a given resolving power around the horizon to be detected. We tested individuals at five stimulus widths, including a uniform control. Animals oriented (upon clearing an obstacle) only to the widest stimuli (45 deg and above). This acuity may suffice for tasks such as finding nearby shelters or distant patches of habitat. We modelled the visual and neuronal processes to recapitulate these responses in P. lividus, by fine-tuning the model of Li et al. (2022), as applied to the sea urchin Diadema africanum. While these species differ morphologically, the model robustly predicts angular sensitivity in keeping with the behavioural experiments. We find that P. lividus (and likely many Echinacea) possesses coarse spatial vision and that the neurosensory model applies broadly to sea urchins.

zoology↗

bioRxiv2022

A model of decentralized vision in the sea urchin Diadema africanum

AO_SCPLOWBSTRACTC_SCPLOWSea urchins can detect light and move in relation to luminous stimuli despite lacking eyes. They presumably detect light through photoreceptor cells distributed on their body surface. However, there is currently no mechanistic explanation of how these animals can process light to detect visual stimuli and produce oriented movement. Here, we present a model of decentralized vision in echinoderms that includes all known processing stages, from photoreceptor cells to radial nerve neurons to neurons contained in the oral nerve ring encircling the mouth of the animals. In the model, light stimuli captured by photoreceptor cells produce neural activity in the radial nerve neurons. In turn, neural activity in the radial nerves is integrated in the oral nerve ring to produce a profile of neural activity reaching spatially across several ambulacra. This neural activity is read out to produce a model of movement. The model captures previously published data on the behavior of sea urchin Diadema africanum probed with a variety of physical stimuli. The specific pattern of neural connections used in the model makes testable predictions on the properties of single neurons and aggregate neural behavior in Diadema africanum and other echinoderms, offering a potential understanding of the mechanism of visual orientation in these animals.

systems biology↗

bioRxiv2021

Modelling the visual world of a velvet worm

In many animal phyla, eyes are small and provide only low-resolution vision for general orientation in the environment. Because these primitive eyes rarely have a defined image plane, traditional visual-optics principles cannot be applied. To assess the functional capacity of such eyes we have developed modelling principles based on ray tracing in 3D reconstructions of eye morphology, were refraction on the way to the photoreceptors and absorption in the photopigment are calculated incrementally for ray bundles from all angles withing the visual field. From the ray tracing, we calculate the complete angular acceptance function of each photoreceptor in the eye, revealing the visual acuity for all parts of the visual field. We then use this information to generate visual filters that can be applied to high resolution images or videos to convert them to accurate representations of the spatial information seen by the animal. The method is here applied to the 0.1 mm eyes of the velvet worm Euperipatoides rowelli (Onychophora). These eyes of these terrestrial invertebrates consist of a curved cornea covering an irregular but optically homogeneous lens directly joining a retina packed with photoreceptive rhabdomeres. 3D reconstruction from histological sections revealed an asymmetric eye, where the retina is deeper in the forward-pointing direction. The calculated visual acuity also reveals performance differences across the visual field, with a maximum acuity of about 0.11 cycles/deg in the forward direction despite laterally pointing eyes. The results agree with previous behavioural measurements of visual acuity, and suggest that velvet worm vision is adequate for orientation and positioning within the habitat. Author summaryIt is difficult to understand the roles that vision may have in animals with visual performance very different to our own. Many invertebrates such as flatworms, polychaetes, onychophorans, gastropod molluscs and numerous arthropods have tiny eyes with unknown visual abilities. At best, behavioural experiments can reveal visual performance limits for specific behaviours but they will not give general information about what is visible to the animal, which is crucial for understanding the roles vision may have. Here we use ray tracing applied to accurate anatomical/optical models of the eyes of a velvet worm to reconstruct the visual acuity in all parts of the visual field. We also use the calculated visual performance to closely simulate what animals would see in their natural habitat. The method can be applied to any (preferably small) eye and offers an alternative strategy that may yield information about the visual capacity that is otherwise hard to obtain.

animal behavior and cognition↗

bioRxiv2018

Orienting to Polarized Light at Night-Matching Lunar Skylight to Performance in a Nocturnal Beetle

For polarized light to inform behaviour, the typical range of degrees of polarization observable in the animals natural environment must be above the threshold for detection and interpretation. Here we present the first investigation of the degree of linear polarization threshold for orientation behaviour in a nocturnal species, with specific reference to the range of degrees of polarization measured in the night sky. An effect of lunar phase on the degree of polarization of skylight was found, with smaller illuminated fractions of the moons surface corresponding to lower degrees of polarization in the night sky. We found that South African dung beetle Escarabaeus satyrus (Boheman, 1860) can orient to polarized light for a range of degrees of polarization similar to that observed in diurnal insects, reaching a lower threshold between 0.04 and 0.32, possibly as low as 0.11. For degrees of polarization lower than 0.23, as measured on a crescent moon night, orientation performance was considerably weaker than that observed for completely linearly-polarized stimuli, but was nonetheless stronger than in the absence of polarized light.\n\nSummary StatementA degree-of-polarization threshold for orientation behaviour is reported for nocturnal dung beetle Escarabaeus satyrus in the context of measurements showing changes in the degree of polarization of skylight with lunar phase.

animal behavior and cognition↗

bioRxiv2017

Zebrafish differentially process colour across visual space to match natural scenes

Animal eyes evolve to process behaviourally important visual information, but how retinas deal with statistical asymmetries in visual space remains poorly understood. Using hyperspectral imaging in the field, in-vivo 2-photon imaging of retinal neurons and anatomy, here we show that larval zebrafish use a highly anisotropic retina to asymmetrically survey their natural visual world. First, different neurons dominate different parts of the eye, and are linked to a systematic shift in inner retinal function: Above the animal, there is little colour in nature and retinal circuits are largely achromatic. Conversely, the lower visual field and horizon are colour-rich, and are predominately surveyed by chromatic and colour-opponent circuits that are spectrally matched to the dominant chromatic axes in nature. Second, above the frontal horizon, a high-gain ultraviolet-system piggy-backs onto retinal circuits, likely to support prey-capture. Our results demonstrate high functional diversity among single genetically and morphologically defined types of neurons.

neuroscience↗