This blog is part of our colourful countdown to the holiday season where we’re celebrating the diversity and beauty of the natural world. Click here to read the rest of the colour countdown series.
Aide Macias-Muñoz and Marina Stoilova of University of California Irvine take us on a journey to the depths of the jellyfish cells to help us understand what they are able to see and how colour plays a role in their ecology.
As simple as jellyfish seem to be, it is strange to think of them being able to see. Even more strange is learning that some jellyfish have eyes. As it turns out, cnidarians, the group composed of sea anemones, corals, and jellyfish have independently evolved eyes at least 9 times! Scyphozoa (true jellyfish) and Cubozoa (box jellyfish) have a sensory structure called rhopalia that has neurons, gravity cells and light receptor (photoreceptor) cells. Eyes in these groups of jellyfish are found in the rhopalia. Their visual systems can be composed of simple eyes or even complex lens eyes, similar to humans. As an example, the moon jellyfish Aurelia has 8 rhopalia each with 2 simple eyes. One box jellyfish called Tripedalia, has 4 rhopalia each with 6 eyes: 2 lens eyes and 4 simple eyes; for a total of 24 eyes (Fig 1)! Our lab is working to investigate the genes that encode for different independently evolved eyes. The question we are trying to answer is whether different jellyfish species use similar genes to make their unique eyes and for vision.
Opsins in Hydra
Opsins are light sensitive proteins typically found in photoreceptor cells. Their genetic sequence determines the wavelengths of light they respond to. The number of opsins that animals have varies between different groups. Hydra is a polyp cnidarian with a simple body plan made up of a hypostome, tentacles, foot, and body column. While Hydra lack eyes, they respond to light stimuli by contracting and expanding their body. In Hydra, we found 45 opsin-like genes. It is interesting that so many gene copies are maintained. Some of these opsins, together with light signalling cascade (phototransduction) genes, had higher expression in the head region (hypostome and tentacles) suggesting Hydra are more sensitive to light near the mouth area. Although opsins are typically thought of as having a role in vision, opsins also have a role in cnidocyte (stinging cell) firing in Cnidaria. When we plotted opsin genes at a single-cell resolution in Hydra, we found them in a whole host of different cell types. Generally, they are expected to be found in neurons such as photoreceptors, but opsin genes can be found expressed even in non-neuronal cells. From gland cells and cnidocytes to epithelial cells, opsin genes have even been found expressed in stem cells! Currently, not much is known about what these opsins genes are doing in these non-neuronal cell types, but the implication of non-neuronal cells being sensitive to light is still very exciting. Some of our future work aims to investigate what the many opsin genes are doing in the different tissues and cell types.
Colour detection and vision in Cnidarians
Cnidarians have a variable number of genes that are typically involved in light detection and colour vision so what do they see? As mentioned above, Hydra don’t have eyes, but they have a behavioural response to light. When bright light is shined on a Hydra polyp, the polyp elongates and contacts its body. If the Hydra is then moved from light to darkness it will continue to periodically contract its body into a tight ball. Electrophysiological recordings of Hydra found that light interrupts a normal rhythmic pulsing of its body, and the effect is double for wavelengths in the blue range. Sensitivity to light decreases significantly for light with wavelengths longer than 500 nm (cyan). These studies thus determined that Hydra is most sensitive to blue.
Another cnidarian that has been used to study visual systems is the box jellyfish Tripedalia, which has 24 eyes and is proposed to have 17 opsins. Tripedalia are found in mangrove swamps in Puerto Rico. Using electrophysiology, the spectral sensitivity curves of the lens eyes in Tripedalia and another box jellyfish had a peak at approximately 500 nm. That means that these lens eyes sense blue-green wavelengths of light. In Tripedalia, an opsin referred to as the “lens eye expressed opsin” was found localised to the lens eyes in the rhopalia and is likely the opsin being used to detect the blue-green light. These studies found that these jellyfish are monochromatic which means they cannot tell apart different colours, so how does vision influence their behaviour? One study compared obstacle avoidance behaviour between Tripedalia and another species of box jellyfish found in shallow sandy waters of Australia. The study could not determine whether the two box jellyfish species used colour vision for obstacle avoidance because avoidance was potentially linked to higher contrast. They did find, however, that Tripedalia was better at obstacle avoidance and hypothesized that it has to do with its need to navigate around mangrove roots. Another study sought to find out whether Tripedalia use their large upper lens eyes to navigate. They conducted an experiment were they placed Tripedalia in a circular tank at various distances from their home canopy and found that while the canopy was visible Tripedalia swam towards it.
Spectral sensitivities in other cnidarians are predicted to also be somewhere in the blue and green wavelengths of light. However, more work in comparative physiology and behaviour needs to be done to determine what these organisms see. Some future aims of our research are to compare the phototransduction cascades between species to gain insight into the ancestral mechanisms for light detection. In addition, we seek to understand the role of the ecology on different eye morphology and colour vision. There is a lot to learn about eye evolution and light detection and these extraordinary organisms with such diversity in eye types provide an interesting group to investigate such topics.
To learn more about cnidarians check out some of the literature we reference:
1. Picciani, N. et al. Prolific origination of eyes in Cnidaria with co-option of non-visual opsins. Curr. Biol. 28, 2413-2419.e4 (2018).
2. Macias-Munõz, A., Murad, R. & Mortazavi, A. Molecular evolution and expression of opsin genes in Hydra vulgaris. BMC Genomics 20, 1–19 (2019).
3. Picciani, N. et al. Light modulated cnidocyte discharge predates the origins of eyes in Cnidaria. Ecol. Evol. 11, 3933–3940 (2021).
4. Passano, L. M. & McCullough, C. B. The light response and the rhythmic potentials of Hydra. Proc. Natl. Acad. Sci. U. S. A. 48, 1376–1382 (1962).
5. Liegertová, M. et al. Cubozoan genome illuminates functional diversification of opsins and photoreceptor evolution. Sci. Rep. 5, 11885 (2015).
6. Coates, M. M., Garm, A., Theobald, J. C., Thompson, S. H. & Nilsson, D. E. The spectral sensitivity of the lens eyes of a box jellyfish, Tripedalia cystophora (Conant). J. Exp. Biol. 209, 3758–3765 (2006).
7. Garm, A., Coates, M. M., Gad, R., Seymour, J. & Nilsson, D. E. The lens eyes of the box jellyfish Tripedalia cystophora and Chiropsalmus sp. are slow and color-blind. J. Comp. Physiol. A Neuroethol. Sensory, Neural, Behav. Physiol. 193, 547–557 (2007).
8. Bielecki, J., Zaharoff, A. K., Leung, N. Y., Garm, A. & Oakley, T. H. Ocular and extraocular expression of opsins in the rhopalium of Tripedalia cystophora (Cnidaria: Cubozoa). PLoS One 9, e98870 (2014).
9. Garm, A., O’Connor, M., Parkefelt, L. & Nilsson, D. E. Visually guided obstacle avoidance in the box jellyfish Tripedalia cystophora and Chiropsella bronzie. J. Exp. Biol. 210, 3616–3623 (2007).
10. Garm, A., Oskarsson, M. & Nilsson, D. E. Box jellyfish use terrestrial visual cues for navigation. Curr. Biol. 21, 798–803 (2011).