Comparative Study of the Effects of Light on Photophore Ultrastructure from Two Families of Deep-Sea Decapod Crustaceans: Oplophoridae and Sergestidae

Counterillumination, the mechanism by which pelagic species produce bioluminescence to replace the light blocked by their bodies to hide their silhouettes, has been known for over 100 years. However, little is known about how these animals are able to so precisely replicate the intensity of downwelling light. The recent discovery of opsins in photophores (Bracken-Grissom et al. 2020) suggests that these autogenic organs (i.e. non-bacterial) may be sensitive to light, in addition to their function of emitting visible light. The study presented here is 1) the first ultrastructural assessment of photophores in species Systellaspis debilis, Janicella spinicauda, Parasergestes armatus, and Allosergestes sargassi and 2) the first study to examine ultrastructural changes in photophore organelles in response to light. The results of this study, demonstrate that photophore organelles exhibit changes in response to light similar to that seen in crustacean photoreceptors, and provides strong support for the hypothesis that the photophores themselves are sensitive to light

Light Organ Photosensitivity in Deep-Sea Shrimp May Suggest a Novel Role in Counterillumination

Extraocular photoreception, the ability to detect and respond to light outside of the eye, has not been previously described in deep-sea invertebrates. Here, we investigate photosensitivity in the bioluminescent light organs (photophores) of deep-sea shrimp, an autogenic system in which the organism possesses the substrates and enzymes to produce light. Through the integration of transcriptomics, in situ hybridization and immunohistochemistry we find evidence for the expression of opsins and phototransduction genes known to play a role in light detection in most animals. Subsequent shipboard light exposure experiments showed ultrastructural changes in the photophore similar to those seen in crustacean eyes, providing further evidence that photophores are light sensitive. In many deep-sea species, it has long been documented that photophores emit light to aid in counterillumination – a dynamic form of camouflage that requires adjusting the organ’s light intensity to “hide” their silhouettes from predators below. However, it remains a mystery how animals fine-tune their photophore luminescence to match the intensity of downwelling light. Photophore photosensitivity allows us to reconsider the organ’s role in counterillumination - not only in light emission but also light detection and regulation

HCNSO Life Sciences Research Symposium Photographs

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Ultrastructural analysis of the effects of light on cuticular pleopod photophores in the deep-sea shrimp Janicella spinicauda

With little refuge and a three-dimensional hunting ground, mesopelagic (200-1000 m) animals have evolved a variety of adaptations to avoid being easy prey. Among these are counterillumination, i.e. a silhouette camouflaged from predators below via ventrally directed bioluminescent light produced in photophore organs. Counterillumination is used extensively throughout the marine environment, but little is known about the mechanisms used to enable animals to precisely replicate the intensity and wavelength of downwelling light. A recent hypothesis suggests that photophore organs may be photosensitive due to the presence of visual opsin proteins, a component of all known visual pigments, that were discovered in photophores from several Oplophoridae shrimp species. This is the first study to investigate photosensitivity in an autogenic light organ in the deep-sea shrimp species Janicella spinicauda (Oplophoridae) by comparing the ultrastructure of unexposed, dim-light exposed, and bright-light exposed photophores. This study has found evidence of photosensitivity in cuticular pleopod photophores as our findings are consistent with well-known cellular responses of dim-light adapted crustacean compound eyes when exposed to dim and ecologically equivalent light intensities, or high and damaging light intensities