If I lived in the deep, dark sea, in the open water column of the mesopelagic habitat, I would want to have eyeballs that took up nearly 50% of my body. Okay, maybe not. It might be hard for me to swim. Then again, the hyperiid amphipod, Paraphronima gracilis, apparently thrives with giant eyes that dominate nearly half its body. And it’s not just that the eyes are gigantic, they are weird, too. This animal has a configuration of compound eyes that has never been described before. Today in Current Biology, Karen Osborn, Sonke Johnsen, and I describe the strange eyes of Paraphronima gracilis and discuss the possible advantages of their novel design.
Paraphronima has a pair of bilobed apposition compound eyes. There is a large upward-facing portion and a smaller lateral-facing portion. Having four eyes isn’t unusual in the mesopelagic habitat, especially amongst the hyperiid amphipods. The unique aspect of the eyes is the multiple, discontinuous retinas. There are 12 retinal groups in each upward-facing portion and 5 retinal groups in the lateral portions, although there is one large, dominating group in the lateral portion. In most animals with compound eyes, the retina is one continuous sheet of photoreceptors, supporting cells and screening pigments.
We suspect the unique eyes of Paraphronima are acting to maximize light capture (and therefore, vision) in the mesopelagic habitat. The mesopelagic habitat could be considered an extreme habitat. In the deep water column, away from surface, shore, or ocean bottom, there are no physical objects to hide amongst, except other animals. Light is dim and blue, extinguished to pitch black after 1000 meters. Bioluminescence flashes and search lights become the major sources of illumination. Animals here are challenged to see, without being seen.
The eyes of Paraphronima are large and nearly transparent, with the exception of the small, orange retinas. The ommatidia of the eyes are arranged in rows perpendicular to the body axis. A single row of ommatidia may function together to boost the light gathering ability of the eye, using a process called spatial summation. In most other animals using spatial summation, a circular group of ommatidia may pool the light signal, boosting light sensitivity at the cost of spatial resolution. In Paraphronima, summation may occur in a linear, not circular, fashion. Spatial summation in one axis would allow the eye to gather more light, while reducing the cost to spatial resolution in one axis. We suspect that Paraphronima is mainly concerned with detecting linear appearing targets, like small siphonophores which could easily be targeted in a single axis.
Further, we studied the spectral sensitivity of the Paraphronima gracilis eye. Spectral sensitivity refers to the color of light maximally absorbed by the photoreceptors, which is 516 nm (basically green) for P. gracilis. The predominant light present in the mesopelagic is around 480 nm (blue-green). Most mesopelagic animals searching in the horizontal plane would benefit from having spectral sensitivity corresponding to the wavelength of dominant light. However, animals looking upward into the down-welling light may have an easier time spotting targets using an offset spectral sensitivity. Light models indicate that mesopelagic animals searching overhead would most benefit from sensitivity near 515 nm. This corresponds closely to that found in P. gracilis, indicating that their eyes are well suited for searching for overhead targets.
The extreme nature of the mesopelagic habitat has generated a number of fascinating eye morphologies amongst its inhabitants shaped by selective pressures to deal with the challenges of dim solar illumination, bioluminescent light sources, and a structure-free habitat. The compound eye of P. gracilis has a novel configuration not described in any other arthropod species. Further investigation into the physiology, neurobiology, and natural history of P. gracilis may help in explaining its novel and intriguing retinal structure.
By Jamie Baldwin Fergus - NMNH Peter Buck Postdoctoral Fellow
See and Read More. . .
Gorgeous Slide Show at the Ocean Portal -- The Hyper Eyes of Hyperiids
Highlight from Smithsonian Science on this same work
Baldwin Fergus JL, Johnsen S & KJ Osborn. 2015. A Unique Apposition Compound Eye in the Mesopelagic Hyperiid Amphipod Paraphronima gracilis. Current Biology. DOI: http://dx.doi.org/10.1016/j.cub.2014.12.010
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