The eye of Orchomene sp. cf. O. rossi , an amphipod living under the Ross Ice Shelf (Antarctica)

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Abstract
The structural organization of the eye of freshly caught O. sp. cf O. rossi is described and compared with that of individuals exposed to sunlight for 2 h or kept in complete darkness for 2 and 7 days. Orchomene sp. of O. rossi occurs under the 400 m thick Ross Ice Shelf in water of -2.degree. C. Its eyes, containing 360 ommatidia, are relatively large and of orange color. External facets are undeveloped, but the diotrio structures, consisting of a cornea 30 .mu.m thick and the bipartite crystalline cones 65 .mu.m long and 55 .mu.m wide, are transparent and show the underlying ommatidial organization and retinula. Rhabdoms, comprising rhabdomeres of 5 retinula cells per omma, are voluminous (up to 50 .mu.m in diameter and 160 um in length) and surround the proximal ends of the crystalline cones. There are a few black screening pigment granules in retinula cell plasma, but their number is too small to effectively shield and insulate neighboring ommatidia from each other. Interstitial cells, containing vesicles of 0.3 .mu.m diameter, occupy the spaces between adjacent ommatidial groups of retinula cells, but do not extend into the layer of retinula cell nuclei. Based on the anatomy, the eye of O. rossi seems to be adapted to maximize photon capture at the expense of acuity. The most orderly arrangement of microvilli was seen in eyes belonging to animals that were fixed immediately after being brought up from under the ice. Evidence is given that the 15-20 min exposure to light during the haul up through the ice hole triggers off membrane breakdown, which, ultimately, results in the total disruption of the rhabdom organization and which can be slowed down but not prevented, by placing the animal in a dark environment. Exposure to sunlight for 2 h produces a very similar ultrastructural rhabdom derangement to that seen in animals kept in the dark for 1 wk. Because of the presence of rhabdom fragments in the interstitial cells, the latter probably play a role in the disposal of visual membrane material. Although the amount and origin of the light available to O. rossi have to remain speculation, it is concluded from simple behavioral observations and the described effects of light on the ultrastructural organization of the retinula that O. rossi possesses photoreception.