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the North Atlantic, Pac That's right, there are several evolved modifications of eyes that are used to compensate for viewing in low light conditions, and that are used in some fish. Water seems optically clear, but has a broad optical transmission with a peak around 490 nm (blue-green) when not optically contaminated by dissolved organic material inshore. This is important only under deep sea water because then wavelengths on either side of this transmission peak have got attenuated noticeably more strongly. Accordingly when it became possible to analyze visual pigments by spectroscopy, a group of marine fish looked at by Herb Dartnall in UK were all found to cluster around a matching 490 nm, therefore enabling the fish to make the most of any available downwelling light. A second strategy used by nocturnal animals, to which deep sea fish are necessarily similar, is to increase the entrance aperture (diameter) of the eye -- as binocular owners know, the light-gathering power is proportional to the square of the aperture (doubling the aperture increases the light gathering power by a factor of 4), particularly important at dusk and dawn. So these eyes typically have low f-numbers, familiar to photographers (f number = focal length/entrance diameter). The Chimaera photos do show quite large diameter eyes. A third modification is to develop a tapetum, or reflecting layer at the very back of the eye, for instance by depositing layers of reflecting guanine crystals in cells there. This is what you are looking at with a cat's or alligator's eyes in your car headlights at night, though moths, crayfish and even scallops also use tapeta (variable, adaptive). Presumably that's what gives rise to ghoulish look of the Chimaera's eyes in one of the photos, though there, the back of the retina looks to have collapsed towards the lens. This reflector trick can potentially (almost) double the light-gathering power of the photoreceptors, because most of the photons lost escaping from the back end get to pass through the absorbing layer twice, on the way in and then on the way out after reflection (you can see the eye-shine because not all the photons are usually absorbed, though the photon relative capture efficiency is high around 66% -- two photons absorbed in rhodopsin for one turned into heat, by absorption in black melanin pigment granules in accessory cells). A fourth trick is to increase the length of the absorbing structure, because absorption in rod outer segments is around only ~1% per micrometer length (not much) and therefore proceeds slowly down the column, decaying exponentially: a short absorber will have wasted light coming out its back end, actually its tip. A long rod-like absorber therefore increases total photon capture, and the tapetum will help additionally. I can't remember the species, but some deep sea fish have also developed a tiered retina with at least 3 layers of long rods in series, so residual light getting through tier 1 gets into tier 2 for extra absorption and so on. Fifth, and probably most important: I don't know if evidence exists for deep sea fish but it's a certainty, based on work on mammals/humans, that large groups of photoreceptors used in dim light are 'pooled' by convergence on to the following neurons. In humans this 'pool' is around 500 rods, so a ~500:1 convergence. The human threshold for just seeing any illumination when dark-adapted is ~5-8 photons, caught one per cell by 5-8 of these rods: the visual threshold lies in the pool, not in the rods themselves. The penalty paid is that the visual system can't tell where in the pool of 500 these photons were caught, so resolution in space is much poorer than when using your green and red cones in the fovea in bright light (there's no convergence in the foveal cone system -- one cone feeds one output neuron). Goldfish also show anatomical convergence of rods on to follower neurons. You'd guess that a fish living on average at a couple of 100 meters where nearly all the light has been absorbed already by the overlying water, would use pooling much greater that 500:1. Steve (Hfx) ________________________________________ From: naturens-owner@chebucto.ns.ca [naturens-owner@chebucto.ns.ca] on behalf of GayleMacLean [duartess@EastLink.ca] Sent: Tuesday, March 8, 2016 11:06 AM To: naturens@chebucto.ns.ca Subject: Re: [NatureNS] Longnose Chimaera Thank you Eric. Was heading down to the library later on today, anyway. Will look for that book. Those eyes are really un-nerving though. Possibly the eyes evolved this way, because of the depth of the ocean where it is usually found? Great information! Cheers! Gayle On 03/08/16 10:51 AM, Eric Mills <E.Mills@Dal.Ca> wrote: Hello Gayle, There are at least 3 species of Chimaeras in the North Atlantic, and two that are similar to this, Longnose Chimaera (Harriotta raleighana) and Knifenose Chimaera (Rhinochimaera atlantica). From the photos it appears to be the latter, which, at least according to W.B. Scott & M.G. Scott (1988), Atlantic Fishes of Canada, is a relatively little known mid-water fish occurring in the North Atlantic, Pacific and Indian Ocean. Maybe there will be more online, but I haven't followed up on that. At any rate, the Scotts' book is a good reference (it should be in most libraries) with pictures , and gives a line on earlier scientific accounts. Apparently there are specimens in the Atlantic Reference Centre at the St. Andrews Biological Station in NB, so it is certainly not unique in the area. As for "evil eyes" - that's a bit of press sensationalism. All the best, Eric Eric L. Mills Lower Rose Bay Lunenburg Co., NS ________________________________ From: naturens-owner@chebucto.ns.ca <naturens-owner@chebucto.ns.ca> on behalf of GayleMacLean <duartess@EastLink.ca> Sent: March 8, 2016 10:15 AM To: naturens Subject: [NatureNS] Longnose Chimaera Hi All, Bev Wigney sent me this link today on Facebook, as she is having a hard time posting to the list from her winter home in Arizona. Anyway, she would love to know more about this very odd fish. And me too! http://www.cbc.ca/news/canada/nova-scotia/lunenburg-fisherman-photographs-creepy-deepwater-fish-1.3479154 [X]<http://www.cbc.ca/news/canada/nova-scotia/lunenburg-fisherman-photographs-creepy-deepwater-fish-1.3479154> Bizarre sea creature with glowing eyes surprises Nova Scotia fisherman<http://www.cbc.ca/news/canada/nova-scotia/lunenburg-fisherman-photographs-creepy-deepwater-fish-1.3479154> www.cbc.ca A fishing trawler off the coast of Newfoundland recently dragged up a creature from the depths of the ocean that made even the most experienced people on board do a double take. Many thanks in a