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=3E Sub --Apple-Mail-9E7C58B2-3A83-4EF0-990F-783B55F442C8 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable And I thank you too! That was so interesting! Sent from my iPad > On Mar 9, 2016, at 6:58 AM, GayleMacLean <duartess@EastLink.ca> wrote: >=20 > Thank you Steve. > This is very well detailed. > =20 > Cheers! > =20 > Gayle=20 > =20 >> On 03/09/16 01:53 AM, Stephen Shaw <srshaw@Dal.Ca> wrote: >> That's right, there are several evolved modifications of eyes that are us= ed to compensate for viewing in low light conditions, and that are used in s= ome fish. >> =20 >> Water seems optically clear, but has a broad optical transmission with a p= eak around 490 nm (blue-green) when not optically contaminated by dissolved o= rganic material inshore. This is important only under deep sea water becaus= e then wavelengths on either side of this transmission peak have got attenua= ted noticeably more strongly. Accordingly when it became possible to analyz= e visual pigments by spectroscopy, a group of marine fish looked at by Herb D= artnall in UK were all found to cluster around a matching 490 nm, therefore e= nabling the fish to make the most of any available downwelling light. =20 >>=20 >> A second strategy used by nocturnal animals, to which deep sea fish are n= ecessarily similar, is to increase the entrance aperture (diameter) of the e= ye -- as binocular owners know, the light-gathering power is proportional to= the square of the aperture (doubling the aperture increases the light gathe= ring power by a factor of 4), particularly important at dusk and dawn. So t= hese eyes typically have low f-numbers, familiar to photographers (f number =3D= focal length/entrance diameter). The Chimaera photos do show quite large d= iameter eyes. >> =20 >> A third modification is to develop a tapetum, or reflecting layer at the v= ery 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 a= lligator's eyes in your car headlights at night, though moths, crayfish and e= ven scallops also use tapeta (variable, adaptive). Presumably that's what g= ives rise to ghoulish look of the Chimaera's eyes in one of the photos, thou= gh there, the back of the retina looks to have collapsed towards the lens. T= his reflector trick can potentially (almost) double the light-gathering powe= r of the photoreceptors, because most of the photons lost escaping from the b= ack end get to pass through the absorbing layer twice, on the way in and the= n 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 effici= ency is high around 66% -- two photons absorbed in rhodopsin for one turned i= nto >> heat, by absorption in black melanin pigment granules in accessory cells= ). >>=20 >> A fourth trick is to increase the length of the absorbing structure, beca= use absorption in rod outer segments is around only ~1% per micrometer lengt= h (not much) and therefore proceeds slowly down the column, decaying exponen= tially: a short absorber will have wasted light coming out its back end, act= ually its tip. A long rod-like absorber therefore increases total photon ca= pture, 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 get= s into tier 2 for extra absorption and so on. >>=20 >> Fifth, and probably most important: I don't know if evidence exists for d= eep sea fish but it's a certainty, based on work on mammals/humans, that lar= ge 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 whe= n dark-adapted is ~5-8 photons, caught one per cell by 5-8 of these rods: th= e visual threshold lies in the pool, not in the rods themselves. The penalt= y paid is that the visual system can't tell where in the pool of 500 these p= hotons were caught, so resolution in space is much poorer than when using yo= ur green and red cones in the fovea in bright light (there's no convergence i= n the foveal cone system -- one cone feeds one output neuron). Goldfish als= o show anatomical convergence of rods on to follower neurons. You'd guess t= hat a fish living on average at a couple of 100 meters where nearly all the l= ight has >> been absorbed already by the overlying water, would use pooling much gre= ater that 500:1. >> Steve (Hfx) =20 >> ________________________________________ >> From: naturens-owner@chebucto.ns.ca [naturens-owner@chebucto.ns.ca] on be= half of GayleMacLean [duartess@EastLink.ca] >> Sent: Tuesday, March 8, 2016 11:06 AM >> To: naturens@chebucto.ns.ca >> Subject: Re: [NatureNS] Longnose Chimaera >>=20 >> Thank you Eric. >>=20 >> Was heading down to the library later on today, anyway. Will look for tha= t book. Those eyes are really un-nerving though. Possibly the eyes evolved t= his way, because of the depth of the ocean where it is usually found? >>=20 >> Great information! >>=20 >> Cheers! >>=20 >> Gayle >>=20 >>=20 >> On 03/08/16 10:51 AM, Eric Mills <E.Mills@Dal.Ca> wrote: >>=20 >> Hello Gayle, >>=20 >>=20 >> There are at least 3 species of Chimaeras in the North Atlantic, and two t= hat are similar to this, Longnose Chimaera (Harriotta raleighana) and Knifen= ose Chimaera (Rhinochimaera atlantica). =46rom the photos it appears to be t= he latter, which, at least according to W.B. Scott & M.G. Scott (1988), Atla= ntic Fishes of Canada, is a relatively little known mid-water fish occurring= in the North Atlantic, Pacific and Indian Ocean. >>=20 >>=20 >> Maybe there will be more online, but I haven't followed up on that. At an= y rate, the Scotts' book is a good reference (it should be in most libraries= ) with pictures , and gives a line on earlier scientific accounts. Apparentl= y there are specimens in the Atlantic Reference Centre at the St. Andrews Bi= ological Station in NB, so it is certainly not unique in the area. As for "e= vil eyes" -