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Many thanks in a This is a multi-part message in MIME format. --Boundary_(ID_H58p+dqDM0NxDlbPlgNAzA) Content-type: text/plain; CHARSET=US-ASCII Content-transfer-encoding: quoted-printable Content-disposition: inline Thank you Steve=2E This is very well detailed=2E = Cheers! = Gayle = = On 03/09/16 01=3A53 AM=2C Stephen Shaw =3Csrshaw=40Dal=2ECa=3E wrote=3A = =3E = =3E That=27s right=2C there are several evolved modifications of eyes th= at are used to compensate for viewing in low light conditions=2C and tha= t are used in some fish=2E =3E = =3E Water seems optically clear=2C but has a broad optical transmission = with a peak around 490 nm (blue-green) when not optically contaminated b= y dissolved organic material inshore=2E This is important only under de= ep sea water because then wavelengths on either side of this transmissio= n peak have got attenuated noticeably more strongly=2E Accordingly when= it became possible to analyze visual pigments by spectroscopy=2C a grou= p of marine fish looked at by Herb Dartnall in UK were all found to clus= ter around a matching 490 nm=2C therefore enabling the fish to make the = most of any available downwelling light=2E = =3E = =3E A second strategy used by nocturnal animals=2C to which deep sea fis= h are necessarily similar=2C is to increase the entrance aperture (diame= ter) of the eye -- as binocular owners know=2C the light-gathering power= is proportional to the square of the aperture (doubling the aperture in= creases the light gathering power by a factor of 4)=2C particularly impo= rtant at dusk and dawn=2E So these eyes typically have low f-numbers=2C= familiar to photographers (f number =3D focal length/entrance diameter)= =2E The Chimaera photos do show quite large diameter eyes=2E =3E = =3E A third modification is to develop a tapetum=2C or reflecting layer = at the very back of the eye=2C for instance by depositing layers of refl= ecting guanine crystals in cells there=2E This is what you are looking = at with a cat=27s or alligator=27s eyes in your car headlights at night=2C= though moths=2C crayfish and even scallops also use tapeta (variable=2C= adaptive)=2E Presumably that=27s what gives rise to ghoulish look of t= he Chimaera=27s eyes in one of the photos=2C though there=2C the back of= the retina looks to have collapsed towards the lens=2E This reflector = trick can potentially (almost) double the light-gathering power of the p= hotoreceptors=2C because most of the photons lost escaping from the back= end get to pass through the absorbing layer twice=2C 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=2C though the photon relative c= apture efficiency is high around 66=25 -- two photons absorbed in rhodop= sin for one turned into =3E heat=2C by absorption in black melanin pigment granules in accessor= y cells)=2E =3E = =3E A fourth trick is to increase the length of the absorbing structure=2C= because absorption in rod outer segments is around only =7E1=25 per mic= rometer length (not much) and therefore proceeds slowly down the column=2C= decaying exponentially=3A a short absorber will have wasted light comin= g out its back end=2C actually its tip=2E A long rod-like absorber ther= efore increases total photon capture=2C and the tapetum will help additi= onally=2E I can=27t remember the species=2C but some deep sea fish have= also developed a tiered retina with at least 3 layers of long rods in s= eries=2C so residual light getting through tier 1 gets into tier 2 for e= xtra absorption and so on=2E =3E = =3E Fifth=2C and probably most important=3A I don=27t know if evidence = exists for deep sea fish but it=27s a certainty=2C based on work on mamm= als/humans=2C that large groups of photoreceptors used in dim light are = =27pooled=27 by convergence on to the following neurons=2E In humans th= is =27pool=27 is around 500 rods=2C so a =7E500=3A1 convergence=2E The = human threshold for just seeing any illumination when dark-adapted is =7E= 5-8 photons=2C caught one per cell by 5-8 of these rods=3A the visual th= reshold lies in the pool=2C not in the rods themselves=2E The penalty p= aid is that the visual system can=27t tell where in the pool of 500 thes= e photons were caught=2C so resolution in space is much poorer than when= using your green and red cones in the fovea in bright light (there=27s = no convergence in the foveal cone system -- one cone feeds one output ne= uron)=2E Goldfish also show anatomical convergence of rods on to follow= er neurons=2E You=27d guess that a fish living on average at a couple o= f 100 meters where nearly all the light has =3E been absorbed already by the overlying water=2C would use pooling m= uch greater that 500=3A1=2E =3E Steve (Hfx) = =3E =5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F= =5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F =3E From=3A naturens-owner=40chebucto=2Ens=2Eca =5Bnaturens-owner=40cheb= ucto=2Ens=2Eca=5D on behalf of GayleMacLean =5Bduartess=40EastLink=2Eca=5D= =3E Sent=3A Tuesday=2C March 8=2C 2016 11=3A06 AM =3E To=3A naturens=40chebucto=2Ens=2Eca =3E Subject=3A Re=3A =5BNatureNS=5D Longnose Chimaera =3E = =3E Thank you Eric=2E =3E = =3E Was heading down to the library later on today=2C anyway=2E Will loo= k for that book=2E Those eyes are really un-nerving though=2E Possibly t= he eyes evolved this way=2C because of the depth of the ocean where it i= s usually found=3F =3E = =3E Great information! =3E = =3E Cheers! =3E = =3E Gayle =3E = =3E = =3E On 03/08/16 10=3A51 AM=2C Eric Mills =3CE=2EMills=40Dal=2ECa=3E wrot= e=3A =3E = =3E Hello Gayle=2C =3E = =3E = =3E There are at least 3 species of Chimaeras in the North Atlantic=2C a= nd two that are similar to this=2C Longnose Chimaera (Harriotta raleigha= na) and Knifenose Chimaera (Rhinochimaera atlantica)=2E From the photos = it appears to be the latter=2C which=2C at least according to W=2EB=2E S= cott =26 M=2EG=2E Scott (1988)=2C Atlantic Fishes of Canada=2C is a rela= tively little known mid-water fish occurring in the North Atlantic=2C Pa= cific and Indian Ocean=2E =3E = =3E = =3E Maybe there will be more online=2C but I haven=27t followed up on th= at=2E At any rate=2C the Scotts=27 book is a good reference (it should b= e in most libraries) with pictures =2C and gives a line on earlier scien= tific accounts=2E Apparently there are specimens in the Atlantic Referen= ce Centre at the St=2E Andrews Biological Station in NB=2C so it is cert= ainly not unique in the area=2E As for =22evil eyes=22 - that=27s a bit = of press sensationalism=2E =3E = =3E = =3E All the best=2C =3E = =3E = =3E Eric =3E = =3E = =3E Eric L=2E Mills =3E = =3E Lower Rose Bay =3E = =3E Lunenburg Co=2E=2C NS =3E = =3E =5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F= =5F=5F=5F=5F=5F=5F=5F=5F=5F =3E From=3A naturens-owner=40chebucto=2Ens=2Eca =3Cnaturens-owner=40cheb= ucto=2Ens=2Eca=3E on behalf of GayleMacLean =3Cduartess=40EastLink=2Eca=3E= =3E Sent=3A March 8=2C 2016 10=3A15 AM =3E To=3A naturens =3E Sub