[NatureNS] Longnose Chimaera

From: GayleMacLean <duartess@EastLink.ca>
To: naturens@chebucto.ns.ca
Date: Wed, 09 Mar 2016 06:58:50 -0400
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Many thanks in a
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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