[NatureNS] Longnose Chimaera

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From: Annabelle <hamst@xplornet.com>
Date: Wed, 9 Mar 2016 07:54:13 -0400
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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" -