next message in archive
next message in thread
previous message in archive
previous message in thread
Index of Subjects
vil eyes" -
This is a multi-part message in MIME format.
------=_NextPart_000_4A16_01D179DC.81E7E780
Content-Type: text/plain;
charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
Ditto.
DW
----- Original Message -----=20
From: Annabelle=20
To: naturens@chebucto.ns.ca=20
Sent: Wednesday, March 09, 2016 7:54 AM
Subject: Re: [NatureNS] Longnose Chimaera
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:
Thank you Steve.
This is very well detailed.
Cheers!
Gayle=20
On 03/09/16 01:53 AM, Stephen Shaw <srshaw@Dal.Ca> wrote:=20
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.
=20
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. =20
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 =3D focal =
length/entrance diameter). The Chimaera photos do show quite large =
diameter eyes.
=20
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) =20
________________________________________
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 =
India