Thank you for referencing the paper below. I was not aware of it.
Functional mapping of ultraviolet photosensitivity during metamorphic transitions in a salmonid fish, Oncorhynchus mykiss
I have read the paper and my critique of the discussion, findings, and conclusions of the paper follows.
They reference previous papers that showed that UV receptors do disappear in adult trout but are present in parr. Therefore, these are the references that you wanted from Sexyloops and Trout University.
"Age-related loss, or reduction, of ultraviolet photoreception appears to occur in many species of fish, including salmonids (Bowmaker and Kunz, 1987; Whitmore and Bowmaker, 1989; Hawryshyn et al., 1989; Loew and Wahl, 1991).
Parr are ultraviolet-sensitive, and possess ultraviolet-sensitive cones in the retinal mosaic (Bowmaker and Kunz, 1987; Browman and Hawryshyn, 1992; Browman and Hawryshyn, 1994; Beaudet et al., 1993; Novales Flamarique and Hawryshyn, 1996; Novales Flamarique, 2000; Parkyn and Hawryshyn, 2000).
.. As parr undergo smoltification, ultraviolet sensitivity appears to decrease and may disappear altogether in some species. In conjunction with this loss of ultraviolet sensitivity, ultraviolet-sensitive cones are no longer present in at least some portions of the retina of salmonid smolts (Bowmaker and Kunz, 1987; Hawryshyn et al., 1989; Beaudet et al., 1993; Novales Flamarique and Hawryshyn, 1996; Novales Flamarique, 2000; and see below)."
They then cite other research that shows some of the UV cones either persist or are regenerated in the dorsal temporal retina.
"Examination of the retina of four species of sexually mature salmonids, however, has revealed the presence of accessory corner (ultraviolet-sensitive) cones, in the dorso-temporal retina (Beaudet et al., 1997). These findings led to the hypothesis that ultraviolet-sensitive cones may regenerate into the retinal mosaic of adult salmonid fish (Beaudet et al., 1997) and, hence, that sexually mature fish may be ultraviolet-sensitive. Because the teleost retina grows throughout life and new photoreceptors are continually added to the retinal mosaic (Lyall, 1957; Johns and Fernald, 1981; Raymond and Hitchcock, 1997), this is certainly a plausible hypothesis. However, it is also possible that the accessory corner cones found in the retina of sexually mature fish are never completely lost. That is, loss of ultraviolet-sensitive cones at smoltification may not occur over the whole retina, and ultraviolet-sensitive cells may be retained in the dorsal retina throughout the life of the fish."
To investigate the UV cones in the dorsal temporal retinal of mature salmonids, they examined the Rainbow trout that they raised from wild eggs. They hormonally treated some Rainbow trout with thyroid hormone thyroxine to simulate the maturation process of the retina. They also raised a control group of non treated rainbow trout.
They use thyroxin to induce smoltification so they can study the retinal changes. However they admit that the issue of whether the changes are due to smolification or thyroxine has never been addressed
and this is one goal of their experiment. That is, can thyroxin be use as a proxy for inducing adult changes? The difference between the thyroxine fish and the control fish noted later in this post, leads me to say no. They have not proven that thyroxine and natural maturation are the same.
Nor do they cite any other references that show that thyroxine treatment is equal to the natural maturation process. Perhaps thyroxine is a true proxy, but they have not provided such evidence in the text of their paper or in the list of references.
"Exposure to exogenous thyroxine can (i) induce smoltification and photopigment changes in salmonids, including rainbow and brook trout (Allen, 1977; McFarland and Allen, 1977; Alexander et al., 1994; Alexander et al., 1998) and (ii) cause a precocial loss, or reduction, of ultraviolet photosensitivity and the number of ultraviolet-sensitive cones in the retina of rainbow trout parr (Browman and Hawryshyn, 1992; Browman and Hawryshyn, 1994). However, whether loss of ultraviolet sensitivity is complete during smoltification (or in response to thyroxine treatment) in O. mykiss has not been addressed."
They found a significant difference at week 6 between the control and the thyroxine treated group
. Thyroxine treatment reduced the UV sensitivity in the ventral retina compared to the control fish. Dorsal UV sensitivity was equal to the control fish.
"At week 6, ultraviolet sensitivity was significantly reduced in all thyroxine-treated fish tested for ventral sensitivity (Fig.4). Ultraviolet sensitivity was significantly lower in the ventral retina of thyroxine-treated fish than in the ventral retina of control fish
. In addition, ultraviolet sensitivity was significantly lower in the ventral retina of thyroxine-treated fish than in the dorsal retina of thyroxine-treated fish
When fitting absorptance curves to the thyroxine-treated, ventral week-6 data, we concluded that only the blue cone mechanism was necessary to explain the data."
They conclude that some UV cones persist after smoltification. Note that they say they persist
that they stay at their original density
. This is critical. Is the density the same as when they were parr or do some of the cones persist at a lower density?
Note that their paper states in the bolded section above, "However, it is also possible that the accessory corner cones found in the retina of sexually mature fish are never completely lost.
" It is clear from this statement that the levels of UV cones are reduced, and that by using persist, they mean persist at a reduced level
"Our finding implies that a population of ultraviolet-sensitive cones reside in the dorsal retina of O. mykiss after smoltification. Ultraviolet-sensitive cones (i.e. accessory corner cones) in the ventral retina disappear from the retinal mosaic during smoltification in O. mykiss (Browman and Hawryshyn, 1992; Browman and Hawryshyn, 1994; Beaudet et al., 1993). However, accessory corner cones have been observed in dorsal regions of the retina (H. I. Browman, personal communication), which apparently impart the regional ultraviolet sensitivity demonstrated here."
The problem with this conclusion is that it implies that the UV cones in the dorsal temporal retina do not later disappear and then regenerate during spawning. It is an inference and not proof that this is the case. They use thyroxine as a proxy for maturation, but can 6 weeks of thyroxine hormone treatment = 2 years of maturation in the wild? That is the fulcrum upon which this experiment rests.
They later go on to try to unify their findings with earlier findings that reported that all UV receptors recede in adults. They theorize that earlier studies examined primarily the ventral retina that does show disappearance of the the UV cones. One would have to go back to the original investigators to see if they agree.
"Earlier studies on ultraviolet sensitivity in both artificially induced O. mykiss smolts (i.e. rainbow trout parr treated with thyroxine) and larger, more developed, juvenile rainbow trout showed a reduction of ultraviolet sensitivity using stimulus presentations that may have illuminated primarily ventral retina (Browman and Hawryshyn, 1992; Browman and Hawryshyn, 1994; Beaudet et al., 1993). Given stimulus design considerations, however, it is unclear how well focused these stimuli were on the retina (see Introduction)."
They conclude is that some UV cones remain in the dorsal retina after smoltification. One would need to examine older wild
fish at 2 years to see if that is true in 2 year old wild
fish since they have not proven that thyroxin is equal to maturation in the wild.
Secondly, even if this were true in wild fish, the visual field of the dorsal retina is that area of vision below the horizontal midline of the field of vision. So if we are to be accurate and accept the persistence of direct UV vision in adult wild fish, it would be approximately 50% of the visual field at a reduced
level of UV cones, and would exclude dry flies that are seen by the ventral retina.
Add your previous post that stated the even with the "persistence" of some of the UV cones in the dorsal retina, the blue cones were responsible for most of the UV sensitivity in adult salmonids, I think they perceive UV as more blue than UV.