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Six of eight species of reef fish with red fluorescent irises showed more intense fluorescence in vivo - under laboratory conditions - when collected at or below 20 metres than did individuals from the same species collected in shallow water of 5 metres or above, according to a report in today's Proceedings of Royal Society B. All species observed have red fluorescent irises. In two of the species there was no difference in brightness at the two sampling depths.
Two hypotheses were being tested: that red fluorescence offers protection from the short wavelength light which penetrates shallow water; and that at depth red fluorescence has a visual function, because it provides light at wavelengths the water column above 20 metres has absorbed.
The authors from the University of Tuebingen in Germany and from the Suez Canal University in Egypt say their results are consistent with an hypothesis that visual, contrast enhancing functions may result from red fluorescent irises, and photoprotection "in at least some marine fish is probably not" the primary reason for red fluorescence. Photoprotection could be of secondary importance.
Why reef fish may benefit from increasing visual contrast using red fluorescence is not answered in the study, according to the authors. Speculations advanced include that red fluorescence at depth where water has absorbed most longer wavelengths (600 - 700 nanometre range) gives a light to help them locate prey.
"The key question," write the authors, "is under which conditions additive fluorescent colouration can be significant for colour vision, given the evolutionary success of subtraction colours." To the authors a subtraction colour is one shown by an organism's cellular pigments or structural colours, as a result of reflection or transmission of unabsorbed spectral wavelengths.
For the experiments reported today the nine authors took samples from the Red Sea, the Mediterranean and Eastern Indian Ocean. Collecting criteria were that the fish have fluorescence in the iris, small size, benthic life style and sufficient abundance at the collecting depth. Eight species from three fish families were selected.
Family Gobiidae: B. natans, E. guttata, E. zebrina, P. Micheli, F. duospilus.
Family Syngnathidae: C. nigripectus.
Family Triptergiidae: H. striata, T. delaisi.
Samples were collected along coral surfaces, working in line with memorandas of understanding and local regulations.
Before being netted the fish were partially anaethetised, then underwent spectroscopic studies in shallow water in Petri dishes, before being transferred to a recovery tank, and back to the wild within 24 hours of being caught.
The results indicate "fluorescence is adjusted to depth", write the authors, and they say it is unclear whether those species (B. natans and F. duospilus) showing no difference in red fluorescence intensity at the two sampling depths lack adaptations for depth.
In discussion the authors posit that red fluorescence at depth is related to the number of pigment-containing cells in the irises of the different species, or to the size of the patch of the iris which is fluorescent, or to phenotypic plasticity during development, or to an adult adaptation.
Fish length was treated as a co-variant because in five species individuals collected from shallow water showed a statistically significant greater length than those from deeper water. Analysis revealed length did not contribute to fluorescence brightness.
Results from the study are not suitable for benchmarking because of the nature of the experiments, which were adequate for qualitative anayses within datasets, but did not provide the kind of number allowing quantitative comparisons among datasets.
Natural fluorescence, its biological purpose in evolutionary, phylogenetic, functional and ecological contexts, as wells as how the phenomenon fits into developmental biology and genetics, are, given the discussion and bibliography, the big pictures within which the authors are striving to isolate useful hypotheses to elucidate the pheneomenon of red fluorescence in reef fish. Their literature search ranges from 1944 to 2014, and covers a significant range of biological and biology-relevant disciplines.
NEWS REPORT BASED ON
Meadows M.G., Anthes N., Dangelmayer S., Alwany M.A., Gerlach T., Schulte G., Sprenger D., Theobald J., Michiels N.K., (2014). Red fluorescence increases with depth, supporting a visual function, not UV protection. Proceedings of the Royal Society B, 16th July, 2014.
http://dx.doi.org/10.1098/rspb.2014.1211
The above paper was embargoed for 00.01 am bst, 16.07.2014, and Science for Biophysics published the above news report 16th July, 2014.
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