title: Red fluorescence in reef fish: a novel signalling mechanism?
abstract: BACKGROUND
  At depths below 10 m, reefs are dominated by blue-green light because seawater selectively absorbs the longer, 'red' wavelengths beyond 600 nm from the downwelling sunlight. Consequently, the visual pigments of many reef fish are matched to shorter wavelengths, which are transmitted better by water. Combining the typically poor long-wavelength sensitivity of fish eyes with the presumed lack of ambient red light, red light is currently considered irrelevant for reef fish. However, previous studies ignore the fact that several marine organisms, including deep sea fish, produce their own red luminescence and are capable of seeing it.
  
  
  RESULTS
  We here report that at least 32 reef fishes from 16 genera and 5 families show pronounced red fluorescence under natural, daytime conditions at depths where downwelling red light is virtually absent. Fluorescence was confirmed by extensive spectrometry in the laboratory. In most cases peak emission was around 600 nm and fluorescence was associated with guanine crystals, which thus far were known for their light reflecting properties only. Our data indicate that red fluorescence may function in a context of intraspecific communication. Fluorescence patterns were typically associated with the eyes or the head, varying substantially even between species of the same genus. Moreover red fluorescence was particularly strong in fins that are involved in intraspecific signalling. Finally, microspectrometry in one fluorescent goby, Eviota pellucida, showed a long-wave sensitivity that overlapped with its own red fluorescence, indicating that this species is capable of seeing its own fluorescence.
  
  
  CONCLUSIONS
  We show that red fluorescence is widespread among marine fishes. Many features indicate that it is used as a private communication mechanism in small, benthic, pair- or group-living fishes. Many of these species show quite cryptic colouration in other parts of the visible spectrum. High inter-specific variation in red fluorescence and its association with structures used in intra-specific signalling further corroborate this view. Our findings challenge the notion that red light is of no importance to marine fish, calling for a reassessment of its role in fish visual ecology in subsurface marine environments.
authors.affiliation: Faculty of Biology, University of Tübingen, Auf der Morgenstelle 28, 72076 Tübingen, Germany. nico.michiels@uni-tuebingen.de
authors.forename: Nico K
authors.initials: NK
authors.lastname: Michiels
authors.forename: Nils
authors.initials: N
authors.lastname: Anthes
authors.forename: Nathan S
authors.initials: NS
authors.lastname: Hart
authors.forename: Jürgen
authors.initials: J
authors.lastname: Herler
authors.forename: Alfred J
authors.initials: AJ
authors.lastname: Meixner
authors.forename: Frank
authors.initials: F
authors.lastname: Schleifenbaum
authors.forename: Gregor
authors.initials: G
authors.lastname: Schulte
authors.forename: Ulrike E
authors.initials: UE
authors.lastname: Siebeck
authors.forename: Dennis
authors.initials: D
authors.lastname: Sprenger
authors.forename: Matthias F
authors.initials: MF
authors.lastname: Wucherer
mesh_terms: D000819:Animal Communication; D000818:Animals; D037421:Anthozoa; D005399:Fishes / Q000502:physiology*; D005453:Fluorescence; D006147:Guanine / Q000737:chemistry; D008027:Light; D008855:Microscopy, Electron, Scanning; D008856:Microscopy, Fluorescence; D010781:Photography; D012160:Retina / Q000502:physiology; D013045:Species Specificity; D013050:Spectrometry, Fluorescence
chemical_list: D006147:Guanine
references.citation: J Exp Biol. 2006 Jun;209(Pt 11):2034-41
references.pmid: 16709906
references.citation: Vis Neurosci. 2007 May-Jun;24(3):345-54
references.pmid: 17822575
references.citation: J Exp Biol. 2003 Oct;206(Pt 20):3607-13
references.pmid: 12966052
references.citation: J Mol Evol. 2006 Mar;62(3):332-9
references.pmid: 16474984
references.citation: Science. 2002 Jan 4;295(5552):92
references.pmid: 11778040
references.citation: Proc Natl Acad Sci U S A. 1933 Jul;19(7):750-4
references.pmid: 16577560
references.citation: Philos Trans R Soc Lond B Biol Sci. 2000 Sep 29;355(1401):1269-72
references.pmid: 11079412
references.citation: J Exp Biol. 2004 Mar;207(Pt 7):1229-40
references.pmid: 14978063
references.citation: J Exp Biol. 2002 Dec;205(Pt 23):3661-7
references.pmid: 12409492
references.citation: Science. 2004 Jan 2;303(5654):51
references.pmid: 14615546
references.citation: FEBS Lett. 2000 Aug 18;479(3):127-30
references.pmid: 10981720
references.citation: Philos Trans R Soc Lond B Biol Sci. 2000 Sep 29;355(1401):1243-8
references.pmid: 11079407
references.citation: Science. 2005 Jul 8;309(5732):263
references.pmid: 16002609