Visual communication between the Hawaiian Cleaner Wrasse (Labroides phthirophagus) and Host Fish in Kaneohe Bay, Hawaii
Date
1996
Authors
Barry, Karen Lynne
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Abstract
The goal of this study was to identify the visual cues which attract host fish to the Hawaiian cleaner wrasse (Labroides phthirophagus) in order to understand how visual communication mediates behaviour between Hawaiian coral reef fishes . The central idea was that the cleaner wrasse has one or more optical signals which maximize its conspicuousness to other reef fish. Specifically, it was hypothesized that the colour pattern of the cleaner wrasse provides high contrast and that the swimming motion, known as the dance, is a distinctive behaviour. Since the visibility of optical signals not only depends on properties of the target but also photic conditions of the habitat and the sensory system of the recipient host fish, these factors were also studied.
To characterize the photic conditions, an underwater spectroradiometer was used to measure spectral and transmission properties of the water at three study sites in Kaneohe Bay, and reflectance properties of coral substrates (300-850nm). Spectral reflectance was measured from the colour patches of the fish and the amounts of luminance contrast (LC) and spectral contrast (SC) were calculated : I) between the colour patches within the pattern, 2) between the colour pattern and backwelling light and 3) between the colour pattern and coral background. The contrast provided by L. phthirophagus was compared to that of three other reef fish ( Chaetodon auriga, Thalassoma duperrey, Zebrasomaflavescens). The dance behaviour was evaluated as a potential optical signal by analyzing the movements which were recorded with an underwater video camera. Finally, to examine the visual ability of host fish, spectral sensitivity of T. duperrey was determined with electrophysiological recordings from the optic nerve.
The two inshore sites had mean diffuse attenuation coefficients (Ko) of 0.89 and 0.41 , while the offshore site was comparatively clear (K0 =0.12). The wavelength at which half of the photons occurred, A.p50, for downwelling light at the Sm and 15m site were 542nm and 524nm respectively, corresponding to the green region of the spectrum. The dominant wavelength at the offshore site occurred in the blue region of the spectrum (A.p50 = 485nm for downwelling light). For coral substrates, about 8 - 20% of downwelling incident light was reflected and the dominant wavelength occurred in the green part of the spectrum p.,p50 = 543 nm). Several factors were found to affect the underwater photic regime including direction, sky conditions and time of day.
The colour pattern of L. phthirophagus showed significantly greater inherent LC than the other fish, while C. auriga had greatest inherent SC. Against water and coral backgrounds, the amount of luminance and spectral contrast provided by the fish did not differ significantly between species. With respect to the dance, it was observed less frequently than expected in adult cleaner wrasses, however the body movements were distinctive and very stereotyped.
T. duperrey was found to possess at least two cone mechanisms. The ON response had a short wavelength sensitivity peak (A.max= 460nm) and a medium wavelength mechanism (A.max= 555nm). The OFF response lacked a short wavelength mechanism, but a medium wavelength mechanism (Amax= 555nm) was present. When the contrast provided by the colour pattern of L. phthirophagus was related to visual abilities of T. duperrey (by restricting the wavelength specification of the contrast calculations to correspond with the sensitivity range of T. duperrey), there was sufficient LC and SC to be visually detectable by this host. In conclusion, the role of the cleaner's dance as an optical signal is unclear, however the conspicuousness of L. phthirophagus due to luminance contrast may be an important optical signal for hosts.