Predation is associated with variation in colour pattern, but not body shape or colour reflectance, in a rainbowfish (Melanotaenia australis)

Abstract

Summary1. In freshwater fishes, inter-population variation in male phenotype is often associated with differences in predation intensity, but these effects can be difficult to disentangle from environmental influences. 2. The western rainbowfish Melanotaenia australis exhibits marked sexual dimorphism -females are plain with a slender body, while males have striking coloration and are deeper in the body. Male traits differ in expression among populations, but this has not been described or explored in the literature. 3. This paper describes a study designed to test for geographic structuring of male phenotype in M. australis and to determine whether between-population variation in male phenotype is attributable to variation in predation regime, after accounting for environment. 4. We collected data describing habitat, and the size, activity and abundance of predators at sites containing M. australis populations. We then used photography, spectrometry and geometric morphometrics to describe colour pattern, spectral reflectance and body shape in males from these populations. Finally, we used permutation-based multivariate statistics to partition variance in these traits according to environment and predation regime. 5. Downstream environments posed higher predation risk to M. australis. Furthermore, males from these sites consistently exhibited larger cheek spots and fewer coloured lateral stripes than those from upstream sites. Variation in predation regime accounted for a significant proportion of the total variance in these traits (30AE9%), after controlling for the effects of environment. 6. Variation in predation regime did not explain variation in reflectance or shape. Environmental variation, however, explained a significant portion of the total variance in reflectance (74AE9%), and there was a strong trend towards it explaining a portion of the total variance in body shape (34AE9%). 7. We conclude that natural selection by predators may be an important determinant of the evolution of colour pattern variation in M. australis, but not of that of body shape or colour reflectance. 8. Further study of M. australis will complement existing models, which show complex relationships between predation regime, environment and phenotype. Understanding these relationships is prerequisite to predicting the evolution of phenotypic variation in natural systems.

Trade-off between camouflage and sexual dimorphism revealed by UV digital imaging: the case of Australian Mallee Dragons (Ctenophorus fordi)

“…The implemented image processing protocol is a modification of a published methodology for the study of animal patterns using image segmentation (Young et al, 2011). Prior to image segmentation, individual linearised images were calibrated in millimetres using the scale included on each picture as a reference, then sample areas were delimited using the elliptical selection tool (Fig.…”

Visual habitat geometry predicts relative morph abundance in the colour-polymorphic ornate rainbowfish

“…Spectral curves were truncated to 325-685 nm for analysis (to avoid secondary fluorescence by aquatic vegetation), representing the majority of the probable visible range of R. ornatus based on related [21,22] and ecologically similar species [23,24]. The area under the resultant curves was standardized to a nominal value of 1 [3,25], and the spectral index (lp 50 , median wavelength) was calculated for each curve as an estimate of the dominant wavelength [26].…”

Functional diversity of the lateral line system among populations of a native Australian freshwater fish

“…The western rainbowfish is endemic to the Pilbara and Kimberley regions of northwest Australia, exhibiting considerable variation in body colouration and body shape across its range (Allen et al, 2002;Lostrom et al, 2015;Young et al, 2011). It is presumed that the diet of the western rainbowfish is similar to that of the eastern rainbowfish (Melanotaenia splendida splendida), which includes macroalgae, aquatic invertebrates and terrestrial invertebrates (Pusey et al, 2004).…”

“…It is presumed that the diet of the western rainbowfish is similar to that of the eastern rainbowfish (Melanotaenia splendida splendida), which includes macroalgae, aquatic invertebrates and terrestrial invertebrates (Pusey et al, 2004). Predators of rainbowfish in the Pilbara catchments include low-risk omnivores such as spangled perch (Leiopotherapon unicolor) and high-risk predators such as Fortescue grunters (Leiopotherapon aheneus) and barred grunters (Amniataba percoides) (Young et al, 2011).…”

“…Fish were categorised as high-or low-risk predators according to the classification of predation risk to M. australis developed by Young et al (2011).…”