Abstract
Mats of the pelagic macroalgae Sargassum represent a complex environment for the study of marine camouflage at the air-sea interface. Endemic organisms have convergently evolved similar colors and patterns, but quantitative assessments of camouflage strategies are lacking. Here, spectral camouflage of two crab species (Portunus sayi and Planes minutus) was assessed using hyperspectral imagery (HSI). Crabs matched Sargassum reflectance across blue and green wavelengths (400–550 nm) and diverged at longer wavelengths. Maximum discrepancy was observed in the far-red (i.e., 675 nm) where Chlorophyll a absorption occurred in Sargassum and not the crabs. In a quantum catch color model, both crabs showed effective color matching against blue/green sensitive dichromat fish, but were still discernible to tetrachromat bird predators that have visual sensitivity to far red wavelengths. The two species showed opposing trends in background matching with relation to body size. Variation in model parameters revealed that discrimination of crab and background was impacted by distance from the predator, and the ratio of cone cell types for bird predators. This is one of the first studies to detail background color matching in this unique, challenging ecosystem at the air-sea interface.
Highlights
The need to hide is a potent evolutionary driver [1]
One of the central topics of camouflage is color—Does an animal match the color of its background? The concept of general background color matching has been integral to the study of camouflage throughout its history, and this strategy is common in the marine environment [1, 4,5,6,7,8,9,10,11]
Tetrachromat birds, by contrast, were able to discriminate between the color of Sargassum crabs and algae because of photoreceptors at far red wavelengths, corresponding to the chlorophyll absorption maximum
Summary
The need to hide is a potent evolutionary driver [1]. Biological camouflage, the ability to avoid detection or recognition by an observer, has evolved in multiple phyla as an adaptation to visually-orienting predators and prey. The study of marine camouflage systems has relevance to ecology and animal behavior as well as human naval operations, and to target discrimination in the water column [2, 3]. A wide variety of strategies exist, and the classification of camouflage techniques is a complex field [4]. One of the central topics of camouflage is color—Does an animal match the color of its background? The concept of general background color matching has been integral to the study of camouflage throughout its history, and this strategy is common in the marine environment [1, 4,5,6,7,8,9,10,11]. One of the central topics of camouflage is color—Does an animal match the color of its background? The concept of general background color matching has been integral to the study of camouflage throughout its history, and this strategy is common in the marine environment [1, 4,5,6,7,8,9,10,11].
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