Fig 1.
Photos of the floating macroalgae Sargassum encountered during this study in the Sargasso Sea, south of Bermuda.
Individual aggregations can be extensive and densely packed (A) or more diffuse (B). Densely packed mats can be solid and partially emergent from the water column (C). The algae may float at several different depths and extend downwards from the surface by tens of centimeters, as depicted with a sample in a small tank (D).
Fig 2.
Photos of representative Sargassum crabs from two different species encountered in the Sargasso Sea.
Portunus sayi display considerable variation in pattern from more uniform and pale (A, small adult male) to highly mottled (B, large adult female). Some aspects of patterning, such as the central dorsal white spot and m-shaped saddle marking, appeared in all adult individuals. The smaller species Planes minutus tended to have a more uniform coloration across the carapace (C, adult female). Some individuals had one or more white patches on the dorsal surface which ranged from small to covering the entire carapace and appendages. Several dark red P. minutus (D, adult male) were found drifting on a plastic bucket.
Fig 3.
Sargassum crabs on algae in a small tank.
Individual crabs (circled in red) did not show any preferred orientations relative to algae during short-term measurements in shipboard aquaria. Note the presence of a small frog fish, Histrio histrio, in the lower left (yellow circle).
Fig 4.
Reflectance of P. sayi pattern elements and subjectively corresponding habitat features.
Reflectance R(λ) of the white, brown, and yellow areas on a randomly selected P. sayi carapace very closely matched the reflectance of calcareous epibionts, senescent brown, and healthy yellow Sargassum. Spectra of yellow and white areas on P. minutus showed similar correspondence. As with carapace and algae mean reflectance, maximum divergence appears in the far red.
Fig 5.
Mean and standard deviation of the measured reflectance spectra, R(λ), for a single randomly selected individual and its background obtained from a hyperspectral image of A) P. sayi and B) P. minutus.
The greatest difference for both species appears around the secondary chlorophyll absorption dip (675 nm) present in Sargassum reflectance, but not observed for the crabs. Variation in R(λ) across the carapace is lowest in the blue and increases towards the red.
Fig 6.
Reflectance R(λ) of individual crabs and associated backgrounds determined from hyperspectral imagery collected from the Sargasso Sea.
Left panels show the reflectance spectra of individual crabs. Middle panels show spectra for the associated background (Sargassum or bucket). Right panels present the difference between the left (crab) and middle (background) panels. Error bars for each spectrum have been omitted for clarity, but were similar to that for the example individuals. Black dashed lines in panels (D, F) represent P. minutus with large white markings and higher reflectance than other crabs of both species. Individual P. minutus collected from a floating red bucket (G) all had substantially lower reflectance than crabs on natural algae. The greatest difference between crabs and algae was centered around the secondary absorption band of Chlorophyll a (675 nm) where a dip was found in the Sargassum reflectance spectrum.
Fig 7.
Normalized visual pigment sensitivities for dichromat fish mahi mahi (A), Coryphaena hippurus (Munz and McFarland 1979), and tetrachromat bird wedge-tailed shearwater (B), Puffinus pacificus (Hart 2004).
C. hippurus is most sensitive to light in the blue and green wavelengths, while P. pacificus is also sensitive to red wavelengths. Sensitivity spectra (limited to 400–700 nm) are superimposed on sample R(λ) for crab and algae.
Fig 8.
Crab chromatic contrast (ΔS) for initial model conditions when viewed by fish (A) and bird (B) predator models.
Units are Just Noticeable Differences (JNDs), and values less than 1 (dashed line) indicate that the organism is not distinguishable from its background. Contrasts for the fish model were mostly below 1, while values for the bird were significantly higher.
Fig 9.
Chromatic contrast (ΔS) plotted against carapace width (CW) for each species under fish (left panels) and bird (right panels) visual models.
A linear regression model was fit to each group. ΔS showed a strong, significant (p = 0.05) negative correlation to CW for both models in P. sayi (A, B). For P.minutus (C, D), significant positive correlation to CW was observed for individuals without large white patches (circles). This correlation was not observed for individuals with white patches (squares), which have been superimposed on the figure and were not used in calculating the regression. Dotted lines represent 95% confidence interval.
Fig 10.
Achromatic contrast (ΔSAC) for initial model conditions when viewed by fish (A) and bird (B) predator models.
Units are Just Noticeable Differences (JNDs), and values less than 1 (dashed line) indicate that the organism is not distinguishable from its background. Achromatic contrast for both crab species were not significantly different (p = 0.05) for either predator model. All individuals were above the threshold value of 1 for both predators.
Fig 11.
Impact of bird model photoreceptor ratios and water column attenuation on chromatic contrast (ΔS).
Bars represent median contrast for each crab group relative to ΔS for the initial condition. Dashed line represents a ratio of 1 (no change). For bird photoreceptor ratios (A), the unity ratio achieved the lowest chromatic contrast. Contrast increased with increasing proportion of red-sensitive photoreceptor types. Chromatic contrast at 1 and 5 m distance in a low (Bermuda/Bmd) and high (Florida/Fld) attenuation water columns were compared to null attenuation (T = 1, Eq 1). Simulating attenuation of light with distance generally decreased contrast values for both fish (B) and bird (C) predators, though this was highly variable on an individual basis with some individuals becoming more distinguishable (> 1). Outliers (+) were observed for both crab species. Decrease in chromatic contrast was most pronounced for the highest attenuation condition (Fld, 5 m).
Fig 12.
Impact of water column attenuation on achromatic contrast (ΔSAC) to fish (A) and bird (B) models.
Bars represent median contrast for each crab group relative to ΔSAC for the initial condition. Dashed line represents a ratio of 1 (no change). Decrease in achromatic contrast was generally low, except for the avian predator under the 5 m conditions, where median ΔSAC of both species decreased by 15% and up to 50% for some individuals. Outliers (+) were observed for both crab species.