A whiter shade of male: Color background matching as a function of size and sex in the yellow shore crab Hemigrapsus oregonensis (Dana, 1851)

Abstract

Juveniles of the shore crab Hemigrapsus oregonensis are highly variable in color, ranging from the typical yellowishgreen of adults to pure white and myriad patterns of white mottling and other disruptive markings, but large individuals with white coloration appear to be very rare. Using image analysis to quantify the relative “whiteness” of beaches, we sampled crabs from nine locations in Washington State that varied widely in their amount of shell fragments and other light-colored material. The total proportion of white individuals in the different locations was strongly correlated to the proportion of white material on the beaches, but there was a striking difference between sexes. Although white specimens of both sexes declined significantly at sizes above 10 mm carapace width, white females generally persisted throughout the entire female size range on lighter-colored beaches while white males larger than 10 mm were virtually absent from all of the sampled populations. Pure white males held on dark backgrounds in captivity remained white, as they lack the dark chromatophores in their hypodermis needed to change color; off-white males became darker and in some cases lightened up again when transferred back to a white background. Behavioral differences between the sexes may result in differential mortality of white individuals by visual predators.

Color Change, Phenotypic Plasticity, and Camouflage

“…First, there is increasing evidence that juveniles can undergo some changes in brightness that may improve camouflage (Powell, 1962b;Stevens et al, 2014a). In addition, they very likely undergo substantial changes in appearance through phenotypic plasticity as they molt (Todd et al, 2006;Stevens et al, 2014b;Jensen and Egnotovich, 2015; Figure 2). Indeed, signaling patterns in fiddler crabs can undergo substantial changes between molts (Detto et al, 2008).…”

“…One of the advantages of phenotypic plasticity is that it can enable animals to change their appearance depending on the habitat where they live, or even to resemble specific background types/microhabitats (Keeble and Gamble, 1899;Gamble and Keeble, 1900;Rosenblum, 2006;Todd et al, 2006;Stevens et al, 2014bStevens et al, , 2015Hultgren and Mittelstaedt, 2015;Jensen and Egnotovich, 2015;Russell and Dierssen, 2015;Duarte and Flores, 2016). This may be particularly valuable in species with high dispersal and planktonic larval stages, such as in many crustaceans, because there may be uncertainty as to where juveniles will settle and what the visual environment is like.…”

“…This may be particularly valuable in species with high dispersal and planktonic larval stages, such as in many crustaceans, because there may be uncertainty as to where juveniles will settle and what the visual environment is like. Another feature of many of these species is that they have high levels of intraspecific diversity in color and pattern (Figures 4, 5) even at the same locality (Keeble and Gamble, 1899;Gamble and Keeble, 1900;Todd et al, 2006;Stevens et al, 2014b;CarvalhoBatista et al, 2015;Hultgren and Mittelstaedt, 2015;Jensen and Egnotovich, 2015;Duarte and Flores, 2016). The reasons for this are as yet largely unclear.…”

“…A further largely unresolved question is why many crabs are highly variable in appearance among individuals as juveniles, but undergo ontogenetic changes as they grow (Figure 3). In many species, adults become less patterned and often darker, and apparently less cryptic (Palma and Steneck, 2001;Todd et al, 2006Todd et al, , 2009Stevens et al, 2014b;Carvalho-Batista et al, 2015;Jensen and Egnotovich, 2015;Russell and Dierssen, 2015). Two possible explanations are that older/larger crabs move to different habitat types, requiring a different type of camouflage, or that as they grow bigger they become less susceptible to predation (Todd et al, 2009).…”

“…Research on Uca has identified similar chromatophore types. However, past work on most species has generally used subjective indices of chromatophore state or qualitative assessments over a short term (Fingerman and Yamamoto, 1967;Shibley, 1968;Jensen and Egnotovich, 2015), rather than detailed histology and pigment staining and identification over different time scales (especially ontogenetic changes), and has not coupled this to quantifying the changes in overall coloration produced. In addition, what happens during molting in crustaceans has not, to my knowledge been explored.…”