Current Zoology 61 (4): 729–738, 2015 Received Mar. 11, 2015; accepted May 22, 2015.  Corresponding author. E-mail: gjensen@uw.edu © 2015 Current Zoology A whiter shade of male: Color background matching as a function of size and sex in the yellow shore crab Hemigrapsus oregonensis (Dana, 1851) Gregory C. JENSEN * , Michael S. EGNOTOVICH School of Aquatic and Fishery Sciences, Box 355020, University of Washington, Seattle, WA 98195, USA Abstract Juveniles of the shore crab Hemigrapsus oregonensis are highly variable in color, ranging from the typical yellowish- green 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 [Current Zoology 61 (4): 729–738, 2015 ]. Keywords Camouflage, Color, Polymorphism, Disruptive patterns, Crabs There are a number of different ways in which the color or pattern of an organism can make them harder for predators to detect. The most straightforward of these, color matching, can be effective in homogeneous habitats but is difficult for motile individuals to achieve in heterogeneous environments due to the constantly changing background. In these circumstances two very different (but not mutually exclusive) strategies are useful: color polymorphism and disruptive patterning. Color polymorphism—color variations within a species —can be the result of pigments incorporated from the diet (Lindberg and Pearse, 1990) or due to genetic va- riability (Palmer, 1985). Such variation can make it dif- ficult for predators to establish a search image (Pietre- wicz and Kamil, 1979, Kono et al., 1998) and as a result can be maintained through apostatic selection (Bond and Kamil, 1998). Color polymorphism is widespread among inverte- brates and has been investigated in a number of differ- ent crab species, including Cancer irroratus (Palma and Steneck, 2001), C. productus (Krause-Nehring et al., 2010), Carcinus maenas (Hogarth, 1975, Todd et al., 2006), Xantho poressa (Bedini, 2002), Pugettia spp. (Hultgren and Stachowicz, 2008) and Gaetice depressus (Murakami and Wada, 2015). A general pattern is emer- ging in which juveniles of large species may exhibit a range of color variants while adults may be monochro- matic (Todd et al., 2009), corresponding with ontoge- netic changes in habitat (Hogarth, 1978) and/or adults growing too large to be susceptible to fish predators (Palma and Steneck, 2001, Krause-Nehring et al., 2010). Conversely, species that never gain a size refuge tend to retain their polymorphism throughout their life (e.g., Xantho poressa; Reuschel and Schubart, 2007, Parax- anthus barbiger; Manríquez et al., 2008). Disruptive coloration (sensu Stevens and Merilaita, 2009) also makes organisms difficult to differentiate from their background, often through the use of false edges that disguise their true shape (Cuthill et al., 2005; Stevens and Cuthill, 2006). Disruptive coloration has rarely been explicitly mentioned or differentiated from color polymorphism in the carcinological literature (Krause-Nehring et al., 2010); notable exceptions are the extensive work involving Carcinus maenas (Todd et al., 2005, 2006, 2009, Stevens et al. 2014a, b) and work by Merilaita (1998) on strikingly patterned morphs of Downloaded from https://academic.oup.com/cz/article/61/4/729/1803176 by guest on 18 December 2022