Improved camouflage through ontogenetic colour change confers reduced detection risk in shore crabs

Abstract

Animals from many taxa, from snakes and crabs to caterpillars and lobsters, change appearance with age, but the reasons why this occurs are rarely tested. We show the importance that ontogenetic changes in coloration have on the camouflage of the green shore crabs ( Carcinus maenas ), known for their remarkable phenotypic variation and plasticity in colour and pattern. In controlled conditions, we reared juvenile crabs of two shades, pale or dark, on two background types simulating different habitats for 10 weeks. In contrast to expectations for reversible colour change, crabs did not tune their background match to specific microhabitats, but instead, and regardless of treatment, all developed a uniform dark green phenotype. This parallels changes in shore crab appearance with age observed in the field. Next, we undertook a citizen science experiment at the Natural History Museum London, where human subjects (“predators”) searched for crabs representing natural colour variation from different habitats, simulating predator vision. In concert, crabs were not hardest to find against their original habitat, but instead, the dark green phenotype was hardest to detect against all backgrounds. The evolution of camouflage can be better understood by acknowledging that the optimal phenotype to hide from predators may change over the life history of many animals, including the utilization of a generalist camouflage strategy. A plain language summary is available for this article.

Using camouflage for conservation: colour change in juvenile European lobster

“…Ontogenetic changes are likely to override such plasticity longer term, as adult European lobsters are much less variable than juveniles. Such shifts from plastic camouflage to ontogenetic changes in coloration have been observed in other marine crustaceans (16,68). These ontogenetic shifts in coloration correspond to improved camouflage across the range of environments experienced by adults (68).…”

“…Such shifts from plastic camouflage to ontogenetic changes in coloration have been observed in other marine crustaceans (16,68). These ontogenetic shifts in coloration correspond to improved camouflage across the range of environments experienced by adults (68). Consequently, the timing of rearing individuals on different backgrounds relative to the timing of release needs careful thought in order to maximise any applied benefits of camouflage for stocking.…”

Gray plumage color is more cryptic than brown in snowy landscapes in a resident color polymorphic bird

“…The use of human observers via online “games” has been recently used in camouflage studies in birds and their eggs (Troscianko, Wilson‐Aggarwal, Griffiths, Spottiswoode, & Stevens, ), as well as crabs (Nokelainen, Maynes, Mynott, Price, & Stevens, ). Similar citizen science approach has also previously been used to study disruptive coloration with, for example, computer‐generated moth images (Fraser, Callahan, Klassen, & Sherratt, ), and detection and learning of camouflage strategies (Troscianko, Lown, Hughes, & Stevens, ) and to quantify the appearance of camouflaged prey (Troscianko, Skelhorn, & Stevens, ).…”

“…The use of human observers via online "games" has been recently used in camouflage studies in birds and their eggs (Troscianko, Wilson-Aggarwal, Griffiths, Spottiswoode, & Stevens, 2017), as well as crabs (Nokelainen, Maynes, Mynott, Price, & Stevens, 2019).…”

Imperfect camouflage: how to hide in a variable world?

“…At least observationally, there are clearly many examples of excellent and highly specialized camouflage phenotypes [3,5,7,15]. Similarly, there are cases that have been suggested as examples of generalist camouflage, including the desert spiny lizard [9], some Aegean wall lizard morphs [29], shore crabs [30,31], and some moth [10] and grasshopper [32] species. These putative generalist species often seem to have relatively drab coloration, which is often greenish-brownish to human observers and appears as if it would broadly match more than one background ( figure 3).…”