Jennifer Colvin

Briffa, Haskell, and Wilding [1] (2008) studied the hermit crab, Pagurus bernhardus, to determine its ability to distinguish between contrasts and its use of cryptic coloring to evade predation by C. maenas.

P. bernhardus and Littorina were obtained and transported back to the laboratory. P. bernhardus (hermit crabs) were removed from their shells by cracking them, and were then sexed and weighed. Only healthy males (no diseases, missing appendages) were used for the study; females were excluded because of the difference in shell selection for females based on the time of the year (for breeding). Littorina were removed from their shells in order to provide the hermit crabs with shell choices which were alike in all respects except for size and color; however, for each trial, a crab was presented with two same-size shells appropriate for them, which only differed in color. The researchers first measured hermit crab preference for shells based on color (either citrina, a light color, or dark reticulata, a dark color) by placing the crab in a tank with either light or dark sand, and both shells with openings upwards at the same distance from the crab. This was done in the presence and absence of chemical signals from a predator, C. maenas. They found that initially, regardless of the predator state, crabs would choose the shell which contrasted the greatest with the substrate they were in. After 24 hours, a significant amount of the crabs had entered a shell that contrasted less with the environment; only the presence of a predator cue indicated that the crabs would stay in the contrasting shell. Additionally, using C. Maenas, the researchers studied whether a highly or poorly contrasting Littorina shell would be more likely to be investigated by a predator. They placed two empty shells, one citrina and the other dark reticulata, on either dark or light substrate behind a divider separating them from C. maenas. They found that overall, when C. maenas was given access to the shells, it would spend a significantly longer amount of time investigating the highly contrasting shell.

The methods, results, and implications that the researchers used and found are significant to the study of visual transmission and reception by both supporting and expanding on existing knowledge in these fields. They found that crabs devoid of physical protection from shells will choose the first shell they see – the shell that contrasts most with the environment – simply to obtain shelter. This is consistent with the idea that visual reception of a signal is based on how much it contrasts with its environment. Additionally, because C. maenas was more likely to investigate shells that contrasted with the substrate, it follows that C. maenas is able to receive information based on differences in luminance. They also found that once the protection requirement was fulfilled, P. bernhardus would investigate other shells and eventually move into a lesser contrasting shell – one that would be the least likely to be perceived and investigated by predators – as long as no predator was currently present. This supports the idea that animals which are cryptically colored transmit only subtle visual signals which are less likely to be perceived by others, as discussed in lecture. However, because lecture and text discussions focused mostly on animals that were able to quickly change their color through physiological processes, this finding expands upon provided information. It shows that if cryptic coloring is a behavioral process, it will only occur if the benefits for crypsis (limited visual signal transmission and reception by predators) outweigh the costs for changing.