Red to White
The synchronised moult that occurs prior to migration results in a colour change from the lobsters’ usual red shell, to a pale pink shell. It is unknown why this moult occurs, however, there are obvious ecological advantages. Being a lighter colour while moving long distances over open (light coloured) sand would prove advantageous in helping to camouflage from predators.
Studies investigating what causes the shell to change colour have found that neither diet nor background substrate colour are able to initiate or prevent this event (Wade et al. 2008). Instead, it would appear that this moult is part of the animal’s circadian rhythm, an internal clock governing a specific moult at a certain life stage or age. Complicating this assumption, however, is the fact that lobsters can undergo multiple white moults over several years. Presumably, a certain age triggers the onset of seasonal white moulting, and then the onset of sexual maturity turns it off. More research would be required to confirm this.
Unique to the Western Rock Lobster
Colour change is common among decapod crustaceans, especially crabs, occurring either slowly over time or quickly in conjunction with a moult. However, colour change in other decapods usually occurs cyclically or in response to the environment, rather than at a particular life stage as it does in western rock lobster. Additionally, while migrations are common among many crustacean species, none have been associated with a corresponding colour change. This colour changing moult, at a determined life stage, and in conjunction with a migration, appears to be unique to the western rock lobster.
White to Red
Once the whites complete their migration, around January/February, they moult again, returning to their deep red colour. Anecdotal evidence from fishers indicates that by February most of the catch is composed of “red” or “coral” (an intermittent colour that occurs between white and red) lobsters. In contrast to the colour change from red to white, which occurs within the single moult, this colour change occurs both gradually, in the lead up to the moult, as well as instantaneously over the moult (Melville-Smith et al. 2003). The gradual change generally takes the lobster from “white” to “coral”, while the moult moves them closer to their normal dark red (Melville-Smith et al. 2003).
Background substrate has been shown to influence this colour change. Animals kept on a white background substrate, did not darken as much in the lead up to the moult as those housed on a dark substrate (Melville-Smith et al. 2003), and in some instances a white substrate actually caused lobsters to become lighter after the white to red moult, not darker as is seen in the wild (Wade et al. 2008). Therefore, the gradual change in shell colour that occurs before moulting may be triggered by a change in substrate background colour, from light; while travelling over sand during migration, to dark; once settled in the deep-water reef habitats. Again, while diet can bring about some changes in shell colour, it does not appear to be a driver of this colour change (Melville-Smith et al. 2003, Wade et al. 2008).