Geographic variation for colour in the sand-burrowing beetle Chaerodes trachyscelides White (Coleoptera: Tenebrionidae) on New Zealand beaches analyzed using CIELAB L values

Chaerodes trachyscelides White is a highly specialized, flightless burrowing beetle confined to the narrow strip of sand at and just above high water level on sandy marine beaches in New Zealand. Although the ventral surface of the beetle is always pale, the dorsal surface varies from pale to almost black. Large samples of this beetle were taken, together with the sand, from 11 beaches on New Zealand's three main islands. The colour of the dorsal surface of each individual beetle and that of the sand samples was measured using reflectance spectroscopy and expressed as CIE L*,a*,b* (CIELAB) values. The L* values, which are objective, quantitative measures of the degree of lightness of the beetles, were subjected to statistical and frequency analysis. Although the species was very variable in colour and the variation appeared to be continuous, a highly significant correlation was obtained between the mean of the L* values for the samples on each beach and that of the sand, the correlation coefficient being 0.961. This close association between the lightness of the beetles and that of the sand suggests the variable melanism functions as cryptic colouration. On most beaches, the distribution oflightness among the beetles sampled conformed to a normal curve. For beetles from sites where the sand was relatively uniform, such as the black Taranaki beaches, the L* frequency distribution curves were narrow and the coefficient of variation of mean beetle colour was relatively small indicating low colour variability. In contrast, the greatest within-site variability occurred on the two Stewart Island beaches sampled, where in each case there was less uniformity in the colour of the sand. At one of these sites, Maori Beach, darker sand present below the high water level is often deposited on the zone occupied by the beetles after storms. On Lonneker's Beach, the distribution of L* values among the beetles sampled was actually bimodal. On this small beach, there was an area of intensely black sand in the zone occupied by the beetles, but most of the rest was covered with light golden sand. These results are interpreted as evidence that the variability of colour of Chaerodes beetles has the effect of populations being able to match the colour of the sand of their home beaches, presumably as a consequence of the differential survival of individuals.