Some evolutionary and ecological implications of colour variation in the sea urchin Heliocidaris erythrogramma

An investigation into the evolutionary and ecological implications of variation in the external colouration of the sea urchin Hefiocidaris erythrogramma was made. Two different pigment systems create a complex polymorphism; red granules of echlnochrome A in the dermis occur in varying densities, and purple and green naphthoquinone pigments are found in the calcareous test and spines. Many morphs may occur within one population, but the proportions of morphs vary markedly between sites. Evidence from the observed variability and chemistry of the pigments strongly indicates that the variation has a genetic basis. Breeding studies which would have resolved this question were unsuccessful, but did show that all crosses between morphs developed and metamorph osed successfully. Repeated sampling of 15 sites showed that morph proportions were stable at most sites ove r the 35 months of the study. Geographic variationin the proportions of morphs was determined from samples from 49 sites. Environmental variables were recorded and the exposure of each site to wave action was estimated using algal communities to develop an Algal Exposure Index (A.E.I.). Stepwise linear regressi on analysis indicated that the A.E.I. and amount of algal cover were the only environmental factors noted that were useful predictors dermis colour proportions. Five hypotheses were developed (two selective and three stochastic) of processes which might be affecting morph proportions in the study area; these were tested using Mantel's non-para metric test. The results suggest that fou r geograph ical regions each ·have different patterns of morph distribution which are controlled by unique combinations of selection (related to exposure) and gene flow. These results are generally supported by what is known of water currents in each region, as most gene flow in H. erythrogramma will occur due t o movement of pelagic larvae. Morphological data showed slight differences between urchins of different dermis colour at one site, but no differences between urchins with different coloured spines. Th ere were significant differences between urchins at different sites. Surveys of urchin microha bitats indicated that (I) urchins of the same derm is colour tend to occur next to each other, (2) white dermis urchins tend to occur under rocks more often than red dermis urchins, and (3) urch ins which are hidden under rocks tend to 'cover' wit h pieces of shell, algae or pebbles to a lesser extent than urchins which occur on the upper surfaces of rocks. A laboratory experi ment indicated that, although the podia (tu be feet) of red and white dermis urchins were initially of compara ble strength, red dermis urchins tended to tire more quickly. No diff erences between morphs were fo und in the time of matu ration of gonads or the size of gonads re lative to body weight.