Investigating the evolutionary significance of morphological variation in the Australian sea urchin Heliocidaris erythrogramma

[Truncated abstract] Marine invertebrates often display remarkable levels of intraspecific morphological variation. This variation can indicate ecological processes, such as adaptation or phenotypic plasticity to local environments, but can also reveal important evolutionary processes driving genetic and reproductive divergence within a species. Examining highly variable species, therefore, can provide valuable insights into the steps that lead to speciation in marine environments. The Australian sea urchin Heliocidaris erythrogramma exhibits variation in colouration and spine morphology. Observations of the latter trait have been used to describe two subspecies that are broadly distributed between east and west coasts of Australia; however the variation characterising these subspecies has never been thoroughly examined, and little is otherwise known of their evolutionary relationship. This thesis aims to quantify the variation between these subspecies and investigate the processes that have contributed to their divergence. Following a brief introduction, Chapter 2 opens the thesis with an examination of morphological variation in H. erythrogramma within Western Australia. Spine morphology was measured using landmark-based geometric morphometrics, colouration (dermis and spine) was assessed qualitatively and both traits were compared to known specimens from eastern Australia. These analyses revealed two morphological groups representing each of the two subspecies, H. e. erythrogramma and H. e. armigera, co-occurring within Western Australia. Unrealised in their original descriptions, these two subspecies were differentiated by both spine morphology and colour pattern. Contrary to expectations, however, there was no consistent pattern of either trait with wave exposure, a parameter often found to correlate with such variation among echinoids. Consequently, any environmental influence(s) upon the divergent morphology between these subspecies remain unclear.