Sea urchins must be good to eat. Why else would they need the hard and/or sharp spines if not to defend against predators? Like their starfish relatives, they are omnivorous scavengers and have tube-feet to take them in search of plant and animal material to browse on. They eat a lot! Dense populations of some species are thought be a serious threat to seagrass and/or kelp beds.
As Anneliese Rosenmayer’s above pic shows, dead sea urchins have often lost their spines, probably because they’ve been smashed about by wave action. The pic also captures the wide range of colours that can be found in Heliocidaris erythrogramma (purple, grey, green and cream). The Baykeeper shell surveys have found Heliocidaris sp. in highly urban (degraded?) locations at Gem Pier in Williamstown and Cunningham Pier in Geelong.
The number of shell species in these locations was found to be quite low compared to most other areas of the Bay.
The Victorian Field Naturalists (Coastal Invertebrates of Victoria, 2006) recorded H. erythrogramma in more locations than other urchin species and at 14 widespread locations in PP Bay.
This suggests it tolerates a wide range of conditions. Their thick, 25mm spines may help them cope with strong wave action in rocky sites. Species with short, fine, spines generally occur in more protected areas. Baykeeper surveys have found many more sea urchins in the west of the Bay than in the east.