SEARCHIN’ FOR URCHINS
Urchins @ Cheetham Wetlands
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.
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Quite a few of my cousins have news on what eats urchins. Not all of their reports relate to Port Phillip Bay, but they give good clues to urchin life.
All those spines. What’s the point? A sea urchin’s spiky outer shell would put a lot of potential predators off, but apparently some can handle the challenge. But first of all, you have to know where to look. Urchins mostly eat plant material, so a predator searchin’ for an urchin will head for the nearest seagrass bed or kelp forest. Predators include: people, cuttlefish, octopus and squid, fish (e.g. large trigger fish and wrasses), rakali, rock lobsters (Jasus edwardsii), and possibly 11 arm seastars.
Tasmanian studies of caged sea urchins found that juveniles were more likely to be consumed by rock lobsters, but only large lobsters could take large urchins. On the other hand, studies of free roaming urchins found larger urchins were more likely to be taken by predators, probably because the younger/smaller urchins have more places to hide.