Long-spined Sea Urchin

Introduction

The expedition journal records details of their rather unusual acquisition, demonstrating the unexpected events which can happen during fieldwork:

'Sunday October 14th, 1962. A long spined urchin Diadema was observed with a flock of little reddish-brown fishes living among the spines. Some of these were collected by throwing the urchin into the air, leaving the fish behind - these being caught in a plastic bag. The remaining fishes, being bereft of their host, looked around for a dark place to hide and quickly swarmed towards Len's dark swim shorts, disappearing up the legs.'

From the unpublished journal of the Australian Museum Expedition to the Swain Reefs, October 1962.

Identification

Sea urchins are characterised by moveable spines that occur in a variety of shapes and colours.

Habitat

They are mostly encountered on rocky shores, beaches and coral reefs.

Distribution

The Long-spined Sea Urchin, Diadema savignyi, is common throughout tropical areas of the Indo-Pacific Ocean from the shoreline to depths of 70 m. In Australia it occurs in a north-eastern arc from southern Western Australia to Sydney.

Feeding and diet

Sea urchins generally feed by grazing on plant material growing on surfaces, or by burrowing in sediments to find decaying material. They can reach high population densities and, through their feeding activities, can be key organisms in determining the community composition and appearance of marine ecosystems.

Other behaviours and adaptations

The spines of this species grow to approximately 20 cm long and contain irritant toxins. The spines may also harbour small fish, such as the Striped Siphonfish Siphamia majimai, which live in association with the sea urchin.

Sea urchins are relatives of starfishes and belong to a group of animals called echinoderms. There are approximately 800 described sea urchin species worldwide, over 200 of which are recorded from Australia.

The Australian Museum collection contains representatives of over 300 sea urchin species and the most recent catalogue of Australian echinoderms, including sea urchins, was compiled by former Australian Museum staff (Rowe and Gates) in 1995.