The tell-tale signs of a ‘herd’ of heart urchins

You are probably familiar with at least one type of sea urchin (hyperlink:  They are round-ish and covered in spines of various shapes and colours. Many can be found on coral reefs and others dig themselves into the sand, dirt or mud of the sea floor.

But did you know that sea urchins of certain species can be found in large groups called ‘herds’? And what tell-tale signs underwater would a herd of sea urchins leave behind to give you a clue that they are nearby?

A few days ago, scientists from AIMS, CSIRO and the WA Museum got to find out!

What were the signs left by the herd?

We were using the AIMS towed video system to film and take photographs of the sea floor (see previous blog Exploring the Sea Floor with Towed Video’ ). Checking the data afterwards, we could see a distinctive pattern in the mud on the sea floor (see below) which was present in almost every one of 250+ photos taken along each of three separate 1.5 km transects.

Distinctive patterns evident in photographs taken by the AIMS towed video system

Examining the photos closely with John Keesing of CSIRO, we realised that the holes in the sea floor were burrows very likely excavated by the pink creatures nestled inside them (see below). From the photos and some research, John has confirmed that these creatures are heart urchins, specifically ‘petalled heart urchins’ (Metalia dicrana).

Close-up of still photograph showing individual burrows with the urchins that created them nestled inside.

Urchins that live on coral reefs have heavy jaws to help them eat seaweed which is tough to chew. Heart urchins live in mud or sand and eat little bites of much softer food. In John’s experience, heart urchins are often found either on the surface or deep inside burrows where they can’t be seen. This was the first time he observed them to sit in shallow burrows – perhaps using the burrows to collect small food particles to eat. In the Indian Ocean, these urchins have previously been observed at the Rowley Shoals (17.33 degrees South, 119.33 degrees East) and Ashmore Reef (12.25 degrees South, 123.14 degrees East).

How could we be sure it was a heart urchin herd?

Our cruise leader, Karen, decided we should collect bottom samples from the area where we saw evidence of the heart urchin herd using the sled (see below).

Clockwise from top left: RV Solander crew secure the sled back on deck, WA Museum scientists search for creatures in the very muddy sample from the sled, some of the many heart urchins that they found wait to be photographed by CSIRO scientist John Keesing.

As expected, we found many urchins in the very muddy sample, and John confirmed that they are heart urchins.

How wide ranging are the heart urchin herds?

So far, we have found evidence of extensive tracts of heart urchin burrows in three locations: two on the western side of the Eclipse Islands (see map below), and one further to the west in the Troughton Passage between Troughton Island and the Eclipse Islands. We did not find many burrows in surveyed locations further west than this, at Troughton Island or along the Bougainville Peninsula, nor further north towards East Holothuria Reef (off the map to the north-west).

Locations where we found extensive tracts of heart urchin burrows near the Eclipse Islands, north-western Australia, 18.85 South, 126.33 East.

Later in our expedition, we will explore the eastern side of the Eclipse Islands and in the north-eastern section of Troughton Passage (further north than the top of the map above).

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