top of page

204. A Radiant Sea Urchin’s ballistic motion (Astropyga radiata)


A beautiful Radiant Sea Urchin (Astropyga radiata) is on the move. These flamboyantly adorned sea urchins are native to Southeast Asia, particularly Indonesia and the Philippines, but are occasionally spotted beyond this region. The Radiant Sea Urchin prefers to traverse sandy and muddy slopes, where it is most active at night.

Their diet primarily comprises algae commonly found in shallow waters, and as a result, this vibrantly hued echinoderm is mainly found at depths shallower than 30 meters or 100 feet.

These urchins, which can occasionally be found in large aggregations, often serve as protective shelters for smaller and more vulnerable creatures. Shrimps and crabs frequently inhabit the spines, and some smaller cardinalfish have also adopted the Radiant Sea Urchin as their refuge.

The urchin featured in this underwater short video clip appears to be in a hurry. Sea urchins propel themselves using the spines on the sides of their bodies and their tube feet underneath. The spines typically move in a rowing motion. In contrast, the tube feet, liquid-filled hollow skeletons located on the underside of the animal, function differently. These tube feet are controlled through the interplay of muscular cover and internal fluid pressure. Muscles in the tube wall enable sea urchins to bend or retract their feet, while increased hydrostatic pressure is used to extend the adhesive tube feet. Thanks to these feet, which feature suction cups at their ends, the urchin can adhere to or move on vertical objects like submerged rocks.

Urchins with regular shapes can move in all directions, exhibiting a random pattern known as “Brownian motion”. This behaviour is typically observed when the animal is at rest or foraging for algae. In contrast, irregular sea urchins like sand dollars, heart urchins, and sea biscuits that can only move forward in a fixed orientation.

However, when a regularly shaped urchin like the Radiant Sea Urchin detects a potential predator, it engages in a rapid escape manoeuvre known as "ballistic motion." Ballistic motion involves swiftly moving in a nearly straight line until the threat has dissipated. Urchins rely on their sense of smell to detect potential dangers.


In this underwater video clip, it is evident that the Radiant Sea Urchin is in a hurry, most likely spooked by the presence of the underwater videographer with his substantial camera rig and submersible video lights.

To accentuate the urgency of its escape, a fast-paced musical tune, combined with the sound of an insect crawling, has been chosen. The insect crawling sound is a sound effect I often use in my video productions to highlight the movement of tiny legs and feet in underwater video clips featuring crabs and shrimps. I couldn't think of a more fitting sound to emphasise the rapid movement of a sea urchin with its numerous tube feet and sharp, erratic spines.

I also attempted to add an eerie or spooky sound to underscore the urchin's fear, but it proved to be excessive, so I decided to stick with the music and the insect crawling sound.

Filming location:

This short underwater videoclip has been filmed in Bali, Indonesia 🇮🇩

More about this subject:

To learn more about Heart Urchins and their locomotion technique please go to our vlog post 98 or click this link https://www.beyondscuba.com/post/a-burrowing-heart-urchin

Join us on a journey of discovery and entertainment as we explore fascinating marine wildlife topics and explore the many exciting underwater adventures on our YouTube channel and our Facebook page!

Bring the beauty of the ocean to life with our online Marine Wildlife Videography course! 


bottom of page