The primary spines on the aboral (upper) surface are short, thick and sharply-pointed, grow in rectangular areas and are clumped together in dense bundles.

[4] The secondary spines are long, flexible, slender and blunt, and mostly found near the base of the test; they are used in locomotion and righting, and have sensory cells at their tip.

[2] Asthenosoma varium is native to the Indo-Pacific region where its range extends from the eastern Indian Ocean to Japan, the Philippines and New Caledonia.

[2] Several different organisms live in association with the Pacific fire urchin; these include the commensal shrimps Periclimenes colemani and Allopontonia brockii, and the parasitic snail Leutzenia asthenosomae, as well as the zebra crab, Zebrida adamsii.

[2] Asthenosoma varium is omnivorous, feeding on the algal film that covers the substrate, as well as on encrusting invertebrates such as sponges and tunicates, and also organic detritus.