Seastars are members of the Phylum Echinodermata, meaning “spiny skin.” All Echinoderms are radially symmetrical, meaning their internal organs are repeated in each “arm” or equivalent section of their bodies. True seastars are members of the Class Asteroidea, while brittle stars (which look a lot like true seastars) are members of the Class Ophiuroidea. The main difference is that seastars’ organs extend into their arms, while brittle stars’ organs are all contained in their central disc. This diagram shows the diversity of forms and sizes exhibited by true seastars. Clockwise from top: ochre star (Pisaster ochraceous), blood star (Henricia sp.), sunflower star (Pycnopodia helianthoides), slime star (Pteraster tesselatus), and the six-armed star (Leptasterias sp.).

Illustration created for publication in The Ocean’s Menagerie (2025) by Drew Harvell.

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