Juvenile Pycnopodia start with the standard asteroid pattern of five arms. As the animal grows, new arms are inserted between the existing ones, and large adults commonly bear between sixteen and twenty-four. The body remains soft and flexible rather than plated, which is part of why the species can move quickly across the seabed.
Speed matters because this is an active hunter. Sunflower sea stars pursue sea urchins, and their presence suppresses urchin grazing on kelp. Where the sea star has disappeared, urchin numbers in some areas have risen and kelp forest has been replaced by barren rock, though the strength of that link varies by site.
Since 2013 a wasting condition has killed sunflower sea stars across the north-east Pacific, and the species was assessed as Critically Endangered in 2020. The disease is often reported as a single confirmed virus, but the causative agent has been argued over and revised, and no single pathogen explanation is settled.