Crown-of-Thorns Sea Star.
A coral-eating sea star covered in venomous spines. Population outbreaks can devastate reefs across the Indo-Pacific.
The crown-of-thorns sea star (COTS) is one of the largest sea stars in the world, sometimes exceeding 60 cm across, with up to 21 arms and a body covered in long, venomous spines. It feeds almost exclusively on living coral by everting its stomach over a colony and digesting the tissue externally.
In healthy numbers, COTS are part of a balanced reef. But periodic population explosions, driven by a mix of nutrient runoff, overfishing of predators and natural cycles, can strip vast stretches of coral. Major outbreaks have devastated reefs across the Great Barrier Reef and parts of the Pacific.
Hawaiʻi has so far avoided large-scale COTS outbreaks, but isolated population spikes have been documented. Reef monitoring programs track them carefully, and divers should never touch one — the venomous spines cause intense pain and infection.
More species in this group.
Day Octopus
A daytime hunter and master shapeshifter, Hawaiʻi's most commonly seen octopus can change color and texture in under a second.
Chambered Nautilus
A living fossil whose ancestors swam alongside dinosaurs, the nautilus drifts through deep reefs in a perfect spiral shell.
Hawaiian Bobtail Squid
A thumb-sized squid that hides in plain sight using bioluminescent bacteria to erase its own shadow under the moon.
Textile Cone Snail
A beautiful but dangerous predator that fires a venomous harpoon to paralyze fish — and whose toxins inspire modern medicine.