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Over 90% of animals are spineless invertebrates including insects, crabs, snails, octopuses, worms, and sea stars.  Hawaiian reefs have fewer species of invertebrates than Indo-Pacific locations like Australia and Thailand; however, about 20% of Hawaiian marine invertebrates are found only in Hawai‘i.  Because of this, Hawai‘i can be called both invertebrate rich and poor.

Most invertebrates fall into the following categories: 

  • Sponges (e.g., fire sponges)
  • Cnidarians (e.g., corals)
  • Worms (e.g., flatworms)
  • Bryozoans (e.g., sea moss)
  • Mollusks (e.g., squids)
  • Crustaceans (e.g., crabs)
  • Echinoderms (e.g., sea stars)
  • Tunicates (e.g., sea squirts).


Most sponges live in shallow saltwater attached to a hard surface.  These colorful creatures filter water through pores to catch microscopic meals of plankton and organic particles.  Hawaiian sponges usually form thin encrustations in caves and crevices, under stones, and on docks. 


A cnidarian, pronounced without the “c”, has two main features: a stomach and a mouth.  Over 300 species of cnidarians, including sea anemones, jellyfishes, and corals, inhabit Hawaiian waters.  Many cnidarians are carnivores, catching small animals with stinging tentacles; others associate with algae called zooxanthellae and live off the products of photosynthesis (the process of using sunlight to make sugar.)


Worms are legless invertebrates with slender or flattened bodies.  Hawaiian snorkelers and divers may see flatworms, ribbon worms, and annelid worms.  Some worms are brilliantly colored like the fuchsia flatworm, Pseudoceros ferrugineus.


Bryozoans are best known as “fouling organisms” that grow on boat bottoms along with barnacles, tunicates, and sponges.  They have a variety of names from “sea moss” to “lace coral.” 


A mollusk is a soft, legless animal with a muscular foot and calcium carbonate shell.  With over 85,000 species, mollusks are the largest group of animals in the sea.  About 160 species of mollusks are found only in Hawai‘i.  This is a popular seafood group with members like clams, oysters, snails, and squids.


Crustaceans—crabs, lobsters, and shrimp—have jointed appendages that they use to eat, breathe, sense, and reproduce.  They have an exterior skeleton or shell made of chitin, the same material found in insect exoskeletons.  To grow, crustaceans must periodically shed their protective shells in a process called molting.  They are especially vulnerable to predators after molting while they wait for their new, soft shells to harden.

Donna Loudons


An echinoderm is an animal without a head, brain, or heart.  All echinoderms, from sea stars to sea urchins to sea cucumbers, have internal calcareous skeletons.  They may be filter-feeders, scavengers, or predators.

Donna Loudons


As larvae, tunicates have primitive backbones which disappear after maturation.  They may be colonial or solitary, attached to the sea floor (sessile) or swimming (pelagic).  They are filter-feeders, have soft bodies, and secrete a protective outer tunic.

For more information on invertebrates, visit Ocean Encyclopedia and Marine Life Profiles at Waikiki Aquarium.


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