Beginner 8 minutes
Marine invertebrates: the ocean's silent majority
Mollusks, crustaceans, echinoderms, cnidarians, and sponges: five of the sea's major groups of invertebrates
A diver comes back from a dive and talks about seeing sharks, turtles, and a school of barracuda. They almost never mention that, on that same dive, they swam right over several thousand animals from dozens of different species: invertebrates, which make up the vast majority of the ocean's animal life.
In this article you'll go through the major groups of marine invertebrates (mollusks, crustaceans, echinoderms, cnidarians, worms, and sponges) with the keys to recognizing which group whatever you're looking at belongs to, and what makes each one unique.
Learning to see invertebrates completely changes what a dive has to offer: it's the difference between 'there wasn't much there' and discovering that one square meter of reef held more life than the rest of the dive combined.
Mollusks: the ocean's most versatile group
The mollusk phylum brings together animals so different from one another that it's hard to believe they're related. Cephalopods (octopuses, squid, and cuttlefish) are the most striking: they have the most complex nervous system of any invertebrate and an unmatched capacity for camouflage, thanks to skin cells called chromatophores that can expand and contract to change color in under a second. An octopus doesn't just change color, it also changes its skin texture to mimic sand, rock, or coral.
Gastropods include marine snails and nudibranchs, those impossibly colorful sea slugs that are one of macro photography's biggest draws. In many cases, those bold colors aren't decorative: they're a warning that the animal is toxic or foul-tasting.
Bivalves (clams, oysters, mussels, scallops) live inside two hinged shells and feed by filtering water. Although they may seem the least interesting of the group, they play a huge role in water quality: a single adult bivalve can filter dozens of liters a day.
Keep reading with full access
This article is part of the full Encyclopedia. A donation from USD 10 unlocks every article, every species photo and the downloadable app, and helps sustain the Scuba Sign Paperless Diving Community project.