Marine invertebrates are sea creatures without backbones. Actually, they have no bones at all. Instead, their bodies are supported by a variety of mechanisms from exoskeletons, shells, spicules, or even just water.
Imagine all the animals you have seen that live primarily in the ocean. Now discard all the fish (16,475 described species), birds (452 described species, give or take), dolphins and porpoises (45 described species), whales (40 described species), seals and sea lions (33 described species), and sea turtles (7 described species). What else is there, some might ask? In fact, the answer is … almost everything!
It turns out that from a sheer numbers game, whether you count the number of species (over 200,000 described species and counting), or you actually count the number of individuals making up those species (bajillions), marine invertebrates have those of us with backbones beat hands down. And it’s not even close.