Marine Invertebrates

Animals in the Sea

There are 11 phyla of animals on land and 28 phyla of animals in the sea. But it was not always that way in the ocean. As many as five mass extinctions, probably caused by asteroids and/or comets that struck the earth, transformed the ocean ecology in the half-billion years since the emergence of multicelled life. Life in the ocean changed from SESSILE (non-moving organisms) to MOTILE (organisms capable of movement on their own). Whole branches from the evolutionary tree were lopped off while others flourished. The greatest transformation occurred 251 million years ago causing the extinction 95 percent of species in the oceans (and most vertebrates on land). Research now suggests that it was followed by an explosion of complexity in marine life, one that has persisted ever since. More motile organisms arose which resulted in more interactions and, ultimately, more complexity.

Click here to read about the change in the complexity of ocean organisms from sessile to motile.