A sea pen may look like an underwater quill, but it's actually a colony of individual polyps — one polyp becomes the stalk of the sea pen and the bulb that attaches it to the sea floor, while the rest form feathery branches. Some sea pens have only a few polyps, while others have as many as 35,000.

A remora uses the sucking disk on top of its head to attach itself to larger marine animals like sharks, rays, and sea turtles — where it can stay for 3 months. The host provides the remora with protection, food, and a fast flow of water over its gills, which the remora needs in order to survive.