Many deep-sea animals, such as corals and sponges, are sessile, meaning they spend most of their lives permanently attached to rocks. As opposed to a flat seafloor, seamounts rise off the ocean bottom and interact with water flowing around them. These water currents can wash off sediment on a seamount, exposing rocks that are ideal habitat for animals that require hard substrate to grow and attach.
Also, because these animals cannot move around to find food, they are dependent upon ocean currents to bring their food to them. The strong currents running over a seamount can deliver animals living along its flanks with a constant supply of planktonic food.
These currents also produce localized upwelling of water around the seamount. Nutrients like nitrates and phosphates, which are critical to the growth of phytoplankton, are lifted from the deep to the sunlit surface waters. These nutrients fuel an explosion of planktonic plant and animal growth and attract larger animals such as whales, sharks, tunas, and seabirds to a veritable feast.
Additionally, those same currents carry larvae, like shipwrecked mariners, from various geographically distant areas. Seamounts thus provide a place for these lost larvae to settle and grow, so animals that may normally be not found together can be found on seamounts living side by side.
All of these factors mean that at seamounts, you’ll likely find high biodiversity, or a wide variety of life.