They are similar in adaptation to aquatic life, but they can be divided into three groups that differ greatly in origin and life patterns. Sirens, manatees, and dugongs are slow-moving herbivores found in warm, shallow coastal habitats. Pinnipeds, seals, sea lions, and walruses give birth on land but spend most of their time at sea. Cetaceans, dolphins, porpoises, and cetaceans that spend their lives in the ocean include both toothed species, which are carnivorous predators, and baleen whales, which feed on large amounts of small plankton, filter feeding. Marine mammals range in size from tiny seals and porpoises to the largest animals that ever lived, the blue whale. Many whales and pinnipeds dive to astonishing depths to forage, something that was only recently discovered using digital tags that record an animal's movement, direction, and depth over time. Hunting pressure has decreased, but marine mammals are still low in numbers and suffer from unintended human activities such as certain fishing methods, boat and ship traffic, and increased sea noise. The WHOI Marine Mammal Center (MMC) sponsors research on behaviour, health, anatomy, cognition, and more in whales, dolphins, and seals. How to release a tangled whale and what causes strandings. Relationships between marine mammal populations and sea conditions and plankton abundance. Captured to near extinction, there are now only 300-400 individuals. Marine mammals are aquatic mammals that depend on the sea and other marine ecosystems for their existence. These include animals such as seals, whales, manatees, sea otters, and polar bears. They are an informal group, united only by their dependence on the marine environment for food and survival. Adaptations of marine mammals to aquatic lifestyles vary greatly between species. Both whales and sirens are absolutely aquatic, as they are both completely aquatic. Seals and sea lions are semi-aquatic. They spend most of their time in the water but must return to land for important activities such as mating, breeding, and molting. In contrast, otters and polar bears are poorly adapted to life in water. Some eat zooplankton, some eat fish, squid, crustaceans, and seaweed, and some eat other mammals. Although marine mammals are few in number compared to terrestrial mammals, they play a major role in various ecosystems and are particularly involved in the conservation of marine ecosystems, such as regulating the population of prey organisms. Given that 23% of marine mammals are currently threatened with extinction, this role in sustaining ecosystems is of particular concern. Marine mammals were the first to be hunted by indigenous peoples for food and other resources. Many have also been targeted by commercial industries, with precipitous declines in all populations of exploited species such as whales and seals. Commercial hunting has exterminated giant manatees, sea ore, Japanese sea lions, and Caribbean monk seals. After commercial hunting ended, some species, such as the gray whale and northern elephant seal, have rebounded in numbers; conversely, other species, such as the North Atlantic right whale, are critically endangered. Marine mammals form a diverse group of 129 species that depend on the ocean for their existence. They are an informal group, united only by their dependence on the marine environment for foraging. Marine mammals are widespread worldwide, but their distribution is sparse, consistent with ocean productivity. Species richness peaks at about 40° latitude in both the north and south. This corresponds to the highest levels of primary production in the Americas, Africa, Asia, and Australia. The total species spectrum of marine mammals is highly diverse. On average, the range of most marine mammals is less than one-fifth the size of the Indian Ocean. The observed range-size variability is a result of species-specific ecological requirements and their ability to cope with varying environmental conditions. The high degree of overlap between marine mammal biodiversity and areas of human impact on the environment is of concern.