Cetacea is a suborder of aquatic mammals that includes whales, dolphins, and porpoises. Their fully aquatic lifestyle, streamlined body shape, often large size, and carnivorous diet set them apart. Cetaceans face a variety of environmental threats, such as underwater noise pollution, entanglement in abandoned ropes and nets, ship collisions, plastic and heavy metal buildup, and accelerating climate change, However, the degree to which they are impacted varies greatly between species. Instead of teeth, baleen whales have keratin bristles that filter krill and other small invertebrates from seawater. Mollusks that live on the seafloor are eaten by grey whales. Throat pleats are used by the Rorqual family to expand their mouths and sieve out water. Cetacean bodies are generally similar to those of fish due to their lifestyle and habitat conditions. Despite having some similarities with other higher mammals, their bodies are well adapted to their environment. The nostrils are located above the eyes on the top of the head, allowing the rest of the body to remain submerged while surfacing for air. The back of the skull has been significantly shortened and deformed. While the vast majority of cetaceans live in the sea, a small subset of them only live in brackish or fresh water.