Cetaceans of the Open Ocean

Whales and dolphins make up this diverse group of marine mammals, with variations in the body design ranging from beautiful to bizarre.

Common Name
Scientific Name
Diet
Size
Depth
Ecosystem/Habitat
Zone
Whales & Dolphins
Cetacea
Various prey from krill & plankton to fish and other cetaceans.
Range: 4.5 ft (1.4 m) to 82 ft (25 m)
Range: 0 - 9,874 feet (2,992 m)
Cetaceans inhabit the waters of the open ocean.
Pelagic Zone

Cetaceans of the Open Ocean

Cetaceans are a diverse group, with variations in the basic body design ranging from beautiful to bizarre. The elongated dorsal fin of a killer whale, the single lance-like tooth of the narwhal, right whales with their distorted head shapes. Distinct features, each useful for specific functions like hunting or communicating. Their incredible success owes itself to these seemingly perfect body structures. Similar to sharks and large fish, whales and dolphins have a streamlined body shape and possess dorsal fins, flippers that mirror pectoral fins, and huge tails. These similarities meant that cetaceans were once known as ‘spouting fish’, but there is a clear difference in the orientation of the tail. A fish tail is vertical and moves from side to side, while a cetacean’s is horizontal and moves up and down. A feature that endures from their four-legged ancestors on land, with backbones that naturally bend up and down.

Cetaceans can be divided into two superfamilies; the Mysticeti, or baleen whales, and the Odontoceti, or toothed whales which includes all dolphins and porpoises. Members of the Mysticeti are characterised by brush-like plates of baleen. A strong material made out of keratin - the same protein that makes up our hair and nails. But in the case of whales, it is used for filtering prey out of the water column. Cavernous mouths allow them to consume vast quantities of prey, be it small schooling fish or krill.

The largest baleen whale, and indeed the largest living animal on the planet, is the blue whale. To survive, it must eat 4 tons of krill each day. A daunting task, for it may seem strange that the largest inhabitants of the ocean feed on the smallest, but blue whales are in fact well-adapted to doing so. They are a form of rorqual, or furrow whale, along with humpbacks and minkes. Species that each have a series of skin folds that run along their underside, allowing the mouth to expand immensely in the process of lunge feeding.

Their large size is useful for another reason, for baleen whales infamously set out on seasonal migrations spanning entire oceans. For example, humpback whales can travel more than 10,000 km every year between the poles and the tropics. By growing to such huge sizes, they become more efficient by losing less energy, with large fat reserves sustaining them on these journeys.

Members of the Odontoceti superfamily are characterised by the presence of teeth in contrast to baleen whales. Ocean dolphins make up the largest proportion of this, as well as being the most diverse family of cetaceans. Famous for their acrobatic displays, spinner dolphins leap 10 feet high out of the water to shake off parasites, as well as to communicate. Their complex social behaviour allows many dolphins to gather in schools of immense sizes, travelling together in search of food.

When it comes to catching food, different cetaceans have mastered a number of unique strategies. Bottlenose dolphins sometimes ‘strand feed’, intentionally beaching themselves to collect fish they have herded and trapped against the shore. What’s fascinating, is that fact that different populations of this species have adopted very different methods of catching prey. One method, seen almost exclusively in Florida, involves bottlenose dolphins stirring up ring-shaped plumes of mud with their tails to corral fish into an ever-tightening circle. The frightened fish then jump out of the water into the waiting mouths of dolphins.

A similar technique has been observed in a few pods of humpback whales in Alaska, which have adopted a method known as bubble-net feeding. Rather than creating rings of mud like the dolphins, these whales instead make the most of their two blowholes. As a group circles a school of small fish or krill, they work as a team to corral the prey into a net of bubbles. One whale dives down and begins to exhale through the blowhole. Others follow, blowing out while circling the prey, in the process creating a net of bubbles to surround the fish and keep them from escaping. One whale sounds a feeding call, commanding the group to rise simultaneously with mouths agape to feed on the trapped fish.

This behaviour is unique to only a few populations because it not instinctual. It is learned, and passed on through generations of humpbacks. Some whales go to greater lengths, or rather greater depths, to find food. The deepest a human has been known to dive is 213 metres. In comparison, the deepest diving mammal, the Cuvier’s Beaked Whale, frequently journeys 2,990 metres or 9.815 feet down on the hunt for squid. The sperm whale comes in at 2nd place, reaching 2,250 metres or 7,380 feet.

To achieve such a feat, these whales have developed some incredible diving adaptations. When diving, the so-called ‘mammalian diving reflex’ conserved oxygen and limits nitrogen uptake into the blood. In this reflex, circulation to the muscles and skin is dramatically reduced, allowing oxygen to be channeled to organs that need it most. The heart and the brain. As they dive, the sperm whales click continuously in a specific sequence, waiting for an echo which reverberates in a fatty sac beneath the mouth. Vocalisations like this play a key role in the life of a cetacean.

Watch the video above to hear the sounds of some cetaceans. Their calls can vary. That of the blue whale is a low, droning song. The lowest frequency sound a human can hear is 20 Hz, but blue whales make calls between 10 and 40 Hz.

The fin whale, however, makes loud pulses, remarkably similar to the sonar of ships.

Humpbacks are known for their long and complex song, repeating a pattern of low notes that vary in amplitude and frequency but in consistent patterns over a period of hours or sometimes days. Its purpose is possibly for attracting females, though there is some debate as to whether this is true.

While the vocalisations of baleen whales tend to be low frequency and loud, the calls of toothed whales are often at far higher frequencies. This allows them to become masters of echolocation. Orcas and other dolphins produce short broad-spectrum burst-pulses that sound to us like "clicks." These are reflected from objects of interest to the whale and provide information on where to find food.

In the case of killer whales, the food they’re searching for can range from fish, to seals and other cetaceans. Transient orcas have been observed bringing down small whales, using group hunting techniques to drown young whales by covering their blowhole and keeping them under the water. But surprisingly, they don’t always attack whales for food. It can sometimes simply be for fun.

In such occurrences as the death of a whale, something quite remarkable happens. The corpse sinks ever-downwards until it rests upon the sea-floor. And here, it brings new life to the often starved community of the deep sea. Scavengers emerge out of the darkness to strip the flesh from the bones. These events, known as whale-falls, provide a concentrated supply of nutrients, and can support a complex community of deep sea creatures for up to 50 years.

Living whales are equally important in the marine ecosystem. They engineer habitats by a simple mechanism known as the whale pump. They feed at varying depths, but tend to defecate at the surface. These gigantic clouds of excrement provide a boost of iron and nitrogen to the marine food chain and boosting biodiversity.

Overall, cetaceans are highly intelligent, well-adapted marine mammals. Their complex social behaviour and intelligence makes them formidable hunters, while the ability of cetaceans to dive to great depths on a single lungful of air show just how well they are adapted to life in the oceans. They are ecosystem engineers, with the whale pump nourishing the open ocean ecosystem in life, and their carrion supporting complex communities on the sea floor after death.

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