Under the sea: marine marvels – mammals making a splash!

The marine environment includes habitats as diverse as the shallow, intertidal coastal zones and  brackish estuarine waters, to catchments and basins, and kilometres-deep trenches in oceans that stretch from the icy poles to the equatorial tropics.

Amazingly, marine mammals have developed a range of adaptations that allow them to hunt, sleep and breed across the varied depths, visibilities, and temperatures that occur in this watery environment. But it’s not always a case of ‘one size fits all’. Think about the adaptations of seals that have a mostly aquatic existence but come ashore to breed and rest, compared to those of the cetaceans (whale and dolphins) and sirenians (dugong and manatees) that live their lives entirely at sea.

A dugong feeding on seagrass underwater
Dugong feeding on seagrass. Credit: Sunphol Sorakul/ Getty Images

So, let’s take a closer look at what it means to be marine.

Living a life at sea means being streamlined to optimise fitness. Marine mammals do this by having their reproductive parts on the inside to prevent drag. They also have a conveyor belt of skin constantly on the slough, which functions as their own in-built, anti-fouling system that stops algae and animals growing on them – no day spa required! Cetacean (dolphin or whale) skin also has the texture and rigidity of a hard-boiled egg to keep them aquadynamic.

Then there’s the issue of finding enough to eat. All marine mammals are carnivores except for the sirenian sea cows (dugong). Whales such as the minke and blue whale filter the water for tiny crustaceans a fraction of their size, while toothed whales, dolphins and seal hunt for prey such as fish and cephalopods. Dolphins have in-built sonar and use clicks to help them find their food in the same way that bats use echolocation to navigate their surroundings.

If finding and catching dinner is not challenging enough, our marine mammals need to eat it without hands. Dolphins are known to toss an octopus rather than a salad. They do it repeatedly in a game of occy hockey until the octopus tentacles are disarmed and rendered safe for consumption. Calamari is sometimes on the menu too, with cuttlefish expertly deboned and de-inked before being diced into bite size pieces.

A snubfin dolphin poking its round head above the water
Snubfin dolphins have a distinctive rounded head and no beak. Credit: Holly Raudino.

Sleeping in this watery world also requires some serious adaptation. Dolphins cat nap by resting one hemisphere of their brain at a time, always keeping one eye open for danger. Seals sleep while at sea on multiple-day foraging forays far from land. Scientists have recently discovered that some species spiral while in these sleepy dives.

Talking underwater involves an impressive array of grunts, whistles and moans sounds produced by the nasal passage. Some whales use these low frequency sounds to communicate with each other over tremendous distances.

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Humpback whale. Credit: George Karbus Photography / Getty Images

Despite these amazing adaptations, the warming climate means marine mammals are faced with an uncertain future. A future where seagrass struggles to survive, and coral reefs are bleached. A future where baleen whales filtering water for krill may be inadvertently ingesting macro and microplastics, and toothed whales and dolphins feed on fish that come with a cocktail of persistent organic pollutants, plastics, and heavy metals that suppress their immune systems and leave them vulnerable to other stressors. This is in addition to the increasing number of marine vessels and fishing gear that present obstacles along their daily commutes and annual migrations. Of course, people continue to have an insatiable appetite for seafood too.

Photo of a group of australian fur seals riding a wave in the ocean.
Australian fur seals. Credit: Vincent Antony

Marine mammals are superbly adapted to their environment, but we are making their already tricky lives perilous with the pressures we’re imposing. Next time you’re at the beach and you see one of these super aquatic marvels, pause and appreciate what it’s like to live and survive in the watery realm. As coastal dwellers, we all have a responsibility to look after our big blue backyards and the inhabitants. Just imagine a world without the wonders of whales, dolphins, dugongs, or seals.

Meet the nominees:

Australian fur seal (Arctocephalus pusillus doriferus), south-eastern coastal Australia, the Bass Strait, and waters surrounding Tasmania

They are the largest of all fur seals in the world, with females weighing 78kg and males 220-360kg.

Australian sea lion (Neophoca cinerea), south coastal waters of Australia including WA and SA

They are the only seal that does not breed annually, instead getting together every 17.5 months.

Australian snubfin dolphin (Orcaella heinsohni), north coast of Australia including WA, QLD, and the NT.

Unlike most other dolphins and whales, they have vertebrae that make them able to flex their necks. This allows them to surface and take a breath without showing their dorsal fin.

Blue whale (Balaenoptera musculus), Australian waters

They are able to produce the loudest sound of any animal on earth, communicating over up to 1,600km.

Dugong (Dugong dugon), coastal northern Australia from WA to QLD (all year), to NSW (summer)

They are the only sea-grass eating mammal that spends all their life in the sea. 

Dwarf minke whale (Balaenoptera acutorostrata), coastal Australian waters

They make sounds that have been likened to the laser gun (blaster) in the Star Wars movies.

False killer whale (Pseudorca crassidens), coastal Australian waters

These dolphins are highly social, forming herds of 20-50 individuals, though they can aggregate in large groups of 100-800.

Humpback whale (Megaptera novaeangliae), the east and west coast of Australia

Male humpback whale song is unique to each population and changes slightly between years.

Indo-Pacific bottlenose dolphin

Killer whale (Orcinus orca), coastal Australian waters

They form pods with family members and these pods remain stable for decades, led by a female matriarch.

Southern elephant seal (Mirounga leonina), southern coastal Australian waters and Macquarie and Heard Islands

They are named after the large nose of the adult males, which is used to make loud roaring sounds.

Southern right whale (Eubalaena australis), southern Australian waters

Males have the largest testicles of the animal kingdom – around 500kg each.

The Ultramarine project – focussing on research and innovation in our marine environments – is supported by Minderoo Foundation.

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