There are no trees to hide in, no burrows you can dig. Death surrounds you in three dimensions and everything larger than you is a predator.
To survive, you have to think big. For some species, this means living as part of a large school of fish. For others, it means actually becoming genuinely, truly enormous. Tiny fish are eaten by small fish. Small fish are eaten by larger fish and so on. In every size bracket, natural selection favours the larger animal over the smaller one. Over millions of years, animal species tend to grow gradually larger and larger until they are too big to fit in anyone’s mouth.
Being big is easier in the sea than on land because the buoyancy of water supports an animal evenly around its body, instead of just through the soles of its feet. An African elephant, for instance, can’t grow much larger than ten tons without fracturing its own legs. A blue whale, meanwhile, will weigh this much before it’s three months old.
Sea giant monsters can get by with much smaller skeletons and their bones don’t need to be so strong as they aren’t subject to so much shock loading. But the density of water also presents some challenges. It’s much harder to move through water than air, so streamlining is essential. A blue whale is 60 times longer than it is wide, compared with only 3.5 times for a hippo. The rear third of the whale’s body provides the muscle to drive the 7.5-metre (25-foot) tail fluke up and down. Why does an animal with no natural predators need to cruise at 32 kilometres (20 miles) per hour? One reason is that it makes it much harder for barnacles to attach. It’s ironic that an animal as large as a whale should be threatened by something as small as a barnacle, but if enough take hold, the extra drag drastically increases the energy required to swim.
Food is the limiting factor for all large sea creatures. Light doesn’t penetrate far in water so there are no grassy plains for large herbivores to graze. Instead the ocean is a thin soup, with the occasional chunk of meat bobbing in it. You can chase after the chunks, but catching them requires more energy, which means you need more food and so on.
The largest animals in the sea have found it is more lucrative to swallow the ‘soup’ instead. This is a mixture of unicellular organisms, fish larvae and shrimp, ie plankton. They are too small to swim against the current, so it’s just a matter of straining them from the water. The lion’s mane jellyfish can do this while expending virtually no energy. It swims slowly up by pulsing its bell and then relaxes to drift down again like a parachute. As it does, its tentacles billow out like hair to cover a wide area and prey gets speared by its stinger cells. Most large whales, along with the whale shark and the manta rays, adopt a slightly more active strategy by either swimming at speed into a dense cloud of plankton or taking huge gulps to suck them in, and then filtering them through a mesh of fibres made from modified teeth or gill bars. Different animals have different sized filter meshes that trap a particular size of plankton. Whales and whale sharks trap only the relatively large krill (a kind of shrimp) and crab larvae. A ton of krill contains about 450 thousand calories -which is about a tenth as much as a ton of chocolate – and an adult blue whale needs 3.5 tons of krill a day.
Very large animals need to protect their young to give them time to grow big enough to fend off predators. Whales are mammals so the embryo develops inside its mother to protect it. Great white sharks and manta rays have abandoned the usual fishy strategy of laying eggs on the seabed and copied mammals; the eggs are retained inside the female and hatch as live ‘pups’. The mating and birthing of the whale shark has never been, but they are believed to use the same technique. Even the giant Pacific octopus will guard her nest of eggs until they hatch. Her month-long vigil is the last thing she does though because the exertion kills her – to compensate she lays around 100,000 eggs in one go!
Huge fish have other tricks normally reserved for mammals too. Large sharks and manta rays have a low surface area compared to their body size so they don’t lose as much heat. This makes them effectively warm-blooded and allows them to maintain a more active lifestyle even in colder seas.
The best-studied ocean giants are those that live in fairly shallow water – above 200 metres (656 feet) – where most of the plankton is. But there are very large animals including squid that live in the perpetual blackness beyond. If you are an air-breathing mammal like a sperm whale that feeds on these squid, you face a unique challenge. To feed you need to dive to depths of up to three kilometres (1.9 miles), but to breathe you need to return to the surface. The pressure change in a round-trip is almost 300 atmospheres! Sperm whales have three times more myoglobin in their muscles to store more oxygen and their ribcage is flexible so that the lungs collapse under pressure and reduce the amount of nitrogen that dissolves into the blood. Despite this, the skeletons of older whales showpitting from the decompression effects of repeated dives.
The shark with the biggest bite
Great white shark – amazing fact: Great white sharks have two stomach compartments and can store food in one compartment for days or weeks without digesting it. Great whites have been found with shoes, wigs, newspapers, licence plates and even cannon balls in their stomachs.
The largest turtle ever found was a leatherback that washed up on a beach in Wales in 1988. It was 2.75 metres (nine feet) long and at least 100 years old when it died.
Top 3 biggest fish ever
Oarfish – 15m (49ft) long and eel-like in appearance. Rare sightings of these normally deepwater fish may have given rise to I legends of sea serpents.
Whale shark – The largest verified specimen was 12.7m (41.5ft) long, but there are unconfirmed reports of whale sharks that reach a staggering 18m (59ft)!
Leedsichthys problematicus – Fossils of this fish from 155 million years ago show it was 22m (72ft) long, making it the largest known fish ever to live.