1. Tasty – In certain Asian cultures the swim bladders of large ocean fish are considered a tasty foodstuff, renowned as a delicacy that is commonly served braised.

2. Origin – Charles Darwin wrote: “There is no reason to doubt the swim bladder has been converted into lungs [and that] all vertebrates with true lungs are descended from an ancient prototype.”

3. Hearing – In some fish the swim bladder is connected to the labyrinth of the inner ear by a bony structure from the vertebrae. This provides a precise sense of water pressure and hearing.

4. Gas – The mix of gases in swim bladders varies. Shallowwater fish bladders tend to approximate that of Earth’s atmosphere, while deep-sea fish have higher oxygen mixes.

5. Bladder-free – Cartilaginous fish like sharks lack both lungs and swim bladders. This has led to postulation that both these organs developed 420 million years ago after such species divided from other fish.

Asked by Mark Wrangham

Like humans, fi sh need oxygen to survive, but unlike us, they are capable of getting the oxygen they need from water. To do this they use an arrangement of filaments on either side of their neck called gills. Fish gulp in oxygenated water and force it through their gills and then out through the gill openings while also continually pumping deoxygenated blood from the body into the gill fi laments. As the oxygenated water passes by the deoxygenated blood, oxygen diffuses into the gills. Efficiency is increased by the blood and water flowing in opposite directions; this is known as a counter-current exchange mechanism. Using this, a fish can extract up to 70 per cent of the oxygen dissolved in the water. Waste products in the blood can also be removed from the blood by the same process, passing from the gills into the water.

James Maclaine, Zoology department, The Natural History Museum, London

Of all the species of fish in the world, one quarter of them shoal and/or school for their entire lives, while about one half participate in the action for limited periods. Together this means that vast selections of fish school at some point or another, coming together to swim in synchronicity.

Fish perform this phenomenon for a number of reasons. The first is to support social and genetic functions, aggregating together to increase the ease of communication and reduce stress – experiments have shown that heart rate reduces significantly in shoaled fi sh compared to those alone. The second advantage of schooling is to boost the group’s foraging success, which has been proven in trials to grow considerably in comparison to a solitary specimen. This is simply because the number of eyes looking for food increases dramatically and, partnered with the ability for each fish to monitor the behaviour of those around it, means that when one fi sh demonstrates feeding behaviour, the others follow.

Finally, the third – and primary – reason why fish school is for protection. By grouping into a tight, regimented pattern, the fi sh minimise their chance of being picked off by generating a sensory overload to a predator’s visual channel. The swirling mass of twisting silvery fish creates a blending effect where the predator struggles to track a single target and becomes confused.

It is the unique arrangement of the long bill with its voluminous distensible pouch that makes pelicans remarkable. The upper mandible is strong and stiff, while the lower mandible is loosely articulated and flexible, to enable considerable distension of the enormous skin pouch which hangs from it. Both the large bill and the massive pouch are evidently adaptations to the bird’s requirement of catching a great quantity of fish. In addition, the bill is very sensitive and can detect fish in murky water. The upper mandible acts as a lid for the pouch when it is used like a fishing net, and the pouch can hold as much as 13 litres (2.9 gallons) of water (including fish) after a catch.

Answered by Hein van Grouw, Department of Zoology, Natural History Museum, London.

Sticking to the question ‘could we’, the answer has to be ‘probably not’. Demand is for the apex predators such as salmon and tuna, high up in the food chains. But they require a lot of food in the form of meals made from other marine fish of lesser market value, which means further depletion of wild fish stocks.

Marine fish farms encounter the problems of any intensive monoculture where individuals of a species are crammed together: disease and pollution. There is some evidence that farmed fish spread diseases to wild populations, that they have a damaging effect on the genetics of wild fish stocks through interbreeding, and that they are less able to survive in the wild. There’s also environmental damage, and it looks as if marine fish farming may be more of a threat to wild fish stocks than a means of replenishing them.

Oliver Crimmen, Senior Fish Curator, NHM London

Bioluminescence is light emitted by living organisms. Yellow-green light is produced by various insects including glow-worms and fireflies, but it is in the oceans where we see most animal lights.

The light is produced in a chemical reaction in which an enzyme, luciferase, stimulates a reaction between molecules called luciferins and oxygen, which results in the emission of light as well as oxyluciferin. It is in effect a cold-light source. Some use their own luciferins to produce light, either from many small sites or in special light organs with reflectors, lenses and even colour filters.

In some of the deep-sea anglerfishes the light is produced by bacteria living within special bulbous light organs. Marine bioluminescence is usually blue or green but there are variations, and a few predatory fishes have the ability both to transmit and to see red light, providing them with a private visual system for detecting prey. Light organs on the undersides of fishes and squids camouflage their owners against down-welling light from the surface, but some organisms’ fl ashing displays and spark-like luminous discharges are designed to confuse predators.

Oliver Crimmen, senior fish curator, Natural History Museum

Unlike man, who has always dreamt of flying, some birds will eagerly shed this evolutionary advantage if it no longer suits them. To remain airborne it helps if you have hollow bones to keep the baggage limit down.

However, if you’re adapted to dive for fish, then having a denser structure helps you become less buoyant. Penguins use their flippers like wings, to guide and propel their torpedo-like bodies through the water to catch fish and evade predators – so they do fly in water, just not in air. Several families of birds have lost the power of flight, including many island forms. On small islands, being able to fly can be a disadvantage if you are likely to be blown out to sea by storms. Although regarded by some as flightless, the chicken and the jungle fowl – the chicken’s Asian ancestor – have limited abilities, which enables them to flutter away from predators.

Amy Lewis, communications officer for The Wildlife Trusts

Adaptive camouflaging to the surrounding environment occurs widely in the animal kingdom, and many examples can be found in the marine world, including cephalopods (octopus, cuttlefish, squid), and flatfish, such as some flounder (particularly in the Paralichthys and Ancylopsetta genera).

These animals all use groups of pigmented cells in the skin collectively called ‘chromatophores’ to alter their colour and simulate their surroundings. In flatfish these cells can be either black (melanophores) or shades of yellow (xanthophores). In conjunction with other groups of cells called ‘iridocytes’, which reflect light to produce a white appearance, the fish can assume the colour (and pattern to a limited extent) of the surrounding background.

To produce these changes within the skin cells, light stimuli is received through the eyes at the retina and passed through nerves to specialised skin cells. The colouration is a response to the ratio of reflected to incident light directed at the retina, and is generally a mix of the inputs from both eyes. While a flounder with one eye is still able to change colour effectively, a fully blind flounder cannot simulate the
background in shade, colour or pattern.

Dan Bayley, MarLIN at the Marine Biological Association

While we may have descended from cavemen and evolved from apes, an unusual source may be the cause of our hiccups: fish.

Neil Shubin, head of the University of Chicago’s Anatomy School, claims in his new book Your Inner Fish that our need to hiccup is ultimately due to our amphibian ancestory.

In his book, Shubin says: “Our tendency to develop hiccups is another influence of our past. There are two issues to think about. The first is what causes the spasm of nerves that initiates the hiccup. The second is what controls that distinctive hic, the abrupt inhalation–glottis closure. The nerve spasm is a product of our fish history, while the hic is an outcome of the history we share with animals such as tadpoles.”

It seems that some of our nerves have been inherited from fish. These can become irritated and trigger hiccups which result after a closing of the windpipe. This action is similar to the breathing motion of amphibians with lungs and gills, and it is to them we should give credit for the annoyance of the hiccup.

As part of The Royal Societies’ 350th anniversary, never-before-seen, super close up colour images of tiny sea creatures are going on display at London Zoo. The amazing images of these creatures are to feature in a book by Royal Society Research Fellow Dr Richard Kirby of the University of Plymouth.

The full range of Dr Kirby’s photographs will go on display from the 16th of February till the end of the year.

Plankton are crucial to the ocean and its environment as they provide an abundant source of food for larger and more familiar aquatic organisms such as fish. This is especially important for fish larvae as they switch from their egg sacks to external feeding, as the abundance and minute size of the plankton, make even the weakest larvae able to feed on them, increasing their survival chances dramatically.

Currently, concern has been raised in the marine biology community over the man-made effects on dwindling plankton populations in certain risk areas.

Image courtesy of Richard Kirby.