Curious Kids is a series by The Conversation , which gives children the chance to have their questions about the world answered by experts. To understand how this works, you first need to know that bony fish have a different concentration of salt in their bodies to their environment. The bodies of marine fish which live in the sea are less salty than the water they swim in, while the bodies of freshwater fish which live in rivers and lakes are more salty than the water they swim in.
Both marine and freshwater fish have to control the amount of water and salt in their bodies, to stay healthy and hydrated. During osmosis, water moves through a membrane like skin , from areas of lower concentration to areas of higher concentration. Remember, the body of a marine fish is less salty than the seawater it swims in — which means it has a lower concentration of salt. So these fish actually lose water through osmosis: it passes from their body, through their skin and gills, out into the sea.
You might be interested to know that the opposite happens in freshwater fish. Water flows into their body through osmosis, instead of out. Dr Karl : Sooner or later, most young children will ask the question, "Why is the sky blue? And another big one for curious, developing minds is: "Do fish drink water?
To save you from blundering through an answer alone, I've done the research and prepared one for you. The solution to this conundrum is that some fish do drink water, and some don't. Put simply, in general, saltwater fish drink lots, while freshwater fish don't. Plus, surprisingly and perhaps entertainingly for the young child asking the question , despite saltwater fish drinking lots of water, they only wee out only a small amount of urine — while freshwater fish do the opposite.
To explain further, there are two things that we need to understand first. Act 1: "getting oxygen to live", and Act 2: "osmosis". We humans have it easy. We are surrounded by oxygen — it makes up one fifth of the air we breathe. So we simply have to breathe in, and inside our lungs oxygen diffuses across some 70 square metres of a very thin membrane into the blood. Problem solved, we've got some oxygen into our blood. But fish have it harder. In their environment, oxygen is dissolved in the water all around them — but it's trickier to get to.
They can't break down the water molecule which is H2O — two atoms of hydrogen and one atom of oxygen. No, they have to rely entirely on oxygen molecules that might happen to be there, lurking in between the water molecules. Fish have been around for about million years, and they've evolved gills to remove the oxygen from the water.
Gills are light feathery structures with lots of surface area on the outside, and blood vessels on the inside. This happens only when a female cannot find a partner for reproduction.
The blue streak cleaner wrasse changes sex from female to male- In the wrasse family Labridae , the largest female of the harem will change into a male and will take over the harem when there is a loss of the dominant male. Most Wrasse bury themselves in the sand at night to sleep and can also perform this behaviour when threatened or startled. The batfish plays dead when danger is near- when a batfish feels scared or threatened it floats motionless on its side making it look like a dead leaf floating on the surface of the water.
The most poisonous fish in the world is the Stone fish. Its sting can cause shock, paralysis, even death if not treated within a few hours. The Syngnathidae family of fish which include Male seahorses, pipefish, and sea dragons are the only species on earth, where the males give birth to their offspring.
The body of a fish acts the same way, either absorbing or losing water depending on its surroundings. Whether a fish absorbs or loses water is based on the fact that all fish must maintain a certain amount of salt in their bodies to stay healthy.
Since fish cannot allow their salt content to be diminished, their kidneys work overtime to expel excess water.
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