Goldfish Digestive System

Goldfish use a simple but efficient and interesting method of absorbing nutrients from their food.  They have no glands, valves, or two sizes of intestine.   Most unusual of all – even among fish – they have no stomach.  Most surprising of all, they have a set of teeth to get the digestion ball rolling.

Once food is in the goldfish’s mouth, it is pushed to the back of the throat where a set of teeth grind and crush it against a hard pad.  There is an excellent photograph of goldfish teeth here.

The ground down food then passes down a tube called the oesophagus (pronounced oss-SOF-a-gus), which squeezes out excess water.  The oesophagus is lined with taste buds, little muscular hairs that brush the food along, and cells that produce mucus to keep things moving on.

The oesophagus empties into an expandable section of the goldfish’s digestive tract that for many years was thought to be a stomach.  However it does not produce any acid or enzymes to distinguish it from any other part of the gut.  It is simply a buffer zone to hold excess food as required.

Just before this expanded section, chemicals from the gall bladder and pancreas are pumped in with the food.  The ones from the gall bladder make up bile, which is used to break down fats; the ones from the pancreas contain enzymes that are used to break down proteins.

All along the digestive tract are cells that secrete enzymes that act on carbohydrates, breaking them down into sugars.

From the expanded section to the goldfish’s anus, lots of mucus is produced and as much useful material as possible is absorbed into the bloodstream to be used for energy, growth, protection and repair.

The whole process takes about 16 hours at an ideal temperature, but will slow to a stop in ponds during cold periods.

This is the reason goldfish should not be fed when it is cold.  Any food they do ingest can end up rotting inside them.  Any they don’t ingest can end up rotting in the pond water.

The nature of the goldfish’s digestive system also illustrates why it is vital to feed small meals throughout the day, rather than one or two big meals.

The food eaten in big meals will not be in the goldfish long enough to be broken down properly.  Most of it will come out the back end before digestion has a chance to act on it.

Related article:

Goldfish diet and nutrition

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