Why is the pufferfish so deadly?

Image credit: Pixabay (Rudy and Peter Skitterians)

A look at why, despite its size and timid appearance, one type of fish can be extremely deadly when it comes to defending itself

The pufferfish is a group of over 100 species that are so-named for their unique line in defence. When cornered, a puffer’s last gasp is to draw in water (or sometimes air) and pump it to the stomach, expanding to three times its normal size; deterring potential predators and when possible, affording it the vital seconds necessary to escape.

To achieve this with the required efficiency and speed, once the puffer has taken on water its gills clamp shut and a powerful bow-door-like valve closes over the inside of the mouth. Once the mouth’s cavity is compressed, this forces the water into its stomach. 

Despite its resulting comic appearance, the tissues and organs of many a puffer are no joke, laced with the potent poison tetrodotoxin – a single pinhead of which could kill a grown man. This makes it ten times more deadly than the black widow spider. The poison is produced as part of a mutually beneficial relationship by common bacteria where nutrients are exchanged as payment for the ultimate deterrent.

Some species such as the porcupine puffer are more sporting than others, covered with spines that offer added protection and ample warning to any would-be attackers. Each spine is attached to the skin by an ingenious tripod-shaped bony base. When the skin stretches, one of the legs is pushed forward and two are pulled back to snap the spine outwards… a point well made in more ways than one.

Second only to the golden dart frog, pufferfish are said to rank as the most deadly vertebrate on Earth. The poison it carries, tetrodotoxin (TTX), is not of its own making; it is produced, in association, by relatively common marine bacteria and dinoflagellates.

In susceptible animals, TTX binds to the sodium channels of nerve cells, halting the influx of sodium and causing a cessation of nerve function; this leads to suffocation, paralysing the diaphragm and causing the death of its victim. There is no known cure.

Humans are most likely to taste its deadly effect from improperly prepared Japanese delicacy, fugu. The diner can expect a deadening of the mouth, dizziness, vomiting and difficulty breathing. This is followed by respiratory failure and coma or death within 24 hours if treatment is not forthcoming. 

How it blows

As water enters the stomach, kilted pleats in the inner-lining allow it to flex and stretch under enormous pressure. As the stomach cavity fills, it balloons above and around the spine, continually pulling the inner-lining fibres so tight that they harden to form an almost perfect impenetrable sphere. 

This article was originally published in How It Works issue  12

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