Why do jellyfish burst when moved from saltwater to fresh water?

Jellyfish burst when moved from saltwater to freshwater mainly due to the differences in osmotic pressure. This process, called osmosis, occurs when there is a difference in water concentration between two solutions, separated by a semipermeable membrane.

When a jellyfish, which has a higher concentration of water inside its body compared to the surrounding saltwater, is placed in freshwater, the water molecules tend to move into the jellyfish's body through the semipermeable cell membranes. This influx of water causes the jellyfish to swell and become turgid.

As the jellyfish continues to absorb freshwater, its internal pressure increases, putting a strain on its delicate body structure. Eventually, the pressure becomes too great, causing the jellyfish to burst and disintegrate.

In summary, the sudden change in osmotic pressure, resulting from the movement from saltwater to freshwater, leads to the rapid influx of water into the jellyfish's body. This causes excessive swelling and ultimately causes the jellyfish to burst.