Sponges can live for thousands of years, grow continuously and lack an immune system, so they should be particularly prone to cancers. But聽sponges exposed to X-rays have survived 100 times the lethal dose for humans without tumours.
鈥淭hey seem to be extraordinarily resistant to radiation,鈥� says Angelo Fortunato at Arizona State University. In fact, sponges have the highest level of radiation resistance ever observed in any animal whose cells keep dividing throughout its lifetime 鈥� as opposed to creatures like tardigrades whose cells stop dividing.
Fortunato鈥檚 team has been studying animals for which there are no records of cancers, such as sponges, comb jellies and placozoa, to see if they really are cancer resistant and if so, why. The researchers exposed young sponges of the species Tethya wilhelma to increasing doses of X-rays to try to induce cancers.
Advertisement
To their surprise, they found that it took 800 gray of radiation to kill most of the sponges. When they were exposed to 600 gray, they had no obvious signs of cell death. For comparison, a chest X-ray exposes a person to around 0.1 milligray.
However, at 600 gray the animals lost their shape and their ability to filter food from the water over the following 20 days or so. 鈥淭hey completely lost their internal morphological structure,鈥� says Fortunato.
The sponges then began reforming and regained their normal structure after around 180 days. This wasn鈥檛 a big surprise, he says, as sponges are known to have the ability to reform even if broken apart into single cells.
After a year, none showed any signs of tumours. 鈥淭hey recovered completely,鈥� says Fortunato. So even though they should in theory be vulnerable to cancers, there is still no record of any of the 8000 species of sponge ever getting any type of cancer.
Instead, these animals appear to have very good mechanisms both for preventing DNA damage in the first place and for repairing damage once it occurs. That might be because their water-filtering lifestyle makes them vulnerable to DNA-damaging toxins, so they may have evolved to cope with such damage, says Fortunato.
Tardigrades can survive radiation doses of around 5000 gray, though . However, like some other tiny animals, tardigrades have a predetermined number of cells in their body, meaning their cells stop dividing once they reach this point. Dividing cells are much more vulnerable to radiation damage, which is why radiation is sometimes used to kill off fast-dividing cancer cells.
Tardigrades also typically live just a few months, and animals with short lifespans are less likely to get cancers.
Reference:
Sign up for Wild Wild Life, a free monthly newsletter celebrating the diversity and science of animals, plants and Earth鈥檚 other weird and wonderful inhabitants>
