Starfish - also known as sea stars are voracious predators. Take the sunflower sea star (Pycnopodia helianthoides) for example. Out in the northeast Pacific where it lives, it will happily munch its way through animals that we think might have good defences, like scallops, barnacles, and snails which all sport a hardy shell, or urchins which are covered in a spiny exterior.
Sunflower sea stars are no top predator though. Unlike their prey, sunflower sea stars have soft, exposed bodies - and there are other animals out there who are quite happy to try to eat them. They are certainly quick (they can move at a meter per minute) but speed isn't always enough to defend themselves. Like most other starfish, sunflower sea stars have a pretty nifty defence - autotomy, basically self-amputation. Just take a look at this photo...
This (deceased) sunflower sea star (Pycnopodia helianthoides) was spotted on a rocky beach in Bamfield, British Columbia in Canada. See how it has a gap where some arms should be. Credit: Samantha Andrews/Ocean Oculus
This rather nifty little trick allows the sea star to make a getaway whilst its hunter feasts on its missing limb. The sea stars ability to 'drop and run' comes down in part to a connective tissue called 'catch connective tissue' which can change in stiffness. When the starfish want to grab onto something, they can make their muscles very stiff. When they want to move through narrow spots, they can make their tissue very soft. If they make their tissue super soft - to the point that the limb really loses all cohesion - off comes the limb.
Whilst autotomy might save a sea star's life, it is not a cost-free exercise. Amongst other things like decreased growth due to the starfish pouring lots of energy into regenerating the missing limbs, scientists in Chile found that the sea star Stichaster striatus stores nutrient in its's limbs so if it drops one, it also loses a vital energy storage. This means if food supply is low, the animal is more likely to suffer from starvation. Over in Wales, scientists looking at the common starfish (Asterias rubens) found that when the starfish autotomised, they weren't as adept at opening mussels as they were before they lost their limb". Yup - more hunger risk.
Surprisingly autotomy isn't just an anti-predator defence. Scientists in California found that when temperatures get hot, the purple sea star (Pisaster ochraceus) will discard legs - apparently to cool themselves down.