It depends both on the type of starfish and how the limbs are lost. As you may know, there are over 1800 species of starfish and most have the ability to regrow lost limbs. They can do this because they keep their vital organs in their arms (rather their body center), with copies in each arm. These organs are connected to the digestive system in the center. If they lose an arm, they still have all the organs they need to survive while they regrow it. Losing a limb is a traumatic experience that leaves the starfish weak and vulnerable to infection, but some starfish will voluntarily give up an arm to a predator if it allows the rest of the starfish to get away.

There is a common misconception that regrown limbs will always be smaller than the original. This is not true - limbs will typically be smaller and weaker as they are growing back, but given enough time (typically a year or so), you won't be able to tell which is the newest arm. So, there is no limit to how many arms a starfish grow back during its lifetime, as long as it remains healthy and is given enough time to heal.

But how many limbs can a starfish lose at once? Regrowing limbs requires time and energy, so most starfish typically need to keep least one arm attached to the digestive system in their main body. This will allow them to eat and move to find food or hide while they recover. However, some starfish species don't even need that much! Instead, a severed arm can grow a whole new body with additional arms, and become its own separate starfish! (The limb can't be too damaged when it is severed, though, or it may not be healthy enough to regenerate.) Since each arm can become another identical starfish, so the original starfish isn't really dead, is it? This ability to evade death is why it is so difficult to get rid of invasive starfish species, such as the Crown of Thorns, that are eating up the Great Barrier Reef in Australia, with starfish reproducing faster than the reef can heal.

For more information on the regeneration process, check out this other ScienceLine question:

For other facts about starfish, check out starfish , too.