[milspec]

That does seem kind of neat, but it still doesn't make sense to me. You have to carry a poison supply that will run out. You need 12 mL per starfish.

Is that really more effective than just injecting lots of seawater? You could pump in seawater by the kL, not mL, and you'll never run out. The starfish will surely die after all bodily fluids have been washed out with seawater.

The other idea that comes to mind is a needle with an electrified tip. Depending on voltage and current, this kills in various different ways. You can heat the animal, zap the nerves, or (via electrolysis of salt) fill the animal with either chlorine or sodium hydroxide.

[stinos]

Water or electricity don't lead to infection in other nearby starfish, which is probably needed here because it's going to be hard for a robot to find and kill all, or enough, of them. Which at the same point is also something to take care of: I hope the inventors checked the range of starfish etc, so that this won't lead to massive starfish death in places far away from the coral reef being meant to be protected.

[dgemm]

Energy. Those are all energy intensive ideas.

[milspec]

12 mL worth of battery ought to be able to handle most of those methods. That's about break-even, but better because you refill the same way you refill the computer and propulsion. Charge the battery and you're ready for more action, mess-free. If you can do the job with less than 12 mL of battery, it's a definite win.