Predasites

My tour of potentially life-sustaining planets had brought me to a pleasant little rock leeward of the Crab Nebula. I always hope, on these jaunts, to encounter intelligent life, and in this, I am seldom not disappointed. This trip was no exception. He appeared to be watching over a herd of white, woolly animals.

"Them's my ships," he said. "I'm the shipherd."

"And a fine and woolly lot they are," I observed affably. But just then, I saw a pack of toothy marauders descending upon the fold. "My goodness" I cried. "Those look like predators, about to attack your ... um, ships!"

"Mm-hm," said the shipherd. "Them's the woofs."

Before I could say another word, the woofs had climbed onto the ships, one woof per ship. Each woof sank teeth and claws into its ship, but then seemed to swoon off into a dreamy sleep. The ships, for their part, staggered around as best they could and continued munching grass, pooping, bleating, and doing all the usual ship things.

"Good heavens!" I gasped. "Those beasts are twice the size of the ships! How do the poor ships manage it?"

"Nature's way," shrugged the shipherd.

It was then I noticed that there were more woof-laden ship than could be accounted for by the recent attack. Apparently, it was an iterative process. Every so often, a depleted ship would collapse. Then its erstwhile passenger would clamber off the spent corpse and waddle on in search of a new host. Bloated as they were with the lives just sucked out of their respective ships, this sometimes took a while.

"On my planet," I observed to the shipherd, "we have predators, prey, and parasites. Predators are powerful, savage beasts that kill and eat the prey quickly. Parasites may weaken and kill their hosts, but they are typically so tiny that their hosts may not even know that they have parasites. You seem to have only prey and a combination predator-parasite. And on my planet, they prey at least have a chance. They can run fast or hide cleverly or hold their own in a fight. And if they are domesticated, they have guardians ... guard animals or ... well, shipherds. Come to think of it, why aren't you protecting your ships?"

"The woofs pay me to look the other way," said the shipherd. "It's better money than you can make guarding ships, I can tell you. Anyway, if the ships stupid enough to let the woofs get them, that's their lookout, isn't it?"