Be small. To get away with this trick, you need to stay inconspicuous. It’s true that the digs aren’t huge—but then again, everything depends on your perspective. For the single pearlfish or loving pair of pea-crabs, the place is spacious and packed with amenities. You just have to be able to weasel your way in—and out. That can be a delicate business. Even a cucumber feels that its anus is its own. However, it’s not as sensitive about this as the rest of us usually are.
In return for your discreet size, you can take up residence inside a cozy, well-ventilated cave that offers good protection from neighbors who might look on you as easy pickings. You can go out every night, but no one’s going to crash your private party. No one’s coming over for ‘just a night or two.’
We emphasize the good ventilation because we can’t say much for the lighting. Since a cucumber breathes by rhythmic expansion and relaxation of its hindgut—just like another animal will breathe by expansion and relaxation of its soft, springy lungs—its tenants will never be short of oxygen. Better yet, cucumbers are often distasteful or even toxic to other animals. To get to you, a predator would have to devour a lot of cucumber first. Chemical defenses are common in cucumbers and their other spiny-skinned relatives—seastars, for instance. If a predator is going to sniff you out, it also has to be able to smell you above the overpowering and possibly noxious aroma of cucumber that surrounds you.
Don’t bite the butt that shelters you. Once you’ve moved in, you may be tempted to take advantage of the fact that you’re basically surrounded by food, 24-7. You’re living in another animal’s gut, right? What could be better than a light snack from time to time? It’s a lot easier than squeezing outside again to forage. It’s room service without a tab. The chef may be serving mud, but it’s free.
Or so you might think.
We recommend that you take the long view, and choose your strategy accordingly. Evolution is a race. There’s definitely no law against taking advantage of your host. Actually, you should go for it, if you think that you can get away with it. But remember that if you do steal food from that host of yours, or maybe even chew on that tasty tender gut once in a while, chances are you’re weakening him or her. If the impact is great enough to reduce the number of offspring your host produces—making babies demands a lot of energy—you’ve just made the world a harder place for your own children. Other cucumbers, especially the ones who keep their anuses tightly closed to the likes of you, will not be suffering any impediments to their fecundity. They will go on having babies, loads of babies, while your host has fewer. Play this out for a hundred generations or so, and then where will your children be? As with most things in life, it’s completely possible to trip yourself up by being too short-sighted.
Of course, plenty of animals do take this tack. Maybe your hosts just don’t have any defense against your residence—no way to seal up their butts, so to speak. Maybe they do develop defenses, but you can outmaneuver them too quickly; the poor hosts never have permanent immunity to your invasion of their bodies. Each time they erect some kind of barrier, you find a way around it in a generation or two. After all, there will always be a few of your relatives who aren’t put off by a defense. Those relatives will succeed and will make babies of their own, and chances are these children will inherit the talents that made their parents successful. It helps if your generations turn over faster than your host’s. If you can have great-grandchildren before they even get around to producing children, you probably have them beat. Even so, it’s best to achieve a balance. You can’t have too negative an impact—if you wipe your hosts out entirely, it’s the end of the story for you too. How successful are cold viruses? And why aren’t there more viruses like Ebola in circulation?
But as it happens, you are not a parasite. You’ve taken the high road. You got where you are—small, discreet, eligible for accommodation up a cucumber’s hindgut—because you have RESPECT for the butt that shelters you. You don’t scavenge more than an occasional careful nibble from your host—or at least, you don’t eat enough to damage its chances. In return, your host tolerates your continued residence up its most private regions.
In fact, you’re a commensal. If you make a little more effort, maybe defend your host against animals that try to prey on it or parasitize it—the relationship becomes one of mutualism. You do good things for your host, and your host does good things for you. You both get ahead. It’s the sweeter outcome to the race.