In the ocean which serves as the habitat for most Qwilfish, they are an enormously fragile pokemon. Their dense hide requires them to contain an enormous amount of air inside their body just to float, and a single puncture wound beneath the surface can send them tumbling to the bottom of the sea as water replaces air, where they are crushed to death by the water pressure. In addition to poisoning their prey, Qwilfish also use their spikes to pop one another to reduce competition for food: it is a common sight in deep waters to see two of them bobbing up and down while ramming another in the hopes of deflating the other before being popped themselves.
There is only one method for a Qwilfish to defend itself from obstacles, and paradoxically, that is to knock itself unconscious with an Explosion. Although the attack will not be used on other Qwilfish, for even if successful it will only knock them out, wild Qwilfish have been known to use it against everything from boats to Gyarados as a last resort. Trained Qwilfish have even been pressed into service as naval mines for this reason, but this is not commonly done, for Qwilfish require a great deal of training not to attack one another on sight.
On land, where most trained Qwilfish battle, their lower spikes instead act as something similar to legs, allowing them to walk with surprising mobility. Indeed, between this and their ability to survive being popped, many wonder why they remain in the water at all; perhaps it is simply that they enjoy the ocean too much to leave except in a pokeball.