Meyer-Rochow wrote an essay that begins:
You can lock arms with someone, you can lock on to something or be locked in or even be locked up or locked out. But this essay is about animals that possess locking mechanisms. In the tropical waters of Ascension Island in the South Atlantic Ocean, for example, I once caught a pair of matchbox-sized trigger fish and observed their behaviour in my aquarium on board of the research vessel “Walter Herwig”. These denizens of the tropical seas as well as their cousins, the file fishes, can wedge themselves into rock cracks and coral crevices in such a way that it is virtually impossible to dislodge them by pulling at their tails.
You might think that these fish must be mighty strong, but in reality they hardly use any energy at all in this process of “locking in”. Their first dorsal fin ray is a spine that possesses a groove on the backside; but when fully erect a smaller second spine behind the first one is pushed home into the groove of the spine in front, so that the first bigger spine cannot be depressed by external forces, except when the second smaller one is retracted first. A very fine and effective device that is….