We, as well as fish themselves, recognize fish from symmetric and recognizable patterns. Some predators mask this fish “bauplan” symmetry by have many irregularities in their skin and shape. Many scorpionfish and most notably the different stonefishes are very irregular and asymmetric in their shape. The purpose of the asymmetry is to break the pattern recognition system prey fish (and humans, many birds, many other mammals) have. This works remarkably well, as shown by the difficulty many divers have with recognizing a stonefish or a scorpionfish even when it is clearly pointed out to them. This was clearly demonstrated yesterday, when Serge quite happily pointed out what I thought was a really common pipe fish. As I am a polite person, I nodded, gave him the OK-sign, waved nicely to him, but thought: Really!. However, Serge kept on pointing, and after quite a while I saw that the pipe fish had been hovering just above a really well camouflaged eustarine stonefish!
Symmetry is a way of expressing that there is order to something. As all of you know, and as the laws of physics clearly state, it is much easier to let disorder rule than to keep things highly organized. If you do not believe me, think about a room full of children, and the order in the room will quickly approach random pattern! There are ways of using this aspect of order, or symmetry, in nature. Mate choice by many species has been shown to partly depend on symmetrical factors of the mate, such that more symmetrical mates are preferred to less symmetrical. Several experiments have shown that people do that as well, preferring partners with a high degree of symmetry between the right and the left side of our faces. It is thought that a high level of symmetry is a sign of good and efficient gene combinations in the bearer, enabling the bearer to exhibit high degrees of symmetry despite the obvious costs for that, which is why the general preference for symmetry exists. However, the flip side of this is that we tend to find lack of symmetry or asymmetry unattractive and ugly.
So, in essence, the ultimate reason for us to find stonefish ugly is that we look for symmetric mates to provide us with great genes! Luckily stone fish have no idea that the fascination for their ugliness is an old remnant from our evolutionary past, where individuals within species have competed with each other for mates with high genetic quality, using the degree of morphological symmetry to estimate mate quality. Probably stonefish couldn´t care less even if they knew!