Fluctuating asymmetry based on fractal geometry does not track environmental stress in an intertidal habitat-forming fucoid.

The availability of easy-to-measure and inexpensive biomarkers is a challenge for monitoring environmental stress in organisms. Although it has been proposed that non-directional deviations from a symmetrical pattern in morphological traits (called fluctuating asymmetry. FA) may be directly related to environmental stress, this postulate has been questioned. Self-similarity symmetry (fractal) is one possible approach to addressing FA analysis, but there are very few studies with this approach, and the results have been inconclusive. Here, we evaluate whether fractal descriptors of thallus morphology in the habitat-forming fucoid Fucus limitaneus reflect environmental stress across multiple contexts, including seasonal variation, vertical gradients in the intertidal zone, and the possible long-term impact of an invasive species. Our results show that fractal metrics did not vary consistently across these contrasting environmental conditions, suggesting that in this system, FA based on fractal geometry does not reliably track environmental stress. This is possibly because stress is expressed through alternative ecological and biomechanical processes rather than through modifications of thallus self-similarity.