1 INTRODUCTION In Mediterranean rocky shores, the purple sea urchin Paracentrotus lividus and the black sea urchin Arbacia lixula, often co-occur in sea urchin barrens devoid of fleshy, frondose algae (herein, FA; Pinna et al., 2020). A. lixula has clear warm-water affinities compared to P. lividus and its frequency has been increasing in the last decades, even in the coldest regions of the Mediterranean Sea (Carreras et al., 2021). There is very little information about the behaviours of adult A.
