| Summary: | The black sea urchin Arbacia lixula (Linnaeus, 1758) is one of the most abundant sea urchins in the Mediterranean. Despite its increasingly recognized ecological significance, including the ability to create and maintain barren zones of decreased diversity and productivity, it has been traditionally less studied than the Atlanto-Mediterranean edible sea urchin Paracentrotus lividus. This thesis is aimed at studying the factors affecting the phylogeography, trophic ecology and biology of the black sea urchin A. lixula in Mediterranean ecosystems, in order to assess its ecological role and its possible future impact in benthic communities. We studied the phylogeography, the trophic relationships, the reproductive cycle and the effects of temperature and ocean acidification on larval development of A. lixula. Our phylogeographical study, based in the mitochondrial marker COI, found evidences of a relatively recent (94 – 205 kya) colonization of the Mediterranean by A. lixula. Our trophic study, using stable isotopes and gut contents analyses, showed that A. lixula is an omnivore tending to carnivory, feeding mainly on sessile invertebrates. This results challenge the previous belief that A. lixula was an herbivorous grazer of encrusting coralline algae. The results of the reproductive cycle study, followed up during four consecutive years, showed that the reproductive timing of A. lixula is regulated by photoperiod, while temperature is a main modulator of its gonad development and thus of its reproductive output. The study of larval development in different conditions of temperature and pH showed that temperature increases accelerated the development and enhanced the larval survival rate, while acidification caused only slight effects in its survival, developmental rate and larval morphology. Considering all these new results about the past history and ecological characteristics of A. lixula, we conclude that this thermophilous species of tropical affinities is probably facing suboptimal conditions in northern ...
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