The Ligurian Sea, situated in the north-east corner of the western Mediterranean, is colder than the Tyrrhenian Sea, situated just to the south. As a consequence, there are differences in the composition and physionomy of their respective marine biota. Episodical records of epibenthic warm-water species, of Tyrrhenian origin, in the colder Ligurian Sea were paradoxically linked to the occurrence of low mean air temperatures. This is explained by the peculiar pattern of water and heat exchanges between the Tyrrhenian and Ligurian basins mediated through the northward-flowing Tyrrhenian Current. In response to the greater cooling of the Ligurian Sea, induced by the more intense water and temperature losses, a seasonal flux of warmer water is drawn from the Tyrrhenian Sea in order to restore the altered budget: the more intense the Ligurian Sea winter cooling, the larger the volume of the warmer flow carried northward by the Tyrrhenian Current, increasing the probability of warm-water species transport into the Ligurian Sea. Survival of warm-water species in cold years is unlikely, and this explains the rarity of findings in the last one and a half centuries. In a few cases, warm-water species have succeeded in establishing adult pseudopopulations. However, this scenario is probably changing: the detection of warm-water species in the Ligurian Sea has become more frequent and nearly constant in recent years, even in the presence of high winter temperatures. It is hypothesized that present sea water warming is allowing former sterile pseudopopulations to reproduce in the Ligurian Sea, thus assuring independence from the larval supply by the Tyrrhenian Current. Future investigations should concentrate on monitoring the population biology of warm-water species established in the Ligurian Sea.