Long-term life cycle and massive blooms of the intertidal hydroid Paracoryne huvei in the North-western Mediterranean Sea

Paracoryne huvei is a rare encrusting hydrozoan living in the intertidal zone of the North-western Mediterranean Sea, from the Catalan coast to the Ligurian Sea. In 2014, a large bloom of P. huvei was recorded along the cliffs of the Marine Protected Area 'Isola di Bergeggi' (Ligurian Sea). The species has shown densities related to important biomass values never previously recorded (wet weight as high as 1600 g m2), creating a continuous belt up to 1 m wide from February to April. At an increasing distance from the promontory, the species decreased in abundance, assuming a spotted aspect comparable to the colonies recorded by earlier researchers. In the Bergeggi area, the species has exhibited an almost complete cover of the lower intertidal zone, and for the first time it was recorded in the infralittoral and upper intertidal zone and in tide pools, demonstrating a wider range of habitat than had previously been observed. The life cycle appeared significantly shorter than cycles reported in earlier literature, and this phenomenon has been compared with similar observations from other shallow water hydrozoans in the same area. The abundance of the species was continually monitored in 2015-2016 to help assess the inter-annual variations in relation to temperature and rainfall trends and, indirectly, to competition with other species. Finally, a comparison of the 59 years of scattered data of occurrence in the literature has shown P. huvei to be a strict stenothermal species, reported only in winters characterized by average temperatures.