The historical reconstruction of distribution of the genus Halecium (Hydrozoa: Haleciidae): a biological signal of ocean warming?

The distribution of 130 nominal species of the genus Halecium, based on published records, has been mapped for the first time in a comprehensive set of marine ecoregions, to analyse their distribution. Most Halecium species are found at mid- and high latitudes, with some overlaps in distribution ranges across regions. The species richness of Halecium is strongly related to the latitudinal gradient, with maximal diversity at polar and temperate latitudes. Previous detailed studies in the Mediterranean Sea show that large Halecium species of coldwater affinity have regressed or disappeared in recent years, probably due to global warming. Worldwide, however, the overall species richness of Halecium has not changed along the latitudinal gradient over recent decades, with some changes in species composition at temperate-tropical latitudes in both hemispheres, even though the majority of the species that have not been recorded for more than 50 years are of coldwater affinity. The genus can be considered an indicator for biological responses to climate changes for the Mediterranean Sea, but the available distribution data do not allow extending this possibility to the rest of the world. A focused evaluation on the distribution of Halecium species with the addition of new field data might reinforce the picture stemming from the present analysis.