Global climate change and regional biotic responses: two hydrozoan tales
Résumé
Global climate change is affecting the planet's biodiversity, altering marine ecosystems and modifying species distributions. Using historical and recently collected data, changes in depth distribution of two hydrozoan taxa (Ectopleura crocea and Millepora spp.) with different life histories, ecology and geographic distributions were compared to trends in sea surface temperature (SST). Ectopleura crocea, a temperate species, used to be very common in Italian seas on shallow water artificial substrata, where it settled mainly between spring and autumn. In the Maldives, three species of the tropical hydrocoral Millepora (M. platyphylla, M. latifolia and M. tenera) were reported as common from the early 20th century until recently. Millepora underwent mass mortality during a bleaching event linked to a thermal positive anomaly caused by El Niño and lasting from April to July, 1998. Similarly, E. crocea disappeared from Italian surface waters where it was once abundant. Both the temperate and the tropical hydrozoans survived at moderate depth and recolonized shallow water habitats in recent years. These two hydrozoan examples support the depth refugia hypothesis, according to which marine organisms may shift to deeper habitats to avoid shallow water heatwaves and increased temperature variability. Return to shallow water has been possible only after acclimation such as shifting season of occurrence from summer to winter-spring (E. crocea in Italian seas) or possibly through species replacement (Millepora in the Maldives). Continued field research is needed to understand and evaluate the effect of climate change on marine species distribution.