Global climate change and regional biotic responses: two hydrozoan tales.

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 croceaandMilleporaspp.) 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 hydrocoralMillepora(M. platyphylla,M. latifoliaandM. tenera) were reported as common from the early 20th century until recently.Milleporaunderwent 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. croceadisappeared 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. croceain Italian seas) or possibly through species replacement (Milleporain the Maldives). Continued field research is needed to understand and evaluate the effect of climate change on marine species distribution. [ABSTRACT FROM PUBLISHER]