Effects of global warming on reproduction and potential dispersal of Mediterranean Cnidarians

Abstract

Water temperature directly affects life cycles, reproductive periods, and metabolism of organisms living the oceans, especially inthe surface zones. Due to the ocean warming, changes in water stratification and primary productivity are affecting trophicchains in sensitive world areas, such as the Mediterranean Sea. Benthic and pelagic cnidarians exhibit complex responses toclimatic conditions. For example, the structure and phenology of the Mediterranean hydrozoan community displayed markedchanges in species composition, bathymetric distribution, and reproductive timing over the last decades. The regional speciespool remained stable in terms of species numbers but not in terms of species identity. When the Scyphozoa group is considered,we observe thatPelagia noctiluca(among the most abundant jellyfish in the Mediterranean Sea and eastern Atlantic waters) hasincreasingly frequent massive outbreaks associated to warmer winters. Variations in metabolic activities, such as respiration andexcretion, are strongly temperature-dependent, with direct increment of energetic costs with jellyfish size and temperature,leading to growth rate reduction. Water temperature affects sexual reproduction through changes in the energy storage andgonad development cycles. Anthozoan life cycles depend also on primary productivity and temperature: gonadal productionand spawning are tightly related in shallow populations (0–30 m depth) with the spring-summer temperature trends andautumn food availability. Overall, the energy transferred from the mother colonies to the offspring may decrease, negativelyaffecting their potential to settle, metamorphose and feed during thefirst months of their lives, eventually impairing thedominance of long-living cnidarian suspension feeders in shallow benthic habitats. In this review, we describe the alreadyongoing effects of sea warming on several features of cnidarian reproduction, trying to elucidate how reproductive traits andpotential dispersion will be affected by the cascade effects of increasing temperature in the Mediterranean Sea.