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Ocean acidification is expected to significantly affect calcifying marine organisms, such as corals. While ocean acidification effects on coral calcification have been widely investigated, the effects on coral gametogenesis are almost unknown. A volcanic vent at Panarea Island (Mediterranean Sea, Italy) characterized by continuous and localized CO2 emission, generates a stable pCO2 gradient from the centre to its periphery, providing a unique natural laboratory for ocean acidification studies. Here we assessed the effects of ocean acidification on gametogenesis and embryogenesis of the Mediterranean non-zooxanthellate coral Leptopsammia pruvoti transplanted along this natural pCO2 gradient. After 3 months under experimental conditions, increasing pCO2 seemed to negatively influence spermary production and development. In the control Site, all spermaries showed the latest (most advanced) maturation stage while in the most acidic Sites they also displayed earlier stages, indicating a delay in spermatogenesis and consequently of fertilization and planulation processes. Considering the negative response showed by this species after a short-term exposure, an even stronger negative effect on sexual reproduction is hypothesized under a long-term exposure.

Oceanic uptake of anthropogenic CO2 is causing significant changes in ocean chemistry, reducing pH and increasing sea surface temperature. A decline of 0.3–0.5 pH units and 2.0-4.5 °C of temperature are expected by 2100. Few information are available regarding the effects of ocean acidification on sexual reproduction in corals and none on temperate corals. This study assessed the effects of ocean acidification on sexual reproduction in three Mediterranean scleractinian corals: Balanophyllia europaea (solitary, zooxanthellate), Leptopsammia pruvoti (solitary, azooxanthellate), and Astroides calycularis (colonial, azooxanthellate). The relationship between reproductive parameters (fecundity rate, spermary abundance, gonadal index, reproductive element size) and pCO2 were assessed at an underwater volcanic crater characterized by continuous emissions of CO2 which create a natural pH gradient, located near the island of Panarea (Tyrrhenian Sea, Italy). Samples of the three species were transplanted along the pH gradient in four sites from the center of crater (pH 7.4) to its periphery (control). The biological responses were analyzed according to different reproductive (gonochoric vs hermaphrodite) and metabolic (zooxanthellate vs non zooxanthellate) strategies of each species.

Despite extensive studies focus mainly on sexual reproductive characteristics in tropical scleractinian species, there is limited knowledge on temperate regions. The Mediterranean is a biodiversity hotspot under intense pressure from anthropogenic impacts. Climatic models further predict that the Mediterranean basin will be one of the most impacted regions by the ongoing warming trend. This makes it a potential model of more global patterns to occur in the world’s marine biota, and a natural focus of interest for research on climate. The present research contributed to increase data on reproductive modes and sexuality of temperate scleractinian corals, highlighting their developmental plasticity, showing different forms of propagation and different responses to environmental change. For the first time, sexuality and reproductive mode in Caryophyllia inornata were determined. An unusual embryogenesis without a clear seasonal pattern was observed, suggesting the possibility of an asexual origin. Sexual reproduction of Astroides calycularis was governed by annual changes in seawater temperature, as observed for other Mediterranean dendrophylliids. Defining the reproductive biology of these species is the starting point for studying their potential response to variations of environmental parameters, on a global climate change context. The results on the influence of temperature on reproductive output of the zooxanthellate (symbiosis with unicellular algae) Balanophyllia europaea and the non-zooxanthellate Leptopsammia pruvoti suggest that the latter may be quite tolerant to temperature increase, since the zooxanthellate species resulted less efficient at warm temperatures. A possible explanation could be related to their different trophic system. In B. europaea thermal tolerance is primarily governed by the symbiotic algae, making it more sensitive to temperature changes. On the contrary, the absence of symbionts in L. pruvoti might make it more resistant to temperature. In a progressively warming Mediterranean, the efficiency on scleractinian reproduction could be influenced in different ways, reflecting their extraordinary adaptability.

Le conoscenze sulla biologia riproduttiva dei coralli delle zone temperate sono ancora molto limitate, in modo particolare per le scleractinie del Mediterraneo. E’ necessario incrementare gli studi sulla riproduzione di questi organismi e quantificarne la resilienza al potenziale impatto dei cambiamenti climatici, che si prevede più importante nella fascia temperata rispetto a quella tropico-equatoriale. Scopo di questo studio è definire il ciclo riproduttivo di Caryophyllia inornata (Duncan, 1878), specie temperata, che ha una distribuzione geografica dal Mediterraneo alle coste Atlantiche nord-orientali. Questa ricerca rientra nel progetto FP7–IDEAS-ERC “CoralWarm” (http://www.CoralWarm.eu/) che si propone di studiare l’influenza dell’aumento della temperatura del mare sulla biologia delle scleractinie del Mediterraneo.

Leptopsammia pruvoti is a common azooxanthellate coral living in the Mediterranean Sea and along the Atlantic coast from Portugal to southwestern of England. It lives in shaded habitats from shallow water to a depth of more than 100 meters. The annual cycle of sexual reproduction in this species is studied in an area nearby Leghorn (Tuscany, Italy). L. pruvoti is a gonochoric and brooder. Testes require 1 yr to reach maturity with a maximum spermary diameter of 340 µm. Oogenesis lasts 2 yr with a maximum diameter of 680 µm. The rate of gonadic development increases significantly during December and January, fertilization takes place from February to May and planulation in June. Embryos have a maximum diameter of 740 µm in the first stages (periblastula), and 860 µm in mature gastrula. Released larvae have a maximum diameter of 1000 µm. The annual cycle of photoperiod and water temperature seems to coincide with the reproductive cycle. Different sex allocation has been detected between males and females of L. pruvoti. Volume occupied by male gonads has been found to be 4 times bigger than volume occupied by female gonads. In the self-fertilizing hermaphrodite Balanophyllia europaea, a close relative species studied in the same area, male and female gonads occupied the same volume. According to sex-allocation theory, a self-fertilizing species require a smaller male allocation than the cross-fertilizing species, in order to guarantee successful reproduction.

Shallow-water corals of the Mediterranean Sea are facing a dramatic increase in water temperature due to climate change, predicted to increase the frequency of bleaching and mass mortality events. However, supposedly not all corals are affected equally, as they show differences in stress susceptibility, as suggested by physiological outputs of corals along temperature gradients and under controlled conditions in terms of reproduction, demography, growth, calcification, and photosynthetic efficiency. In this study, gene expression and induction of a 70-kDa heat shock protein (HSP70) was analyzed in five common shallow-water hard corals in the Mediterranean Sea, namely Astroides calycularis, Balanophyllia europaea, Caryophyllia inornata, Cladocora caespitosa, and Leptopsammia pruvoti. The main aim was to assess the contribution of this evolutionary conserved cytoprotective mechanism to the physiological plasticity of these species that possess different growth modes (solitary vs colonial) and trophic strategies (zooxanthellate vs azooxanthellate). Using quantitative real-time PCR, in situ hsp70 baseline levels and expression profiles after a heat-shock exposure were assessed.Levels of hsp70 and heat stress induction were higher in zooxanthellate than in azooxanthellate species, and different heat stress transcriptional profiles were observed between colonial and solitary zooxanthellate corals. On the whole, the hsp70 transcriptional response to heat stress aligns with stress susceptibility of the species and suggests a contribution of trophic strategy and morphology in shaping coral resilience to stress. Understanding these molecular processes may contribute to assess the potential effects and relative resilience of Mediterranean corals under climate change. published

This study investigates for the first time the transcriptional regulation of a stress-inducible 70-kDa heat shock protein (hsp70) in the scleractinian coral Pocillopora verrucosa sampled at three locations and two depths (3 m and 12 m) in Bangka Island waters (North Sulawesi, Indonesia). Percentage of coral cover indicated reduced habitat suitability with depth and at the Tanjung Husi (TA) site, which also displayed relatively higher seawater temperatures. Expression of the P. verrucosa hsp70 transcript evaluated under field conditions followed a depth-related profile, with relatively higher expression levels in 3-m collected nubbins compared to the 12-m ones. Expression levels of metabolism-related transcripts ATP synthase and NADH dehydrogenase indicated metabolic activation of nubbins to cope with habitat conditions of the TA site at 3 m. After a 14-day acclimatization to common and fixed temperature conditions in the laboratory, corals were subjected for 7 days to an altered thermal regime, where temperature was elevated at 31°C during the light phase and returned to 28°C during the dark phase. Nubbins collected at 12 m were relatively more sensitive to thermal stress, as they significantly over-expressed the selected transcripts. Corals collected at 3 m appeared more resilient, as they showed unaffected mRNA expressions. The results indicated that local habitat conditions may influence transcription of stress-related genes in P. verrucosa. Corals exhibiting higher basal hsp70 levels may display enhanced tolerance towards environmental stressors.