Master Thesis

Black Corals (Hexacorallia: Antipatharia) in shallow and mesophotic reef ecosystems from Madagascar, and in a global perspective

Erika Belen Gress (05/2019-11/2019)

Corals in the order Antipatharia subclass Hexacorallia, commonly known as black corals, are crucial ecosystem engineers on shallow (0-30 m depth), mesophotic (30-150 m depth) and deep-sea (>200 m depth) reefs. These poorly studied colonial animals provide habitat to a vast number of marine faunae, enhancing and supporting corals reefs biodiversity globally. Coral reefs – the most biologically rich and productive marine ecosystems on earth – are being severely impacted by current global changes. One of these global alterations is climate change, which has caused detrimental effects on shallow and mesophotic reefs around the world. It is known that climate change alters the mutualistic endosymbiotic association corals in the subclass Hexacorallia have with dinoflagellates in the family Symbiodiniaceae. Alterations of this mutualistic relationship, due to oxidative stress, can cause corals mortality. Numerous studies have investigated this phenomenon on scleractinian (hard corals), however, almost nothing is known about how antipatharians are being or will be affected in this respect. Limited studies documented absence or low abundance of Symbiodiniaceae in antipatharians, and due to their ability to thrive in depth reefs and low-light environments, concerns of oxidative stress affecting antipatharians were latent. This changed with recent findings of high density of dinoflagellates within three shallow reef antipatharians; therefore, methodical studies with higher sample size are needed. I present here a study that investigates the presence, abundance, identity and location of Symbiodiniaceae within two species of antipatharians - Cupressopathes abies and Stichopathes maldivensis - from shallow and mesophotic reefs of SW Madagascar. This study represents the first systematic approach to corroborate the presence and estimate density of Symbiodiniaceae cells in antipatharians. Dinoflagellates were found within the tissues of the two antipatharian species from both shallow and mesophotic reefs, although the overall density of Symbiodiniaceae was very low (0-4 cell/μl). Yet, a trend of higher dinoflagellate densities in antipatharians on shallow reefs was observed. Both, overall low dinoflagellate density, although microalgae higher densities within antipatharians on shallow reefs, align with previous observations and studies. Further research on the symbiotic relationship between antipatharians and microalgae will be relevant on shallow reefs that are exposed to high light irradiance and that are vulnerable to increased water temperature. In addition, more detail studies on antipatharians and their association with dinoflagellates could help elucidate some of the mechanisms behind the disruption of the coral-algae mutualistic relationship - which are still not fully understood and are threatening shallow and mesophotic reefs health globally.