High coral reef connectivity across the Indian Ocean is revealed 6-7 Ma ago by a turbid-water scleractinian assemblage from Tanzania (Eastern Africa).

The present centre of coral diversity in the Western Indian Ocean is defined by the northern Mozambique Channel with an extension northward to Mafia Island in Tanzania (Eastern Africa). The geological and evolutionary history of this hotspot of marine biodiversity remains so far completely obscure, because Cenozoic fossil reef communities of this area are not well known. This study presents a new fossil scleractinian fauna from the Mikindani Formation in southern Tanzania. It comprises 16 symbiotic coral taxa of which nine could be identified to the species and five to the genus level. Coral habitat consisted of low-relief biostromes that developed in shallow water at the front of the Rovuma Delta under conditions of variable sediment input. The biostromes are dated to be Messinian based on associated calcareous nannoplankton and planktic foraminifers. The studied coral assemblage shows close affinities with the Recent Western Indian Ocean biogeographic province and Central Indo-West Pacific biogeographic region as well as with the Miocene of Indonesia. Faunistic relations with the Oligocene-early Miocene of Somalia and Iran do not exist. The patterns of species distribution document a major palaeobiogeographic change in the Indian Ocean that correlates with the onset of the Miocene Indian Ocean Equatorial Jet during the middle Miocene. The clear Indonesian affinity of the Messinian coral fauna from southern Tanzania implies that this westerly oceanic surface current provided high biogeographic connectivity across the Indian Ocean during the late Miocene. Today, the coastal waters of Indonesia are located in the Coral Triangle. Diversification of this global epicentre of marine biodiversity started in the early Miocene and it was established already during the middle Miocene. Our results indicate that the East African hotspot of coral biodiversity originated as an offshoot of the Coral Triangle in the middle to late Miocene.

Impact of ocean acidification on crystallographic vital effect of the coral skeleton.

Distinguishing between environmental and species-specific physiological signals, recorded in coral skeletons, is one of the fundamental challenges in their reliable use as (paleo)climate proxies. To date, characteristic biological bias in skeleton-recorded environmental signatures (vital effect) was shown in shifts in geochemical signatures. Herein, for the first time, we have assessed crystallographic parameters of bio-aragonite to study the response of the reef-building coral Stylophora pistillata to experimental seawater acidification (pH 8.2, 7.6 and 7.3). Skeletons formed under high pCO2 conditions show systematic crystallographic changes such as better constrained crystal orientation and anisotropic distortions of bio-aragonite lattice parameters due to increased amount of intracrystalline organic matrix and water content. These variations in crystallographic features that seem to reflect physiological adjustments of biomineralizing organisms to environmental change, are herein called crystallographic vital effect (CVE). CVE may register those changes in the biomineralization process that may not yet be perceived at the macromorphological skeletal level.

A unique coral biomineralization pattern has resisted 40 million years of major ocean chemistry change.

Today coral reefs are threatened by changes to seawater conditions associated with rapid anthropogenic global climate change. Yet, since the Cenozoic, these organisms have experienced major fluctuations in atmospheric CO2 levels (from greenhouse conditions of high pCO2 in the Eocene to low pCO2 ice-house conditions in the Oligocene-Miocene) and a dramatically changing ocean Mg/Ca ratio. Here we show that the most diverse, widespread, and abundant reef-building coral genus Acropora (20 morphological groups and 150 living species) has not only survived these environmental changes, but has maintained its distinct skeletal biomineralization pattern for at least 40 My: Well-preserved fossil Acropora skeletons from the Eocene, Oligocene, and Miocene show ultra-structures indistinguishable from those of extant representatives of the genus and their aragonitic skeleton Mg/Ca ratios trace the inferred ocean Mg/Ca ratio precisely since the Eocene. Therefore, among marine biogenic carbonate fossils, well-preserved acroporid skeletons represent material with very high potential for reconstruction of ancient ocean chemistry.

Are coral reefs victims of their own past success?

As one of the most prolific and widespread reef builders, the staghorn coral Acropora holds a disproportionately large role in how coral reefs will respond to accelerating anthropogenic change. We show that although Acropora has a diverse history extended over the past 50 million years, it was not a dominant reef builder until the onset of high-amplitude glacioeustatic sea-level fluctuations 1.8 million years ago. High growth rates and propagation by fragmentation have favored staghorn corals since this time. In contrast, staghorn corals are among the most vulnerable corals to anthropogenic stressors, with marked global loss of abundance worldwide. The continued decline in staghorn coral abundance and the mounting challenges from both local stress and climate change will limit the coral reefs' ability to provide ecosystem services.