The influence of natural particulate organic matter (POM) input on sedimentary oxygen consumption (SOC) in permeable carbonate and silicate sediments close to a coral reef was investigated in front of the Marine Science Station in Aqaba, Jordan (northern Red Sea). We conducted 7 in situ experiments in stirred benthic chambers. Without additional POM input, SOC rates were similar and not significantly different (p > 0.5) in carbonate and silicate sands, with average rates of 20 ± 4 (n = 10) and 16 ± 2 (n = 3) mmol O 2 m -2 d -1, respectively. Gross photosynthesis in the carbonate and silicate sands accounted for 15 to 23 mmol produced O 2 m -2 d -1, characterising both sands as largely independent of allochthonous carbon input. SOC of unamended carbonate sands showed no significant variation in 5 in situ experiments conducted within a period of 19 d. Addition of 2 energyrich sources of naturally occurring POM (coral mucus and clam eggs) resulted in significantly (p < 0.0001) increased SOC rates in the carbonate sands, but not in the silicate sands. Addition of a suspension containing high concentrations of zooxanthellae did not result in higher SOC in the carbonate sands, indicating that zooxanthellae cannot easily be degraded in reef sediments. Our results highlight the short cut between coral mucus production and degradation in the adjacent reef sands. Suspended particles are initially trapped by the cohesive mucus on the coral surface, and ensuing mucus strings sink to the seafloor at a short distance from the mucus-producing coral. Carbonate sands as porous filter systems obviously harbour more active heterotrophic microbial communities than silicate sands, and thus may constitute a major site of organic matter degradation in the reef ecosystem.

Giant clams are among the most spectacular but also the most endangered marine invertebrates. Their large size and easy accessibility has caused overfishing and collapse of the natural stocks in many places and local extinction in some of the species [1, 2]. The diversity of giant clams is extremely low because of reliction in this Tethyan group [3, 4]. The latest additions of living species date back almost two decades [5-7], fixing the number of extant Tridacna at seven species [3]. Here, we report the discovery of a new species of giant clam: Tridacna costata sp. nov. features characteristic shells with pronounced vertical folds, is genetically distinct, and shows an earlier and abbreviated reproduction than its Red Sea congeners. This species represents less than 1% of the present stocks but up to >80% of the fossil shells. The decline in proportion and shell size (20x) indicates overharvesting [8] dating back to the early human occupation of the Red Sea >125,000 years ago [9]. This earliest depletion reported so far of a shallow-water megafaunal invertebrate has important ramifications for human dispersal out of Africa [10]. Its oversight in one of the best-investigated reef provinces [11-13] illustrates the dearth of knowledge on marine biodiversity.

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Tropical scleractinian corals are particularly vulnerable to global warming as elevated sea surface temperatures (SSTs) disrupt the delicate balance between the coral host and their algal endosymbionts, leading to symbiont expulsion, mass bleaching and mortality. While satellite sensing of SST has proved a reliable predictor of coral bleaching at the regional scale, there are large deviations in bleaching severity and mortality on the local scale that are poorly understood. Here, we show that internal waves play a major role in explaining local coral bleaching and mortality patterns in the Andaman Sea. Despite a severe region-wide SST anomaly in May 2010, frequent upslope intrusions of cold sub-pycnocline waters due to breaking large-amplitude internal waves (LAIW) mitigated coral bleaching and mortality in shallow waters. In LAIW-sheltered waters, by contrast, bleaching-susceptible species suffered severe bleaching and total mortality. These findings suggest that LAIW benefit coral reefs during thermal stress and provide local refugia for bleaching-susceptible corals. LAIW are ubiquitous in tropical stratified waters and their swash zones may thus be important conservation areas for the maintenance of coral diversity in a warming climate. Taking LAIW into account can significantly improve coral bleaching predictions and provide a valuable tool for coral reef conservation and management.

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