Carin Jantzen

Collapse of a New Living Species of Giant Clam in the Red Sea

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.

Large-amplitude internal waves benefit corals during thermal stress.

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.

Metabolic plasticity of the corals Porites lutea and Diploastrea heliopora exposed to large amplitude internal waves

The metabolic plasticity of the two mounding coral species Porites lutea (Milne-Edwards and Haime, 1860) and Diploastrea heliopora (Lamarck, 1816) was investigated in the Similan Islands (Thailand), an offshore Andaman Sea island group subjected to large amplitude internal waves (LAIW). Nutrient concentrations were highly correlated with LAIW intensity and contributed to 3- and 10-fold higher symbiont densities in P. lutea and D. heliopora, respectively, along with elevated pigment concentrations, protein content, host tissue, and symbiont biomass. The comparison of LAIW-exposed and LAIW-sheltered island faces, and LAIW-intense and LAIW-weak years suggests a species-specific metabolic plasticity to LAIW, where D. heliopora benefits more from increased nutrient and organic matter availability than P. lutea. The ubiquitous LAIW in Southeast Asia and beyond may provide so far unexplored clues to coral acclimatization to disturbances on various scales, and hence, a potential key to coral resilience to climate change.

Benthic reef primary production in response to large amplitude internal waves at the Similan Islands (Andaman Sea, Thailand).

Coral reefs are facing rapidly changing environments, but implications for reef ecosystem functioning and important services, such as productivity, are difficult to predict. Comparative investigations on coral reefs that are naturally exposed to differing environmental settings can provide essential information in this context. One prevalent phenomenon regularly introducing alterations in water chemistry into coral reefs are internal waves. This study therefore investigates the effect of large amplitude internal waves (LAIW) on primary productivity in coral reefs at the Similan Islands (Andaman Sea, Thailand). The LAIW-exposed west sides of the islands are subjected to sudden drops in water temperature accompanied by enhanced inorganic nutrient concentrations compared to the sheltered east. At the central island, Ko Miang, east and west reefs are only few hundred meters apart, but feature pronounced differences. On the west lower live coral cover (-38 %) coincides with higher turf algae cover (+64 %) and growth (+54 %) compared to the east side. Turf algae and the reef sand-associated microphytobenthos displayed similar chlorophyll a contents on both island sides, but under LAIW exposure, turf algae exhibited higher net photosynthesis (+23 %), whereas the microphytobenthos displayed reduced net and gross photosynthesis (-19 % and -26 %, respectively) accompanied by lower respiration (-42 %). In contrast, the predominant coral Porites lutea showed higher chlorophyll a tissues contents (+42 %) on the LAIW-exposed west in response to lower light availability and higher inorganic nutrient concentrations, but net photosynthesis was comparable for both sides. Turf algae were the major primary producers on the west side, whereas microphytobenthos dominated on the east. The overall primary production rate (comprising all main benthic primary producers) was similar on both island sides, which indicates high primary production variability under different environmental conditions.

In situ short-term growth rates of a cold-water coral

Cold-water corals are known to grow much slower than their tropical counterparts. However, this assumption is mainly based on laboratory measurements exposing specimens to conditions that differ from their natural environments. The cosmopolitan scleractinian Desmophyllum dianthus forms dense banks below 18 m in northern Patagonia, Chile. So as to measure in situ growth rates of this cold-water coral, specimens were collected from two sites, weighed and deployed on holders in their natural headlong orientation at the respective collecting site. Corals exhibited a calcium carbonate (CaCO3) mass increase of 5.44 ± 3.45 (mg (cm2 projected calyx area)–1 day–1) after 2 weeks, equivalent to a mass gain of 0.25 ± 0.18 s.d. % day–1. In comparison, D. dianthus specimens from the same collection sites maintained in an on-site flow-through aquarium system showed lower growth rates that were third of the in situ rates. In situ CaCO3 precipitation of D. dianthus extrapolated for 1 year (kg m2 year–1) displays the same order of magnitude as reported for massive growing tropical scleractinians, e.g. Porites sp.

Mineralization of particulate organic matter derived from coral-reef organisms in reef sediments of the Gulf of Aqaba

In situ and laboratory incubation experiments in a fringing reef in the Gulf of Aqaba were performed to study degradation rates of particulate organic matter in reef sediments. Coral mucus, clam eggs, and zooxanthellae were used as model particulate organic compounds for these experiments. Aerobic and anaerobic mineralization rates were calculated by dissolved inorganic carbon (DIC) and O2 fluxes from the sediments under different particulate organic matter additions. Fast enhancement (approximately twofold) of O2 and DIC fluxes were found with the addition of coral mucus and clam eggs compared with control incubations without addition. Most of the degradation is believed to have occurred anaerobically rather than aerobically (DIC:O2 ratios were 4.3–28.1). Higher degradation rates of coral mucus and clam eggs were estimated in carbonate sediment than in silicate sediment (1.2–1.6-fold), which was attributed to the different physical and chemical properties of both sediments. Our study shows the significance of the reef sediment as a suitable site for microbial degradation of particulate organic material excreted from different reef community organisms. This may increase the regeneration of nutrients in the reef environment necessary to sustain high biological productivity.

Turf algae-mediated coral damage in coastal reefs of Belize, Central America

Many coral reefs in the Caribbean experienced substantial changes in their benthic community composition during the last decades. This often resulted in phase shifts from scleractinian coral dominance to that by other benthic invertebrate or algae. However, knowledge about how the related role of coral-algae contacts may negatively affect corals is scarce. Therefore, benthic community composition, abundance of algae grazers, and the abundance and character of coral-algae contacts were assessed in situ at 13 Belizean reef sites distributed along a distance gradient to the Belizean mainland (12–70 km): Mesoamerican Barrier Reef (inshore), Turneffe Atoll (inner and outer midshore), and Lighthouse Reef (offshore). In situ surveys revealed significantly higher benthic cover by scleractinian corals at the remote Lighthouse Reef (26–29%) when compared to the other sites (4–19%). The abundance of herbivorous fish and the sea urchin Diadema antillarum significantly increased towards the offshore reef sites, while the occurrence of direct coral-algae contacts consequently increased significantly with decreasing distance to shore. About 60% of these algae contacts were harmful (exhibiting coral tissue damage, pigmentation change, or overgrowth) for corals (mainly genera Orbicella and Agaricia), particularly when filamentous turf algae were involved. These findings provide support to the hypothesis that (turf) algae-mediated coral damage occurs in Belizean coastal, near-shore coral reefs.

Large-amplitude internal waves sustain coral health during thermal stress

Ocean warming is a major threat for coral reefs causing widespread coral bleaching and mortality. Potential refugia are thus crucial for coral survival. Exposure to large-amplitude internal waves (LAIW) mitigated heat stress and ensured coral survival and recovery during and after an extreme heat anomaly. The physiological status of two common corals, Porites lutea and Pocillopora meandrina, was monitored in host and symbiont traits, in response to LAIW-exposure throughout the unprecedented 2010 heat anomaly in the Andaman Sea. LAIW-exposed corals of both species survived and recovered, while LAIW-sheltered corals suffered partial and total mortality in P. lutea and P. meandrina, respectively. LAIW are ubiquitous in the tropics and potentially generate coral refuge areas. As thermal stress to corals is expected to increase in a warming ocean, the mechanisms linking coral bleaching to ocean dynamics will be crucial to predict coral survival on a warming planet.

Gaining Ground—Cleaning with Mesenterial Filaments by the Cold-Water Coral Desmophyllum Dianthus?

The solitary cosmopolitan cold-water coral Desmophyllum dianthus (Esper, 1794) (A, scale bar approximately 1 cm) occurs as a deep-water emergent species in Patagonian fjords, Chile, and can form extensive banks (Forsterra et al. 2005). This coral functions as a bioengineer by providing a 3D habitat for a diverse benthic community (Forsterra et al. 2005). With skeletal growth, the living tissue is retracted by the organism to the upper part of the calyx, leaving the lower skeleton uncovered. Endolithic algae are then able to enter the tissue covered skeleton from below and start to encrust the tissue covered part of the skeleton (Forsterra et al. 2005, 2008). They are visible as a greenish coat on the skeleton (A). The algae are able to bore into the skeleton (Forsterra et al. 2005) and thus could interfere with the coral’s calcifying process. This process could result in a parasitic behavior (Forsterra et al. 2012) rather than the previously hypothesized (Forsterra and Haussermann 2008) symbiotic or even mutualistic relationship. This infection by algae may be the primary reason for the peculiar behavior we observed on several specimens both in situ (A) and in our aquaria system (B–D, scale bar approximately 1 cm). After an acclimatization time in our aquaria system, wild collected corals were well adapted and well fed, and secondarily expanded their tissue toward their lower skeleton (about 1 cm in 4 mo, CJ and GMS pers obs), recovering an area that had been without any tissue. Concomitantly, the corals everted their mesenterial filaments between this re-entered outer-skeleton and the covering tissue layer (A, B; C, D: both enlargements of B). During the eversion of the mesenterial filaments the new tissue generally appeared unusually pleated (C, D) in contrast to a less common smooth tissue surface (A). This was not a short-term or rare mechanism: corals maintained this behavior in the aquaria over multiple months. Several tropical coral species are known to eject their mesenterial filaments to digest prey too large to be swallowed (Yonge 1930). The tropical, branching coral Acropora pulchra (Brook, 1891) has even been observed to clean the substrate before it expands its tissue and skeleton (Roff et al. 2008). Our preliminary observations suggest a combination of both mechanisms in this cold-water coral. Mesenterial filaments could be digesting the growing algae below their tissue, but also may be cleaning the substrate (their own skeleton) prior to further tissue expansion.