Pedro Medina-Rosas

Host-symbiont recombination versus natural selection in the response of coral-dinoflagellate symbioses to environmental disturbance.

Mutualisms between reef-building corals and endosymbiotic dinoflagellates are particularly sensitive to environmental stress, yet the ecosystems they construct have endured major oscillations in global climate. During the winter of 2008, an extreme cold-water event occurred in the Gulf of California that bleached corals in the genus Pocillopora harbouring a thermally ‘sensitive’ symbiont, designated Symbiodinium C1b-c, while colonies possessing Symbiodinium D1 were mostly unaffected. Certain bleached colonies recovered quickly while others suffered partial or complete mortality. In most colonies, no appreciable change was observed in the identity of the original symbiont, indicating that these partnerships are stable. During the initial phases of recovery, a third species of symbiont B1Aiptasia, genetically identical to that harboured by the invasive anemone, Aiptasia sp., grew opportunistically and was visible as light-yellow patches on the branch tips of several colonies. However, this symbiont did not persist and was displaced in all cases by C1b-c several months later. Colonies with D1 were abundant at inshore habitats along the continental eastern Pacific, where seasonal turbidity is high relative to offshore islands. Environmental conditions of the central and southern coasts of Mexico were not sufficient to explain the exclusivity of D1 Pocillopora in these regions. It is possible that mass mortalities associated with major thermal disturbances during the 1997–1998 El Niño Southern Oscillation eliminated C1b-c holobionts from these locations. The differential loss of Pocillopora holobionts in response to thermal stress suggests that natural selection on existing variation can cause rapid and significant shifts in the frequency of particular coral–algal partnerships. However, coral populations may take decades to recover following episodes of severe selection, thereby raising considerable uncertainty about the long-term viability of these communities.

Genetic Connectivity Patterns of Corals Pocillopora damicornis and Porites panamensis (Anthozoa: Scleractinia) along the West Coast of Mexico

Abstract: Genetic connectivity was studied in two scleractinian corals, Pocillopora damicornis (branching and broadcast spawner) and Pontes panamensis (massive and brooding type), along the Pacific coast of Mexico. Allelic diversity between adults and juveniles, the latter recruited after the El Niño—Southern Oscillation (ENSO) 1997–1998 event, was determined, and level of genetic connectivity among populations was assessed. There were no significant differences in allelic diversity between adults and juveniles from the same location. Seascape spatial genetic analysis suggested two or three clusters, depending on the species: (1) Bahías de Huatulco, (2) south of the Baja California Peninsula and Bahía de Banderas, and (3) locations in the Gulf of California. The most important barrier to gene flow was detected between Bahía de Banderas and Bahías de Huatulco and corresponds with a major coastal stretch of sandy beaches and lagoons. Moderate to high gene flow was found inside and at the entrance of the Gulf of California (Nem = 62–250), possibly favored by seasonal circulation patterns and sexual reproduction. In contrast, low gene flow was observed between southern populations and the rest of coastal Mexico (Nem < 1.7) based on high local recruitment and habitat discontinuity. A close genetic relationship of corals from the southern part of the Baja California Peninsula and severely damaged Bahía de Banderas coral communities confirmed that exchange of propagules could have taken place between the localities after the ENSO 1997–1998 event. Despite different reproductive strategies, both species showed similar patterns, suggesting the importance of surficial currents and habitat discontinuity to predict connectivity among coral reefs.

Reclutamiento de Porites (Scleractinia) sobre sustrato artificial en arrecifes afectados por El Niño 1997-98 en Bahía de Banderas, Pacífico mexicano

Mexican Pacific coral reefs were severely damaged by the sea surface warming observed during the 1997–98 El Niño. In Banderas Bay, massive coral mortality was as high as 97%, and the surviving colonies were exposed to sublethal temperatures and thermal stress. In this coral recruitment study we attempted to estimate the recovery and potential repopulation of corals in the Mexican Pacific. After the El Niño event, we used terracotta tiles to monitor the settlement of coral larval propagules at nine reefs in the region. The recruitment study took place from December 1998 to July 1999. Settlements of nine specimens of Porites were recorded at two localities. This is the first settlement record for this coral genus in the eastern tropical Pacific and the one with the largest number of specimens recruited during a study involving artificial substrate. The results indicate that these reefs do not totally depend on the arrival of allocthonous coral propagules from other regions of the tropical Pacific to maintain their development. This study also shows that corals maintain their reproductive activity despite the environmental disturbances. Therefore, sexual reproduction of reef corals of this region provides an effective alternative for the recovery of Mexican Pacific coral reefs.

CO2 enrichment and reduced seawater pH had no effect on the embryonic development of Acropora palmata (Anthozoa, Scleractinia).

The effects of decreased pH, caused by carbon dioxide (CO2) dissolution in seawater (known as ocean acidification (OA)), on the development of newly fertilized eggs of the Caribbean reef-building coral, Acropora palmata, was tested in three experiments conducted during the summers of 2008 and 2009 (two repeats). Three levels of CO2 enrichment were used: present day conditions (400 µatm, pH 8.1) and two CO2-enriched conditions (700 µatm, pH 7.9, and 1000 µatm, pH 7.7). No effects on the progression or timing of development, or embryo and larval size, were detected in any of the three experimental runs. The results show that the embryos and larvae of A. palmata are able to develop normally under seawater pH of at least 0.4 pH units lower than the present levels. Acropora palmata larvae do not usually begin to calcify after settlement, so this study only examined the non-calcifying part of the life cycle of this species. Most of the concern about the effects of OA on marine organisms centers on its effect on calcification. Negative effects of OA on the embryonic development of this species were not found and they may not manifest until the newly settled polyps begin to calcify.