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ABSTRACT.A grand geological experiment with a global reach to its biological impact, the formation of the isthmus of Panama between 15 and 3 million years ago split the tropical Interamerican Seaway into two and substantially changed the physical oceanography of each part. That event subjected the now-divided halves of the neotropical marine biota to new environmental conditions that forced each along a different evolutionary trajectory. For the past 45 years the Smithsonian Tropical Research Institute (STRI) marine sciences program has taken full advantage of this event by sponsoring research on a great diversity of topics relating to the evolutionary effects of the formation of the isthmus. That research, which has been supported by multiple laboratories on each coast and a series of research vessels, has produced more than 1,800 publications. Here we provide an overview of the environmental setting for marine research in Panama and an historical perspective to research by STRI's scientifi c staff at the different marine facilities.

The authors investigated the response to experimentally elevated water temperature in genotypes of PociUopora damicornis from three coral reefs in the upwelling Gulf of Panama and four coral reefs in the non-upwelling Gulf of Chiriqui, Panamanian Pacific. Sea-surface temperature in the Gulf of Panama declines below 20 °C during seasonal upwelHng, while in the thermally stable Gulf of Chiriqui, the temperature ranges from 27 to 29 °C. Genotypes of P. damicornis from the seven locations were determined by allozyme electrophoresis. The most abundant genotype at each location was selected for a thermal tolerance experiment where corals were exposed to water temperature of 30 °C (1 °C above ambient) for 43 days. Four site coral genotypes can be uniquely differentiated by the GPI locus, two by the LGG-2 locus, and two by a combination of the MDH-1, LGG-2, and LTY-3 loci. A visual assessment of the coral condition after exposure to an elevated temperature showed that corals from localities in the non-upwelling environment retained a normal to slightly pale appearance, while corals from the upwelling environment bleached and their polyps were mostly retracted. A two-way ANO VA confirmed that corals were significantly affected by water temperature and locality. The zooxanthellae were also significantly affected by the interaction of elevated temperature and locahty of the corals. Mean zooxanthellae density decreased by 25 and 55%, respectively, in experimentally heated corals from the non-upwelling and upwelling environments. Low concentrations of photosynthetic pigments per five area of the corals were the norm in corals under elevated temperature. The mean concentration of chlorophyll a per live area of the corals was reduced by 17 and 49%, respectively, in heated corals from the non-upwelling and upwelling sites. Coral genotypes from the upwelhng Gulf of Panama demonstrated higher vulnerability to thermal stress than coral genotypes from the nonupwelhng Gulf of Chiriqui. However, the latter showed greater differences in their responses. Thus, even at small geographic scales, corals can display different levels of tolerance to thermal stress. The difference in thermal tolerance between corals from upwelling and nonupwelling environments is concomitant with greater genetic differences in experimental corals from the thermally stable Gulf of Chiriqui compared with corals from the upwelling Gulf of Panama.

Advances in our knowledge of eastern tropical Pacific (ETP) coral reef biogeography and ecology during the past two decades are briefly reviewed. Fifteen ETP subregions are recognized, including mainland and island localities from the Gulf of California (Mexico) to Rapa Nui (Easter Island, Chile). Updated species lists reveal a mean increase of 4.2 new species records per locality or an overall increase of 19.2 % in species richness during the past decade. The largest increases occurred in tropical mainland Mexico, and in equatorial Costa Rica and Colombia, due mainly to continuing surveys of these under-studied areas. Newly discovered coral communities are also now known from the southern Nicaraguan coastline. To date 47 zooxanthellate scleractinian species have been recorded in the ETP, of which 33 also occur in the central/south Pacific, and 8 are presumed to be ETP endemics. Usually no more than 20-25 zooxanthellate coral species are present at any given locality, with the principal reef-building genera being Pocillopora, Porites, Pavona, and Gardineroseris. This compares with 62-163 species at four of the nearest central/south Pacific localities. Hydrocorals in the genus Millepora also occur in the ETP and are reviewed in the context of their global distributions. Coral community associates engaged in corallivory, bioerosion, and competition for space are noted for several localities. Reef framework construction in the ETP typically occurs at shallow depths (2-8 m) in sheltered habitats or at greater depths (10-30 m) in more exposed areas such as oceanic island settings with high water column light penetration. Generally, eastern Pacific reefs do not reach sea level with the development of drying reef flats, and instead experience brief periods of exposure during extreme low tides or drops in sea level during La Niña events. High rates of mortality during El Niño disturbances have occurred in many ETP equatorial areas, especially in Panama and the Galápagos Islands during the 1980s and 1990s. Remarkably, however, no loss of resident, zooxanthellate scleractinian species has occurred at these sites, and many ETP coral reefs have demonstrated significant recovery from these disturbances during the past two decades.