Coral Reefs of the Eastern Tropical Pacific

During the Nineteenth and early Twentieth Centuries several renowned researchers—e.g., C. Darwin, J. Dana, A. Agassiz, A.E. Verrill, and T.W. Vaughan—remarked on the absence of coral reefs in the eastern tropical Pacific (ETP), concluding this was due mainly to the influence of cool currents and upwelling centers. From later surveys in southern Mexico in the 1920s (R.H. Palmer), and the Allan Hancock Expeditions across the eastern tropical Pacific in the 1930s (C. Wyatt Durham), workers began to recognize the presence of modest structural coral reef development at several sites in the ETP. A new era of coral reef studies began in the 1970s, and has increased in pace and intellectual depth to the present. This latest surge in coral science was a result of locally active researchers, especially in Mexico, Costa Rica, Panama, and Colombia, and more recently in Ecuador and Chile. Progress in coral research in El Salvador and Nicaragua was thwarted from the 1970s to the early 1990s because of political turmoil and violence in this sector of the eastern Pacific. The establishment of national research funding agencies in Latin American countries, such as CONACYT (Mexico), CONICIT (Costa Rica), COLCIENCIAS (Colombia), and FONDECYT (Chile), has greatly benefitted coral reef research as well as scholarship opportunities for resident students to pursue advanced studies in marine science. The availability of vessels to support research in remote areas and at offshore sites has also facilitated ETP coral studies. Contrary to early views, structural coral reefs were discovered in major upwelling centers—southern Mexico (Huatulco), Costa Rica (Guanacaste), and Panama (Gulf of Panama). The number of known coral species approximately doubled from the late 1940s to 2000, from 60 (total scleractinians) and <20 (reef-building or zooxanthellate corals) to >120 and >40 species, respectively. Additionally, reports of the appearance or disappearance of extralimital coral species (e.g., in the genera Montipora, Siderastrea, Acropora, and Millepora) have added to the excitement and adventure of eastern Pacific studies. On site observations during two very strong El Niño-Southern Oscillation (ENSO) disturbances, 1982–83 and 1997–98, demonstrated that acute temperature stress can cause widespread coral bleaching and mortality and interference with reef growth. More recently, ETP waters have been found to have low pH and aragonite saturation state, which can also weaken carbonate structures and interfere with reef development. Recognizing the economic, scientific, and esthetic value of ETP coral reefs, several national and international agencies are now supporting coral research, which will have a salutary effect in increasing our knowledge and understanding of these long-neglected ecosystems.

Distribution, composition and functioning of eastern Pacific (EP) coral communities and reefs have resulted from modern and ancient processes. While much has been learned from living systems, still many questions remain regarding how pre-Holocene events shaped modern EP coral communities and reefs. From the late Paleocene to late Miocene, fossil outcrops were spatially restricted to the Washington-Seattle and California regions, whereas from the late Miocene to late Pleistocene corals have been mainly recovered from the Gulf of California, but scarcely (just two outcrops) from western Mexico and Central America. From the Paleocene to Recent, 191 reef-building species, including living taxa without fossil representatives, have inhabited the eastern Pacific region. Of the 53 identified genera, 11 are living and 42 are regional (i.e., currently restricted to the Indo-Pacific or Caribbean and western Atlantic) or globally extinct; 49 species are living and 142 are extinct. Fourteen of the 48 living EP species have fossil records that extend back to the early (Porites panamensis) and middle (Pocillopora capitata) Pliocene, while the remainder appeared during the middle to late Pleistocene in the Gulf of California. Considering the age and number of taxa per assemblage, genera and species ranged from 1 to 21 (mean = 2.98 ± 3.75 SD), and 1–39 (mean = 3.87 ± 6.12 SD) respectively. The highest numbers of genera (>11) and species (>12) correspond with the middle Eocene to early Miocene of Central Chiapas, and the late Eocene of Panama. As such, EP coral communities and reefs probably never attained high species richness; in addition, depositional evidence and paleoenvironmental reconstructions of coral-bearing deposits from Washington-Seattle, Central California and the Gulf of California suggest that from late Paleocene to late Pleistocene, reefs were small and paucispecific as are today’s formations. Based on coral occurrences, species richness increased during the Late Eocene/Early Oligocene (55 species), decreased during the early and middle Pleistocene (5 species), rose again during the late Pleistocene (13 species) and finally peaked again during Recent times (46 species). Bootstrap tests of the temporal changes in species richness indicate that except for the numbers recorded in Late Eocene-Early Oligocene, Late Pleistocene and Recent, changes elsewhere were non-significant. From the spatio-temporal distribution of the fossil outcrops, our knowledge regarding events directly related to the origin and evolution of the current coral fauna is biased since it represents the evolutionary history of subtropical coral communities and reefs from the Gulf of California. Quantitative analysis of presence/absence coral data suggests that during the last six million years, Gulf coral communities and reefs experienced dramatic turnover, in particular (a) the extinction of Caribbean-related regional endemics, and (b) since the middle Pleistocene, the steady arrival of Indo-Pacific taxa likely via the North Equatorial Counter Current. Lastly, morphologic, electrophoretic and molecular analyses clearly support a strong affinity of the eastern Pacific endemics, Porites panamensis and Porites sverdrupi, with the Caribbean and western Atlantic Porites clades, suggesting that the eastern Pacific poritid fauna resulted from local speciation of Caribbean/Atlantic populations and long-distance dispersal of Indo-Pacific taxa (e.g., Porites australiensis, Porites lutea). Hence, the vicariance and dispersal hypotheses regarding the origin of the eastern Pacific coral fauna, at least for some taxa, are complementary.

The eastern Pacific warm pool supports reef-building corals, as well as distinct communities of plankton, fishes, marine mammals and birds. This habitat is characterized by warm, low-salinity surface water lying on top of a strong, shallow thermocline. It is bounded by the South Equatorial Current and equatorial cold tongue to the south, cooler and more saline subtropical water to the northwest, and cold eastern boundary currents to the north and south (California and Peru Currents). The continental boundary influences atmospheric forcing by gap winds during winter and by causing the rainy Intertropical Convergence Zone to be located north of the equator and over the warm pool. Patterns of waves, tides and tropical cyclones impinging on coral reefs are described. The structure and variability of water masses and circulation are determined by solar and atmospheric processes, both within and outside of the region. To the west of the Galápagos, surface circulation is predominantly the east-west equatorial currents. Near the coast, surface circulation is modified by the coastal boundary, local winds, eddies, and interaction with eastern boundary currents. Primary productivity depends on oceanic upwelling along the equator and local centers of upwelling and wind mixing in coastal waters. Eastern tropical Pacific surface waters are moderately productive. Phytoplankton productivity is limited by a lack of the micronutrient dissolved iron, except where local coastal processes provide a source, so that macronutrients such as nitrate are never depleted. Seasonal changes in solar forcing, winds, rainfall, surface temperature and salinity, and other environmental characteristics are described, although seasonality in this region is not as pronounced as at higher latitudes. In contrast, interannual variations caused by the El Niño-Southern Oscillation across the entire tropical Indo-Pacific are very important in this region (Chap. 4). Oxygen depletion is extreme below the sharp thermocline, with consequences for mesopelagic and subthermocline benthic organisms. Surface waters are relatively low pH and marginally carbonate-saturated. Climate change is predicted to lead to future oceanographic changes in this region: warming and acidification of surface waters, increased stratification and reduced productivity, and upwelling/mixing of hypoxic waters into the surface layer. These changes are likely to affect organisms and populations living in the eastern tropical Pacific.

The ENSO observing system in the tropical Pacific plays an important role in monitoring ENSO and helping improve the understanding and prediction of ENSO. Occurrence of ENSO has been explained as either a self-sustained and naturally oscillatory mode of the coupled ocean-atmosphere system or a stable mode triggered by stochastic forcing. In either case, ENSO involves the positive ocean-atmosphere feedback hypothesized by Bjerknes. After an El Niño reaches its mature phase, negative feedbacks are required to terminate growth of the mature El Niño anomalies in the central and eastern Pacific. Four negative feedbacks have been proposed: reflected Kelvin waves at the ocean western boundary, a discharge process due to Sverdrup transport, western Pacific wind-forced Kelvin waves, and anomalous zonal advections. These negative feedbacks may work together for terminating El Niño, with their relative importance varying with time. Because of different locations of maximum SST anomalies and associated atmospheric heating, El Niño events are classified as eastern and central Pacific warming events. The identification of two distinct types of El Niño offers a new way to examine global impacts of El Niño and to consider how El Niño may respond and feedback to a changing climate. In addition to interannual variations associated with ENSO, the tropical Pacific SSTs also fluctuate on longer timescales. The patterns of Pacific Decadal Variability (PDV) are very similar to those of ENSO. When SST anomalies are positive in the tropical eastern Pacific, they are negative to the west and over the central North and South Pacific, and positive over the tropical Indian Ocean and northeastern portions of the high-latitude Pacific Ocean. Many mechanisms have been proposed for explaining PDV. Changes in ENSO under global warming are uncertain. Increasing greenhouse gases change the mean states in the tropical Pacific, which in turn induce ENSO changes. Due to the fact that the change in mean tropical condition under global warming is quite uncertain, even during the past few decades, it is difficult to say whether ENSO will intensify or weaken, but it is very likely that ENSO will not disappear in the future.

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.

Contrary to early assessments, the eastern tropical Pacific (ETP) is not devoid of well-developed reefs. Significant accumulations of Holocene reef framework are present throughout the region, although they tend to be poorly consolidated, lack the submarine cementation common on most reefs elsewhere in the world, and are subject to considerable bioerosion. These reef frameworks began accreting as early as 7000 years ago. The thickest accumulations of Pocillopora frameworks occur in coastal areas of Mexico, Costa Rica, Panama, and Colombia, but reefs composed of massive corals—species of Porites, Pavona, or Gardineroseris—are present throughout the region. Reef development in the ETP is limited by a variety of characteristics of the physical environment. Because of high turbidity in most areas, reef development is generally restricted to less than ~10 m depth. The spatial extent of reefs in the ETP is also limited from the combined influences of wave action and upwelling. Most reefs in the ETP are only a few hectares in size and the best-developed reefs generally occur in areas sheltered from strong oceanic influence. Upwelling also influences long-term trends in reef development in the region. There does not appear to be a significant impact of upwelling on the millennial-scale growth rates of Panamanian reefs; however, reefs in upwelling environments typically have thinner frameworks than nearby reefs in non-upwelling environments. Furthermore, upwelling may have contributed to a historic shutdown of reef development in Costa Rica and Panama. Although both ecological and oceanographic disturbances have had some impact on the long-term development of reefs in the ETP, the most important control on reef development in this region throughout the Holocene has most likely been the El Niño–Southern Oscillation (ENSO). ENSO activity—especially that of the 1982–83 and 1997–98 El Niño events—has shaped the landscape of coral reefs across the ETP both in recent decades and in the past. Reefs in Pacific Panama and Costa Rica experienced a 2500-year hiatus in vertical growth beginning ~4100 years ago as a result of enhanced ENSO activity. Although the degree of framework accumulation and rate of reef accretion in some parts of the ETP are more similar to that of the western Atlantic than previously thought, the region still remains a marginal environment for reef development. Given the dominant role that climatic variability has played in controlling reef development in the past, the future of reefs in the ETP under accelerating climate change remains uncertain.

The eastern tropical Pacific (ETP) is an isolated oceanic region exposed to extreme oceanographic conditions, including low salinity, low pH, high temperatures during El Niño, and low temperatures during La Niña and seasonal upwelling. The coral reefs in this region have a relatively limited suite of species compared to other coral reef areas of the world, but much like more diverse reefs the species present interact in complex ways. Here we synthezise the knowledge of taxonomic groups of reef organisms from prokaryotes to vertebrates, including algae, sponges, cnidarians, annelids and other worms, molluscs, crustaceans, echinoderms and fishes. We also present summaries on the biodiversity of associated functional groups and habitats, including (a) reef zooplankton and cryptic fauna, and (b) soft benthic environments, rhodolith beds and mesophotic environments. Several factors that structure the biodiversity of ETP coral reefs are explored, including biological, physical and chemical controls. ETP coral reefs are relatively simple systems that can be used as models for studying biodiversity and interactions among species. We conclude this review by highlighting pressing research needs, from very basic inventories to more sophisticated studies of cryptic assemblages, and to investigations on the impacts of natural and anthropogenic effects on ETP coral reef biodiversity.

The sudden and sporadic occurrence of anomalous conditions associated with El Niño-Southern Oscillation (ENSO) can precipitate diverse, immediate and long-term effects on eastern Pacific reef-building corals and associated organisms. ENSO is manifested in two complementary phases, namely warm (El Niño) and cool (La Niña) events, which have contrasting and potentially negative effects on coral reef ecosystems. Of the two distinct types of El Niño activity—Eastern-Pacific (EP) and Central-Pacific (CP)—the former exhibits maximum SST anomalies and related climate and weather impacts that affect eastern Pacific coral reefs. Relevant ENSO conditions, with direct or indirect effects, include (a) high and low sea temperature extremes, (b) thermocline and nutricline depths, (c) high and low sea level stands, (d) storm activity, (e) rainfall patterns (and terrestrial runoff), and (f) deviances in surface current direction, velocity and spatial extent. The first sign of ENSO stress to zooxanthellate corals is tissue blanching or bleaching, which may occur during periods of elevated (El Niño) or depressed (La Niña) thermal anomalies. Earlier studies of thermally induced coral bleaching in the Galápagos Islands and Panama are updated to 2012 with attention to anomalous warm and cool events that are stressful to reef-building corals. When thermal conditions normalize, surviving reef-building corals typically re-gain their usual complement of zooxanthellae (symbiotic dinoflagellates) and pigmentation. Long-term ecological effects from extreme ENSO activity, particularly during El Niño sea warming events, may occur over months or years following initial impacts. Such effects can markedly reduce coral cover, cause local species disappearances and significantly change the abundances of coral-associated taxa. The bioerosion of dead corals and carbonate frameworks can eliminate essential habitat space for a multitude of species if coral recruitment is suppressed and recovery unduly prolonged. Coral reef recovery and resiliency are examined in the context of recent ENSO disturbances. Since the last very strong 1997–98 El Niño coral reef bleaching event, live coral cover has increased significantly on many but not all equatorial eastern Pacific (EEP) reefs.

Trophic interactions on eastern Pacific coral reefs are complex and highly dynamic, ever changing due to numerous biological and physical factors. In this chapter, we first address the sources of energy at the base of food webs, i.e. photosynthetic carbon fixation by benthic algae and endosymbiotic zooxanthellae, secondarily derived organic deposits, detritus, and fecal matter, as well as demersal (within reef) and allochthonous plankton food sources. Next we consider consumers, covering the major reef trophic guilds in the eastern Pacific—suspension feeders, deposit and detritus feeders, herbivores, carnivores (predators and carnivorous grazers), as well as scavengers. The diversity and relative abundance of consumer taxa are described and considered in terms of their ecological roles in community processes. The complex interplay of these guilds is examined through food webs constructed for Panama, Cabo Pulmo reef in the Gulf of California, Mexico, and the Floreana Island rocky reef in the Galápagos Islands. Finally, the effects of physical and biotic perturbations on food webs, interactions, indirect effects, and trophic cascades conclude this review.

Eastern Pacific reef ecosystems are home to a diverse assemblage of corallivorous fishes and invertebrates. It is therefore not surprising that there is a rich history of research on corallivores in the eastern Pacific. In fact, much of what is known today on the topic of corallivory has built upon studies from the eastern Pacific region. Here we review the progression of our understanding of eastern Pacific corallivory and corallivores. We discuss the behavior and ecology of these specialized consumers, dividing our analysis into the larger conspicuous taxa such as the crown-of-thorns sea star (Acanthaster planci) and the guineafowl puffer (Arothron meleagris), as well as into the smaller cryptic species such as the pustulate egg shell (Jenneria pustulata) and coral crustacean guards (Trapezia spp., Alpheus lottini). The majority of species that consume coral tissues are facultative corallivores, feeding on corals only incidentally. Both the negative and positive interactions of corallivores to their prey/hosts are reviewed. We address detrimental direct consumption of coral and how it can ultimately influence growth form, species distributions, population structure, and the asexual reproduction of corals. We examine the cleaning behavior of some corallivorous species as well as their territorial tendencies, which may potentially lead to the exclusion of more lethal coral predators. Despite the high diversity of corallivore taxa, no population outbreaks have been observed in the eastern Pacific; coral colony growth rates and reef accretion proceed apace. Finally, we explore the far-reaching implications of the corallivore feeding strategy, touching on the connections that ultimately link coral biomass with higher trophic levels and the rest of the reef ecosystem.

Algae from two kingdoms and four major phyla form productive benthic communities that play key ecological roles on coral reefs of the eastern tropical Pacific (ETP). The diversity of algae that comprise these communities is being actively explored throughout the region, with new species still being discovered. Due to its physical setting, there are several distinct algal habitats in the ETP, including reef crests dominated by crustose coralline algae and algal turfs, reef flats that support a higher abundance of foliose macroalgae, unconsolidated accumulations of dead coral rubble that may support rhodoliths and cryptic macroalgae, and deeper-water refuges from herbivory dominated by more of the palatable forms of macroalgae rarely found on reefs. Past work has identified a number of ecological processes that control algal populations and communities at play in the ETP, but research elucidating their relative roles is still limited in this region. Recent work suggests that these reefs are supported by allochthonous nutrient supplies such as upwelling and thermocline shoaling, though reduced light during these events may limit productivity to shallow reef zones. Though at least partially sheltered from major disturbances that structure other reef systems, such as hurricanes and outbreaks of crown-of-thorns sea stars, reefs of the ETP are strongly affected by sea surface temperature extremes associated with El-Niño-Southern Oscillation (ENSO) leading to major coral mortality with the opening of space for colonization by algae. Biotic processes that control algal communities include herbivory and competition with coral, both of which limit algal proliferation and may enhance recovery after disturbance. However, facilitative interactions within algal communities, such as reduced herbivory and amelioration of nutrient limitation by internal recycling that act as positive feedbacks within areas dominated by algae, may stabilize algal communities after disturbance. There is some empirical and modeling evidence that suggests these processes may drive reefs of the ETP to display alternative stable states. This evidence includes a bifurcated pattern of recovery after ENSO disturbance, the patchy nature of this recovery within certain reefs, and the existence of stabilizing feedbacks that support resilience of both coral and algal states. Further, a simulation model (cellular automaton) of ETP reefs parameterized with experimentally-derived feedback relationships suggests these reefs exist under environmental conditions that produce bistability. However, much research is still needed to fully understand the physical and ecological processes that structure algal communities, and how these controls may shift with anthropogenic impacts.

Bioerosion, the weakening and erosion of hard substrates by boring, etching, and grazing organisms, is a major structuring force on coral reefs of the Eastern Tropical Pacific (ETP). Bioerosional processes are the main source of reef erosion, and facilitate recycling of reefal carbonate. In healthy reefs, a dynamic balance exists between destructive (i.e. bioerosion) and constructive (i.e. bioaccretion) processes, allowing for maintenance and growth of reef frameworks. In changing environments, however, bioerosion rates can exceed those of coral calcification, leading to reduced reef development and the destruction of reef frameworks. In the ETP, high rates of bioerosion are promoted by nutrient-rich upwelling and high primary productivity conditions, recurrent coral bleaching and mortality events, and a chemical environment characterized by high-pCO₂ and low aragonite saturation state. Here we examine bioerosion in ETP coral habitats and the variable roles of reef-dwelling bioeroder taxa: microbial euendoliths (microendoliths), sponges, polychaetes, sipunculans, crustaceans, molluscs, echinoids, and reef fishes. Among these agents of bioerosion, sponges, sipunculans, bivalves, and echinoderms have been relatively well studied in this region, while information is currently lacking or limited for microendolith assemblages, polychaetes and reef fishes. The frequency of coral invasion by clionaid sponges (e.g., Cliona vermifera and Thoosa mismalolli) is variable between ETP coral habitats. Dense boring sponge assemblages can lead to high rates of carbonate losses exceeding those of bioaccretion. Boring bivalves (i.e., species of Lithophaga and Gastrochaena) are very abundant on many actively accreting reefs and are generally more prominent contributors to reef erosion in the ETP than in other regions. Sea urchins are by far the most destructive grazers of coral substrates in habitats where abundant. Following ENSO-associated coral mortality events, intense bioerosion by sea urchins has impeded coral recovery and compromised reef health at many eastern Pacific sites. This chapter reviews factors important in ETP bioerosion, and current knowledge of bioeroder populations in the region.

Coral reefs of the eastern tropical Pacific (ETP) are unique in being the only reef region in the world that has experienced multiple episodes of mass coral bleaching while also being exposed to chronically depressed aragonite saturation states as a result of regional upwelling. These characteristics make them ideal case studies for the continued effects of climate change on coral reefs, and in particular for the responses of reef corals and their dinoflagellate algal symbionts (Symbiodinium spp.) to combined climate stressors. As a result, the diversity, distribution and stability of Symbiodinium in ETP corals have been studied since the mid-1990s, shortly after contemporary molecular methods to identify Symbiodinium were first developed. ETP reefs have been instrumental in the discovery that certain members of Symbiodinium in clade D impart bleaching resistance to their coral hosts. In the ETP, clade D is represented by a single symbiont type (D1, also referred to as S. glynni), which has been shown to be tolerant of both episodic El Niño-driven high temperature stress (e.g., 1997 in the Gulf of Chiriquí, Panama), and low temperature stress during an unusually cold winter (e.g., 2008 in Baja California, Mexico). Virtually all studies in the region have focused on corals in the genus Pocillopora, which is both the dominant reef-building genus and the most symbiotically diverse, being the only coral genus in the ETP that routinely hosts heat tolerant D1 symbionts at high abundance. There is debate over the mechanisms by which D1 becomes dominant on pocilloporid reefs, with evidence for both differential mortality of corals, and dynamic change in symbiont communities in response to thermal history and/or disturbance. The relative importance of these two mechanisms is likely to be critical in determining reef survival trajectories over the coming century, as is the degree to which non-pocilloporid corals in the region can associate with, or become dominated by, D1. Dynamic change in symbiont communities may allow corals to survive recurrent bleaching events if stepwise increases in the abundance of D1 following each bleaching event allow these thermotolerant symbionts to accumulate. However, controlled experiments and continued monitoring are required to resolve the debate over symbiont stability versus lability, and there may be significant differences in coral response across the region. Finally, understanding whether different Symbiodinium can alter coral response to high CO₂ or depressed aragonite saturation state (Ω_arag) remains a priority research area for the region, especially given the potential for strong interactions between Ω_arag, coral growth, and symbiont community structure.

As one of the most widely distributed and most studied scleractinian genera in the world, Pocillopora encompasses an important group of corals. In the eastern tropical Pacific, Pocillopora species thrive and are the major reef-building scleractinian taxon, even though conditions are considered suboptimal for coral growth and reef development. Early observations on reproduction and species distributions appear to be complicated by high phenotypic diversity and often inaccurate species identifications. New genetic-based evidence reorganizes species classifications within Pocillopora by delimiting boundaries to genetic recombination. Such improvements toward a natural and accurate taxonomy have further revealed important patterns in Symbiodinium diversity and distribution associated with Pocillopora in the eastern Pacific. Here, I review work on genetic connectivity and symbiosis ecology that may explain physiological, ecological and evolutionary characteristics that account for the differential success of this coral genus in the marginal eastern tropical Pacific.

Sexual reproductive activity has been demonstrated in all reef-building (zooxanthellate) scleractinian corals examined from Mexico to the equatorial eastern Pacific (Galápagos Islands). Eleven of 13 species spawn gametes, six are gonochoric, three hermaphroditic, and four exhibit significant mixed sexuality (both gonochoric and hermaphroditic). Four or 30.1 %, two species each of Pocillopora and Porites, produce autotrophic ova. Porites panamensis is the only known zooxanthellate brooder. Also sexually active are the azooxanthellate scleractinian Tubastraea coccinea and the zooxanthellate hydrocoral Millepora intricata. Reproductive structures, sex ratios, age at sexual maturity, sexuality, and developmental mode have been determined from largely histological evidence. Agariciid corals, comprising more than one-third of investigated species, exhibit predominantly mixed sexual systems with sequential cosexual hermaphroditic cycles in four species. Mixed sexuality is also minimally exhibited in populations of two dominantly gonochoric species. Several eastern Pacific corals spawn mostly on lunar day 17 and 1–2 days following; however, multispecific spawning has not been observed probably because of seasonal, diel, and variable timing in spawning behavior. Factors contributing to the high fecundity of eastern Pacific corals include (1) seasonally prolonged reproductive activity, (2) small size of mature gametes allowing for production of high numbers, (3) split spawning with bimonthly gamete production in some species, (4) alternation of sex maturation in gamete development, and possibly (5) their low latitudinal location under relatively constant and high thermal conditions. Coral community persistence, reef growth and recovery are highly dependent on both sexual and asexual reproductive processes. Asexual fragmentation by physical and biotic causes is particularly important, especially for branching pocilloporid species and the fungiid coral Diaseris distorta. Asexual propagation in massive and encrusting poritid and agariciid species is also common-place, often the result of bioerosion and colony breakage by foraging reef fishes. Some research areas in need of attention are noted, for example (a) timing of spawning and the behavior of gamete release of several species, (b) life cycles of Pocillopora spp. and Millepora intricata, and (c) effects of anthropogenic stressors on eastern Pacific coral reproduction and recruitment.

Gene flow can provide cohesion between conspecific populations. In order to obtain an indirect measure of gene flow between coral reef species in the eastern tropical Pacific (ETP) and between these populations and those of the rest of the Pacific we compiled available data from sequences of DNA and microsatellites for corals, gastropods, echinoderms and fishes, and calculated F_ST statistics. The ETP consists of a narrow strip of continental shelf along the coast of the Americas and a deeper water gap between the coast and the outer eastern Pacific Islands; a large expanse of deep ocean separates the ETP and the closest islands in the central Pacific. We have, therefore, compared populations in four major directions: (1) between the eastern and the central Pacific, (2) between the coast and the outer islands, (3) among the outer islands, and (4) along the coast and nearshore islands. The available data are biased in favor of showing high levels of gene flow because they contain an excess of transpacific species, which are a minority among ETP biota. Despite this bias, shallow water populations of the ETP are isolated from the rest of the world’s oceans. Occasional breaching of the expanse of water between the ETP and the Central Pacific by some species is also possible. Gene flow between the outer eastern Pacific islands and the mainland coast is variable, depending on the species examined. Gene flow among populations at the outer eastern Pacific islands is high except for those at Easter Island (Rapa Nui), in which all but one sampled species show large and significant values of F_ST in comparisons with populations from all other islands. Gene flow rates among populations along the ETP coast are high. There is no evident genetic break resulting from the Central American Gap (southern Mexico to the Gulf of Fonseca, Honduras) in any of the sampled species. A trend of isolation by distance along the coast is evident in corals and fishes.

Refugia could provide the essential conditions for coral and coral reef persistence in the marginal eastern tropical Pacific Ocean (ETP) by allowing corals to avoid extinction after disturbance events and by maintaining populations that can rapidly recolonize disturbed habitats. The low diversity and restricted ranges of ETP corals make them vulnerable to seawater warming caused by positive phases of the El Niño-Southern Oscillation (El Niño). However, at spatial scales ranging from individual colonies (<10 m) to regions, (>100 km) there are areas or habitats that avoid stress or support corals that are more resistant to stress or with the capacity to recover rapidly. These habitats may form a network of refugia that ensures coral persistence through severe disturbance. This chapter explores evidence for the existence of refugia at multiple spatial and temporal scales across the ETP, with recent examples showing refugia may be common and perhaps even necessary for many species to survive the highly fluctuating environment in this eastern Pacific tropical ocean basin.

Eastern tropical Pacific (ETP) coral reefs provide a real-world example of reef growth, development, structure, and function under the high-pCO₂, low aragonite saturation state (Ω_arag) conditions expected for the entire tropical surface ocean with a doubling to tripling of atmospheric CO₂. This provides a unique opportunity to examine various aspects of calcium carbonate (CaCO₃) budgets in low-Ω_arag conditions in the present day. Unlike anywhere else in the world, the ETP displays a continuum of thermal stress and CO₂ inputs up to levels at which reef building is terminated and reef structures are lost. The response of coral reef CaCO₃ budgets to El Niño warming across the ETP shows that reefs can be completely lost after experiencing a 2–3 °C thermal anomaly sustained in excess of two months during the warmest time of the year at Ω_arag values expected for the rest of the tropics when atmospheric CO₂ doubles. ETP coral reefs have persisted and shown resilience to this level of thermal stress or acidification when acting alone, but the combination of the two corresponded with the complete elimination of reef framework structures in the southern Galápagos Islands over the decade after the 1982–83 El Niño warming event. Reef carbonate degradation is exacerbated also by diverse agents of bioerosion such as sea urchins, boring bivalves, and excavating sponges, with experimental evidence demonstrating that the latter may even increase their activities during ocean warming and pH decline. This chapter reviews the CaCO₃ budget of ETP coral reefs and discusses how a high-CO₂ world may impact the major biotic and abiotic factors responsible for the cycling of carbonate materials. Coral reefs of the ETP serve as a model for conditions that will occur in other regions within a few decades.

In the eastern tropical Pacific, large spatial gradients in climate conditions are associated with oceanic upwelling and the position of the Intertropical Convergence Zone (ITCZ). Dramatic shifts in these systems occur during extremes of the El Niño-Southern Oscillation (ENSO), which also reverberate throughout the global climate system. Motivated by the need to understand ENSO history, and underpinned by ecological research, many of the earliest coral paleoclimate studies originated from the eastern Pacific. Early work in Galápagos, Costa Rica, and Panama highlighted the usefulness of a diverse array of tracers that record past environmental change. Longer records of coral δ¹⁸O have delineated the range of natural variability in ENSO and identified a rich spectrum of tropical Pacific variance that extends into multidecadal and century time scales. Coral-based reconstructions of ocean temperatures generally show strong warming trends, except in Galápagos, where existing records do not span the full 20th century and where strong interannual variability impedes detection of smaller trends. Coral records complement sediment-based and terrestrial records in terms of their length, resolution, and sensitivity. A more complete understanding of variability in the eastern tropical Pacific should emerge from ongoing and future work that integrates reef-based paleorecords with other paleoclimate reconstructions and model simulations. In addition, analysis of additional proxies—e.g. of circulation, pH, and salinity—is recommended to complement temperature reconstructions and provide useful reconstructions of changes in climate, oceanography, and reef stress.

Coral reefs world-wide have been impacted by direct and indirect human activity and natural disturbances. This has led to the degradation and disappearance of many reef structures. On a basin-wide scale, the natural impact of El Niño warming has been the main cause of reef decline in the eastern tropical Pacific (ETP). At local scales, human activity has also taken its toll although only limited observations are available on specific impacts to ETP coral reefs. The main direct causes of damage are the extraction of corals and other reef organisms, nonregulated tourist activity, ship groundings, anchor damage, and eutrophication. The main indirect sources of damage to coral reefs are coastal alteration, sedimentation, pollution (including eutrophication), oil pollution, agrochemicals, other pollutants, and plankton blooms. Climate change can impact coral reefs directly (sea warming, sea level rise, ocean acidification, increased storm activity, and possibly stronger and more frequent El Niño events), and indirectly (coastal erosion, increased fresh water runoff and elevated nutrients). Even though human impacts on ETP reefs are low compared to other regions, significant damage has been documented. Since ETP coral reefs are relatively small and few in number, a redoubled effort is necessary for their protection.

This chapter reviews and evaluates coral reef conservation strategies along the eastern tropical Pacific (ETP), a narrow biogeographic region on the Pacific American coast that extends from southern Baja California, Mexico to northern Peru, including several oceanic islands. The ETP is a natural laboratory, a model for understanding the development of coral biotopes in a changing environment. We evaluate conservation strategies in seven countries in the ETP region (Peru was not included for apparent lack of coral habitats). A survey of current Marine Protected Areas (MPAs) highlights great variation in the number, scale and management approaches. Generally, MPAs with no-take areas are relatively uncommon, with multi-purpose areas favored. The Cabo Pulmo MPA in Baja California, Mexico demonstrates that when a local community is involved in the creation and enforcement measures of an MPA, conservation success can be achieved. Despite such apparent successes, inadequate and confusing legal practices have generally forestalled effective protection of coral ecosystems in the ETP. However, in several instances nongovernmental organizations (NGOs) have assisted with planning, negotiation and stakeholder engagement. Nonetheless these findings underscore how the establishment of an MPA does not guarantee that conservation goals will be achieved. This calls for a new approach that incorporates contributions from ecological studies along with a high investment in capacity development and training to ensure that the goals of MPAs better complement effective fisheries and ecosystem management within and outside their borders.

Eastern tropical Pacific (ETP) coral habitats host functionally complex biological communities. Corals and their associated biota, from charismatic reef fishes to cryptic invertebrate infauna, drive and maintain diverse ecosystem functioning in reef environments. This chapter provides a photographic guide to key and otherwise notable species present in ETP coral habitats. The objectives of this inclusion are to both facilitate field identification and to make accessible key information (i.e. habitat, relative abundance, ecological role, geographic and depth distributions, and pertinent references) available for these coral-associated species. Represented in this guide are 11 species of algae, 102 invertebrates of which 53 are cnidarians, and 28 fishes, with bias towards comprehensive inclusion of cnidarians (corals) and fishes. Effort was made to include cryptic species easily overlooked, warranting select ex situ photographs, as well as to visually describe polymorphism and behavior encountered in the field.