First National Consensus on the Safe Practice of Medical Tourism for Bariatric Surgery in Mexico.

BACKGROUND: Medical tourism is increasing every year, especially in fields such as bariatric surgery. Global concerns rely on the fact of its safety, mainly because of a lack of standardization, unknown number of cases, and the type of specialists performing such procedures. Mexico continues to be among the top countries receiving patients from abroad, but there is no national consensus, guidelines, or recommendations about the practice in such fields. METHODS: A group of 32 bariatric surgeons with an accumulated experience of 25,196 procedures in the medical tourism modality were gathered to perform a national consensus in bariatric surgery tourism. The Delphi methodology was used for this project, with a total of 52 items applied. A consensus was reached when the experts' position was ≥ 70% for each statement. The items included pre- and postoperative phases, patient selection, follow-up, and ethical and legal considerations. RESULTS: Of the 52 statements, 40 (76.9%) reached a consensus of ≥ 70%; 36 in favor, 5 against, and 11 with indeterminate responses. CONCLUSIONS: Although more than two-thirds of the statements related to high standard practice reached a consensus, weak points in the medical tourism modality were identified. This first consensus serves to establish the basis of safe practices, eventually reach national guidelines, and define the top standards of care when performing bariatric surgery tourism.

Ranking the ecological effects of coastal armoring on mobile macroinvertebrates across intertidal zones on sandy beaches.

Coastal armoring is widely applied to coastal ecosystems, such as sandy beaches, in response to shoreline erosion and threats to infrastructure. Use of armoring is expected to increase due to coastal population growth and effects of climate change. An increased understanding of armoring effects on those ecosystems and the services they provide is needed for impact assessments and the design of these structures. We investigated the following hypotheses: 1) impacts of coastal armoring on beach macroinvertebrates increase from lower to upper intertidal zones and 2) location of an armoring structure on beach profiles affects the number of intertidal zones, using comparative surveys of armored and unarmored beach sections in Chile and California. The effects of armoring were greater for upper intertidal (talitrid amphipods) and mid-intertidal species (cirolanid isopods) than for lower shore fauna (hippid crabs). Our surveys of sections of armoring structures located higher and lower on the beach profile (with and without interactions with waves and tides), showed loss of upper zone talitrid amphipods and mid-zone isopods and a reduction of lower zone hippid crabs in sections where the structures were lower on the beach profile and interacted with waves, compared to non-interacting sections. Our results support the hypothesis that impacts of armoring on intertidal macroinvertebrates increase from the lower to the upper intertidal zones of sandy beaches and also suggest that the relative position of an armoring structure on the beach profile, determines the number of intertidal zones it affects. Our findings also imply that by altering the position of existing armoring structures on the shore profile and increasing the amount of interaction with waves and tides, sea level rise and regional factors, such as coseismic coastal subsidence, can be expected to exacerbate the impacts of these widely used coastal defense structures on sandy beach ecosystems.

Dietary habits of the black-necked swan Cygnus melancoryphus (Birds: Anatidae) and variability of the aquatic macrophyte cover in the Río Cruces wetland, southern Chile.

The black-necked swan Cygnus melancoryphus is an aquatic herbivorous bird whose dietary habits depend on the dominance and accessibility of macrophyte banks in shallow areas of coastal and limnetic wetlands in southern South America. The swans from the Río Cruces wetland in southern Chile (ca. 39°S) feed mainly on the macrophyte Egeria densa from the water column between depths from less than 0,5 and 2,0 m. A micro- histological analysis of black-necked swan feces (N = 152) collected during six sampling occasions between 2012 and 2017 confirms the preferred consumption of E. densa and highlights the impact of temporal changes in the cover of these macrophytes on the swan's diet. The dietary composition of black-necked swans appears as a reliable proxy for temporal changes in the distribution of the most common aquatic macrophytes in the Río Cruces wetland. These results highlight the importance of preserving shallow wetlands as the habitat for aquatic macrophytes that provide the main food source for these herbivorous water birds.

Resilience of an aquatic macrophyte to an anthropogenically induced environmental stressor in a Ramsar wetland of southern Chile.

In mid-2004, anthropogenically induced changes in water quality of the Río Cruces wetland, a Ramsar site located in southern Chile (ca. 40°S), enhanced the resuspension of iron-enriched sediments, which were subsequently deposited over the most abundant aquatic macrophyte of the wetland (Egeria densa Planch. 1849). This event triggered the formation of brownish, necrotic patches and increased iron contents in the leaves and stems of E. densa, which contributed to a significant demise of the plant within the wetland. In this study, we estimate the recovery time as a proxy for resilience of this macrophyte at organismal and population levels. Macro- and micro-optical characteristics, as well as iron contents in tissues of E. densa, were documented in four time windows (2004, 2008, 2012, and 2014). In addition, the size of the macrophyte population and its spatial occurrence were monitored from 2008 to 2016 across 36 study sites within the wetland. Our results suggest necrotic patches and high iron contents in E. densa persisted at least until 2008. After 2013, a significant increase in the spatial occurrence of E. densa was observed within the wetland, reaching full recovery of the population during 2015. The health of plant tissues and iron contents in leaves and stems showed recovery period close to 4 years, while the recovery of the spatial occurrence of E. densa took approximately 9 years. While the monitoring of plant health was not performed on a strict annual basis, the recovery rates estimated here are slower than those described for other macrophytes. This finding might reflect the long-lasting effects of the disturbance from 2004 and the interaction with biotic processes, such as foraging by waterbirds recolonizing the Río Cruces wetland. These results show that full recovery of E. densa was achieved through a cascade of effects starting with abiotic factors (water quality) and passing through physiological and individual levels, to finally reach the population level. A key aspect of this response is the invasive nature of the macrophyte, which likely contributed to its recovery as a consequence of improved water quality. Less successful macrophyte species in other systems may not reach the specific population recovery, and become subdominant species instead, or even be eradicated from the wetland either as the result of herbivory or due to competition with other macrophytes.

Recovery of black-necked swans, macrophytes and water quality in a Ramsar wetland of southern Chile: Assessing resilience following sudden anthropogenic disturbances.

In 2004 migration and mortality for unknown reasons of the herbivorous Black necked swan (Cygnus melancorhyphus (Molina, 1782)) occurred within the Río Cruces wetland (southern Chile), a Ramsar Site and nature sanctuary. Before 2004, this wetland hosted the largest breeding population of this water bird in the Neotropic Realm. The concurrent decrease in the spatial occurrence of the aquatic plant Egeria densa Planch. 1849 - the main food source of swans - was proposed as a cause for swan migration and mortality. Additionally, post-mortem analyses carried out on swans during 2004 showed diminished body weight, high iron loads and histopathological abnormalities in their livers, suggesting iron storage disease. Various hypotheses were postulated to describe those changes; the most plausible related to variations in water quality after a pulp mill located upstream the wetland started to operate in February 2004. Those changes cascaded throughout the stands of E. densa whose remnants had high iron contents in their tissues. Here we present results of a long-term monitoring program of the wetland components, which show that swan population abundance, body weights and histological liver conditions recovered to pre-disturbance levels in 2012. The recovery of E. densa and iron content in plants throughout the wetland, also returned to pre-disturbance levels in the same 8-year time period. These results show the temporal scale over which resilience and natural restoring processes occur in wetland ecosystems of temperate regions such as southern Chile.

Macroscale patterns in body size of intertidal crustaceans provide insights on climate change effects.

Predicting responses of coastal ecosystems to altered sea surface temperatures (SST) associated with global climate change, requires knowledge of demographic responses of individual species. Body size is an excellent metric because it scales strongly with growth and fecundity for many ectotherms. These attributes can underpin demographic as well as community and ecosystem level processes, providing valuable insights for responses of vulnerable coastal ecosystems to changing climate. We investigated contemporary macroscale patterns in body size among widely distributed crustaceans that comprise the majority of intertidal abundance and biomass of sandy beach ecosystems of the eastern Pacific coasts of Chile and California, USA. We focused on ecologically important species representing different tidal zones, trophic guilds and developmental modes, including a high-shore macroalga-consuming talitrid amphipod (Orchestoidea tuberculata), two mid-shore scavenging cirolanid isopods (Excirolana braziliensis and E. hirsuticauda), and a low-shore suspension-feeding hippid crab (Emerita analoga) with an amphitropical distribution. Significant latitudinal patterns in body sizes were observed for all species in Chile (21° - 42°S), with similar but steeper patterns in Emerita analoga, in California (32°- 41°N). Sea surface temperature was a strong predictor of body size (-4% to -35% °C-1) in all species. Beach characteristics were subsidiary predictors of body size. Alterations in ocean temperatures of even a few degrees associated with global climate change are likely to affect body sizes of important intertidal ectotherms, with consequences for population demography, life history, community structure, trophic interactions, food-webs, and indirect effects such as ecosystem function. The consistency of results for body size and temperature across species with different life histories, feeding modes, ecological roles, and microhabitats inhabiting a single widespread coastal ecosystem, and for one species, across hemispheres in this space-for-time substitution, suggests predictions of ecosystem responses to thermal effects of climate change may potentially be generalised, with important implications for coastal conservation.

Calibrating coseismic coastal land-level changes during the 2014 Iquique (Mw=8.2) earthquake (northern Chile) with leveling, GPS and intertidal biota.

The April 1st 2014 Iquique earthquake (MW 8.1) occurred along the northern Chile margin where the Nazca plate is subducted below the South American continent. The last great megathrust earthquake here, in 1877 of Mw ~8.8 opened a seismic gap, which was only partly closed by the 2014 earthquake. Prior to the earthquake in 2013, and shortly after it we compared data from leveled benchmarks, deployed campaign GPS instruments, continuous GPS stations and estimated sea levels using the upper vertical level of rocky shore benthic organisms including algae, barnacles, and mussels. Land-level changes estimated from mean elevations of benchmarks indicate subsidence along a ~100-km stretch of coast, ranging from 3 to 9 cm at Corazones (18°30'S) to between 30 and 50 cm at Pisagua (19°30'S). About 15 cm of uplift was measured along the southern part of the rupture at Chanabaya (20°50'S). Land-level changes obtained from benchmarks and campaign GPS were similar at most sites (mean difference 3.7±3.2 cm). Higher differences however, were found between benchmarks and continuous GPS (mean difference 8.5±3.6 cm), possibly because sites were not collocated and separated by several kilometers. Subsidence estimated from the upper limits of intertidal fauna at Pisagua ranged between 40 to 60 cm, in general agreement with benchmarks and GPS. At Chanavaya, the magnitude and sense of displacement of the upper marine limit was variable across species, possibly due to species-dependent differences in ecology. Among the studied species, measurements on lithothamnioid calcareous algae most closely matched those made with benchmarks and GPS. When properly calibrated, rocky shore benthic species may be used to accurately measure land-level changes along coasts affected by subduction earthquakes. Our calibration of those methods will improve their accuracy when applied to coasts lacking pre-earthquake data and in estimating deformation during pre-instrumental earthquakes.

Responses of dune plant communities to continental uplift from a major earthquake: sudden releases from coastal squeeze.

Vegetated dunes are recognized as important natural barriers that shelter inland ecosystems and coastlines suffering daily erosive impacts of the sea and extreme events, such as tsunamis. However, societal responses to erosion and shoreline retreat often result in man-made coastal defence structures that cover part of the intertidal and upper shore zones causing coastal squeeze and habitat loss, especially for upper shore biota, such as dune plants. Coseismic uplift of up to 2.0 m on the Peninsula de Arauco (South central Chile, ca. 37.5º S) caused by the 2010 Maule earthquake drastically modified the coastal landscape, including major increases in the width of uplifted beaches and the immediate conversion of mid to low sandy intertidal habitat to supralittoral sandy habitat above the reach of average tides and waves. To investigate the early stage responses in species richness, cover and across-shore distribution of the hitherto absent dune plants, we surveyed two formerly intertidal armoured sites and a nearby intertidal unarmoured site on a sandy beach located on the uplifted coast of Llico (Peninsula de Arauco) over two years. Almost 2 years after the 2010 earthquake, dune plants began to recruit, then rapidly grew and produced dune hummocks in the new upper beach habitats created by uplift at the three sites. Initial vegetation responses were very similar among sites. However, over the course of the study, the emerging vegetated dunes of the armoured sites suffered a slowdown in the development of the spatial distribution process, and remained impoverished in species richness and cover compared to the unarmoured site. Our results suggest that when released from the effects of coastal squeeze, vegetated dunes can recover without restoration actions. However, subsequent human activities and management of newly created beach and dune habitats can significantly alter the trajectory of vegetated dune development. Management that integrates the effects of natural and human induced disturbances, and promotes the development of dune vegetation as natural barriers can provide societal and conservation benefits in coastal ecosystems.

Ecological implications of extreme events: footprints of the 2010 earthquake along the Chilean coast.

Deciphering ecological effects of major catastrophic events such as earthquakes, tsunamis, volcanic eruptions, storms and fires, requires rapid interdisciplinary efforts often hampered by a lack of pre-event data. Using results of intertidal surveys conducted shortly before and immediately after Chile's 2010 M(w) 8.8 earthquake along the entire rupture zone (ca. 34-38°S), we provide the first quantification of earthquake and tsunami effects on sandy beach ecosystems. Our study incorporated anthropogenic coastal development as a key design factor. Ecological responses of beach ecosystems were strongly affected by the magnitude of land-level change. Subsidence along the northern rupture segment combined with tsunami-associated disturbance and drowned beaches. In contrast, along the co-seismically uplifted southern rupture, beaches widened and flattened increasing habitat availability. Post-event changes in abundance and distribution of mobile intertidal invertebrates were not uniform, varying with land-level change, tsunami height and coastal development. On beaches where subsidence occurred, intertidal zones and their associated species disappeared. On some beaches, uplift of rocky sub-tidal substrate eliminated low intertidal sand beach habitat for ecologically important species. On others, unexpected interactions of uplift with man-made coastal armouring included restoration of upper and mid-intertidal habitat seaward of armouring followed by rapid colonization of mobile crustaceans typical of these zones formerly excluded by constraints imposed by the armouring structures. Responses of coastal ecosystems to major earthquakes appear to vary strongly with land-level change, the mobility of the biota and shore type. Our results show that interactions of extreme events with human-altered shorelines can produce surprising ecological outcomes, and suggest these complex responses to landscape alteration can leave lasting footprints in coastal ecosystems.

Reconocimiento y prevención del error en los cuidados de salud / Awareness and avoidance of error in health care

Los errores de ocurrencia progresivamente creciente en los cuidados de salud, de acuerdo a datos universales, ofrecen un panorama epidemiológico alarmante en la medida que se percibe mayor complejidad de las patologías a atender, sus diagnósticos y tratamientos requieren permanente actualización de conocimientos y habilidades clínicas y tecnológicas, y las demandas de satisfacción y cobertura de costos sufren igualmente exigencias previamente impensadas. En la literatura abunda información sobre las causas y mecanismos de estos incidentes, permite caracterizarlos, medir sus alcances y consecuencias, sistematizar sus efectos y definir conductas para su prevención y manejo. En el presente estudio nos hemos dedicado a mirar los errores surgidos en el área, por incomprensión de su significado y del propio diagnóstico de errores en el cuidado de la salud, y de las probabilidades y formas de resolver los dilemas en asignar responsabilidad a los diferentes participantes del equipo de salud involucrados en esa particular contingencia. También analizamos algunos de los aspectos judiciales derivados y explicitados al clarificar las implicancias legales de estas acciones y la interpretación del clima ético surgido de la percepción de daño que sigue a la trasgresión del contrato virtual médico-paciente. Finalmente se comenta la necesidad de recuperar los valores del compromiso incluido en el cuidado al paciente y la dignidad de las acciones de salud comprendidas en el marco del profesionalismo promovido por el ACP y otras instancias internacionales comprometidas con las responsabilidades asumidas por los profesionales sanitarios.

Errors in Health Care are shown to occur at increasing rates in proportion to the new knowledge, the technological and communication display used in controlling disease and human health impairment. Medical publications and reports, as those coming from allied professions devoid to health care, give plenty information of causes, mechanisms, tendencies, most privileged effects and consequences, classification, characteristics and impact of these issues on the progress of diseases involved and the way it affects the patient - professional relations. These aspects have been the main interest of numerous authors in the pertinent scientific literature. Instead of that focus, we committed our search to look over the errors raised in the field because missed understandings on the significance and the proper diagnosis of errors in health care, and the probability and ways to solve the dilemmas imposed by assigning responsibilities to each participant or the working team involved in the alluded contingency. We also address some of the judicial aspects derived and defined to clarify the legal implications of these actions and the interpretation of the ethic climate raised by the feeling of harm that follows transgression in the professional - patient virtual contract. Finally we comment on the need to restore the high values of the professional endeavour to the patients’ requirements and the dignity of the health actions included under the frame of professionalism promoted by the ACP and other international medical societies of internal medicine and bestowed to the personal responsibility of the sanitary agent.

El hospital docente del siglo XXI: [revisión] / The teaching hospital of the 21st century: [review]

Since their creation, teaching hospitals been associated to complex health care institutions. They have been able to give education and training to health care professionals while providing patient care. Teaching Hospitals have been the home of medical training in formal and informal curriculum. In the past Century, social and economic changes have resulted in pressure to change the mission of academic medical centers. Educational ranking and quality of care do not always correlate, and the learning process mostly occurs through personal example. Several models have been developed to sustain the academic mission at a time of financial challenge. Still, the need to sustain the basic triad of patient care, teaching and research as the core of the academic mission remains constant.

Desarrollo del Departamento de Educación en Ciencias de la Salud en la Facultad de Medicina de la Universidad de Chile / Development of the Department of Education in Sciences of the Health in the Medicine Faculty of the University of Chile

Practically, since it was born, an essential interest of the Faculty of Medicine of the University of Chile has been the continuous actualization of its learning objectives, and the educationimprovement of methodologies to teach and practice competences, therefore accomplishing advanced forming of professional resources, to cover every requirement in sanitary needs in the country. This has been a distinct seal, in the institutional history of the State University of the nation, that has been kept effectively radiating its action, from the more than two hundred years following its foundation age and early incorporation to the patron institution. The main stream orientation of its studies and advances has been, naturally, evolving; surging at the beginning, from the individual initiative of spontaneous academic leaders; thereafter from commended working groups or straight assessors to the executive authorities. Finally, starting the present century, the Department of Health Sciences Education was installed to guide advanced programs design to build the educational fortress needed by vocational selection of teachers, called from the same institution or coming for seemingly sake, from other universities in the country or foreign. The narrative and description of the evolution and actual standing of the Unit, its proposals, projects and perspectives, are offered concisely but thoroughly founded.

[Plagiarism. Document from the Ethics Commission of the Medical School, University of Chile]. / En referencia al plagio intelectual. Documento de la Comisión de Etica de la Facultad de Medicina de la Universidad de Chile.

Plagiarism is defined as the intellectual fraud in which an individual attempts to unduly appropriate, for his/her own benefit, the knowledge, ideas or discoveries of someone else. It is not uncommon in academic settings where research is conducted and a creative work is carried out. Due to the dismal consequences of plagiarism, cautionary measures and sanctions are required to avoid it. This paper is intended to warn and promote a discussion about plagiarism. The Faculty of Medicine of the University of Chile and its ethics committee believe that a fight against these type of actions will contribute to prevent their detrimental effects on the moral and intellectual patrimony of our society.

En referencia al plagio intelectual. Documento de la Comisión de Ética de la Facultad de Medicina de la Universidad de Chile / Plagiarism

Plagiarism is defined as the intellectual fraud in which an individual attempts to unduly appropriate, for his/her own benefit, the knowledge, ideas or discoveries of someone else. It is not uncommon in academic settings where research is conducted and a creative work is carried out. Due to the dismal consequences of plagiarism, cautionary measures and sanctions are required to avoid it. This paper is intended to warn and promote a discussion about plagiarism. The Faculty of Medicine of the University of Chile and its ethics committee believe that a fight against these type of actions will contribute to prevent their detrimental effects on the moral and intellectual patrimony of our society.

Histone deacetylase-related protein inhibits AES-mediated neuronal cell death by direct interaction.

Histone deacetylase-related protein (HDRP), an alternatively spliced and truncated form of histone deacetylase-9 that lacks a C-terminal catalytic domain, protects neurons from death. In an effort to understand the mechanism by which HDRP mediates its neuroprotective effect, we screened for proteins in the brain that interact with HDRP by using a yeast two-hybrid assay. One of the HDRP-interacting proteins identified in this screen was amino enhancer of split (AES), a 197-amino acid protein belonging to the Groucho family. Interaction between HDRP and AES was verified by in vitro binding assays, coimmunoprecipitation, and colocalization studies. To investigate the significance of the HDRP-AES association to the regulation of neuronal survival, we used cultured cerebellar granule neurons, which undergo apoptosis when treated with low potassium (LK) medium. We found that in contrast to HDRP, whose expression is markedly reduced by LK treatment, AES expression was not appreciably altered. Forced expression of AES in healthy neurons results in cell death, an action that is blocked by the coexpression of HDRP. AES is a truncated version of larger Groucho-related proteins, one of which is transducin-like enhancer of split (TLE)-1. We found that the expression of TLE1 is reduced in LK-treated neurons and the forced expression of TLE1 blocks LK-induced neuronal death as well as death induced by AES. Our results show that AES has apoptotic activity in neurons and suggest that neuroprotection by HDRP is mediated by the inhibition of this activity through direct interaction.

Bedside diagnostic laparoscopy in the intensive care unit: a 13-year experience.

BACKGROUND: Frequently, critically ill patients suffer from intraabdominal pathology, such as sepsis or ischemia, either as a cause of a critical illness or as a complication from another illness requiring an intensive care unit (ICU) admission. These complications are associated with high rates of morbidity and mortality (between 50% to 100%). The diagnosis of these problems can be difficult in these very ill patients because it may require transport of unstable patients to additional departments outside the ICU setting. One option in the diagnosis of these difficult patients is bedside laparoscopy, as it avoids patient transport, is very accurate, and maintains ICU monitoring. METHODS: From 1991 to 2003, 13 patients underwent bedside diagnostic laparoscopy in the ICU to diagnose intraabdominal pathology in critically ill patients. All the procedures were done at the bedside in the ICU with the patient under local anesthesia and intravenous sedation. RESULTS: Mean procedure time was 36 minutes (range, 17 to 55). Mean patient age was 75.5 years (range, 56 to 86). There were 8 males and 5 females. Forty-six percent of the patients were diagnosed with mesenteric necrosis and died within 48 hours with no further testing or procedures. One patient with massive fecal contamination died the same day. Thirty percent of patients had a normal intraabdominal examination; of these, 2 died of unrelated illnesses and 2 survived their nonabdominal illness. Fifteen percent were diagnosed with acute acalculous cholecystitis as a complication of their ICU illness, which resolved satisfactorily. No intraoperative complications occurred with the ICU procedure. CONCLUSION: Bedside diagnostic laparoscopy in the ICU is feasible, safe, and accurate in the assessment of possible intraabdominal problems in properly selected, critically ill patients.

Needlescopic cholecystectomy: lessons learned in 10 years of experience.

OBJECTIVES: Needlescopic cholecystectomy (NC) is a refinement of laparoscopic cholecystectomy (LC) using 2-mm instruments compared with the standard 5-mm and 10-mm ports. We review our experience with needlescopic cholecystectomy. METHODS: From 1994 to 2004, 303 patients underwent NC. All patients were operated on using 2-mm instruments and one 10-mm trocar for the laparoscope. The characteristics of patients, total operation time, complications, postoperative pain, and hospital course were documented. RESULTS: Patients' average age was 41.86 years; 262 were female and 41 were male. Mean BMI was 25.7. Mean length of surgery was 59.33 minutes. Intraoperative cholangiography was performed in all cases. Mean blood loss was 14.88 mL. One intraoperative complication occurred. Mean hospital stay was 22.68 hours. Postoperative pain was measured on a 0-10 pain scale; on day 0 it was 4.4 and on the first day it was 1.7. Analgesic doses required were 0 doses in 6.89%, 1 in 20.68%, 2 in 24.13%, 3 in 34.48%, 4 in 13.79%, and > 4 doses was not required. No postoperative complications occurred. At 3-month follow-up, patient satisfaction was 100%, and in 99% of patients scars were imperceptible. CONCLUSIONS: NC is safe and feasible without increased operative risk, with better cosmetic results, less pain, and good acceptance among patients.

Competitive interactions in macroinfaunal animals of exposed sandy beaches.

The influence of biotic interactions in structuring macroinfaunal communities of exposed sandy beaches, an unstable habitat characterized by strong physical forces, is generally considered negligible. We investigated the hypothesis that competitive interactions during burrowing could potentially affect the intertidal distribution and abundance of macroinfaunal animals of sandy beaches using two species of invertebrates, a hippid crab, Emerita analoga, and a bivalve, Mesodesma donacium, common along the coast of Chile. Spatial overlap in the intertidal distributions of these species was dynamic, varying with abundance, location, time of year and tide. Highest density zones of each species were often distinctly separated at low tide and spatial overlap in their distributions decreased significantly with increasing density, suggesting density dependence of the interactions. Negative relationships between densities of the two species at the smallest spatial scale examined also suggested active interactions among individuals. Over a tidal cycle, peak densities of the two species overlapped suggesting that interactions could occur frequently. Burrowing performance of E. analoga varied between size classes in three experimental densities of clams (5, 10 and 15 clams 0.008 m(-2)) and in controls with no clams. Burrowing times of large crabs were significantly longer (approximately twofold) in all densities of clams than in controls, while those of small crabs did not differ significantly among treatments and controls. Large crabs also displaced clams from the sand while burrowing suggesting that two mechanisms of direct interference can occur, both of which could increase exposure of individuals involved to active swash and transport across or along the beach with potentially negative consequences. Our results suggest that competitive interactions capable of affecting zonation and population and community biology on a number of scales can occur among burrowing macroinfauna on exposed sandy beaches. Those interactions could be more ecologically significant than previously appreciated and may contribute to patterns observed in community structure and zonation on sandy beaches. Our results illustrate the potential importance of negative biological interactions in a physically stressful environment.