Testing a new version of the size-advantage hypothesis for sex change: sperm competition and size-skew effects in the bucktooth parrotfish, Sparisoma radians

A variety of field studies suggest that sex change in animalsmay be more complicated than originally depicted by the size-advantagehypothesis. A modification of the size-advantage hypothesis,the expected reproductive success threshold model, proposesthat sperm competition and size-fecundity skew can stronglyaffect reproductive pay-offs. Size-fecundity skew occurs ifa large female's fecundity is markedly higher than the aggregateof the other members of her social group and, together withpaternity dilution from sperm competition, can produce situationsin which large females benefit by deferring sex change to smallerfemales. Deferral by large females can create sex-size distributionscharacterized by the presence of large females and small sex-changedmales, and it is precisely these distributions that the traditionalsize-advantage model cannot explain. We tested the predictionsof the new model with the bucktooth parrotfish, Sparisoma radians,on coral reefs in St. Croix, U.S. Virgin Islands. Collectionsand spawning observations determined that the local environmentalregime of S. radians is characterized by pervasive sperm competition(accompanying 30% of spawns) and factors that can produce substantialsize-fecundity skew in social groups. Dominant male removalexperiments demonstrate that the largest females in social groupsoften do not change sex when provided an opportunity. Instead,smaller, lower-ranking females change sex when a harem vacancyarises. This pattern of sex change is in contrast to virtuallyall previous studies of social control of sex change in fishes,but provides strong support for the general predictions of theexpected reproductive success threshold model.     

A Theoretical Model of Pattern Formation in Coral Reefs

We present a mathematical model of the growth of coral subject to unidirectional ocean currents using concepts of porous-media flow and nonlinear dynamics in chemical systems. Linear stability analysis of the system of equations predicts that the growth of solid (coral) structures will be aligned perpendicular to flow, propagating against flow direction. Length scales of spacing between structures are selected based on chemical reaction and flow rates. In the fully nonlinear system, autocatalysis in the chemical reaction accelerates growth. Numerical analysis reveals that the nonlinear growth creates sharp fronts of high solid fraction that, as predicted by the linear stability, advance against the predominating flow direction. The findings of regularly spaced growth areas oriented perpendicular to flow are qualitatively supported on both a colonial and a regional reef scale. Received 1 October 2001; accepted 22 May 2002.     

Shade avoidance and reproductive strategies of an early successional species Penthorum chinense in relation to shade treatments

• Shade avoidance is expected to be favoured under moderate light. However, in previous studies, shade avoidance was highest in the deepest shade, despite the fact that the plants incur the costs of shade avoidance without the benefits of being exposed to increased light.
• We performed shading experiments under different light intensities to understand: (i) how shade avoidance traits of Penthorum chinense could peak in moderate light, and (ii) if there was a trade‐off between plant height and allocation of seeds along the light gradients.
• Penthorum chinense increased shade avoidance traits such as height per total dry mass as the amount of light decreased. Side stem number per total dry mass of P. chinense decreased as shade became deeper, from full light to low light. Regressions on seed mass fraction and height were significant with a linear model (y = ?0.0006x + 0.1338). There were more resources allocated to seeds under low light than under moderate light.
• Penthorum chinense increased shade avoidance traits with the decrease in light amount, as found in previously studied species. There was a trade‐off between height and production of more seeds. The reproductive strategy of P. chinense was to increase seed mass fraction under low light more than under moderate light. This species might be able to expand established populations by both rhizomes and seeds under low light environments.

     

Thresholds in seascape connectivity: the spatial arrangement of nursery habitats structure fish communities on nearby reefs

Ecosystems are linked by the movement of organisms across habitat boundaries and the arrangement of habitat patches can affect species abundance and composition. In tropical seascapes many coral reef fishes settle in adjacent habitats and undergo ontogenetic habitat shifts to coral reefs as they grow. Few studies have attempted to measure at what distances from nursery habitats these fish migrations (connectivity) cease to exist and how the abundance, biomass and proportion of nursery species change on coral reefs along distance gradients away from nursery areas. The present study examines seascape spatial arrangement, including distances between habitats, and its consequences on connectivity within a tropical seascape in Mozambique using a seascape ecology approach. Fish and habitat surveys were undertaken in 2016/2017 and a thematic habitat map was created in ArcGIS, where cover and distances between habitat patches were calculated. Distance to mangroves and seagrasses were significant predictors for abundance and biomass of most nursery species. The proportions of nursery species were highest in the south of the archipelago, where mangroves were present and decreased with distance to nurseries (mangroves and seagrasses). Some nursery species were absent on reef sites farthest from nursery habitats, at 80 km from mangroves and at 12 km from seagrass habitats. The proportion of nursery/non-nursery snapper and parrotfish species, as well as abundance and biomass of seagrass nursery species abruptly declined at 8 km from seagrass habitats, indicating a threshold distance at which migrations may cease. Additionally, reefs isolated by large stretches of sand and deep water had very low abundances of several nursery species despite being within moderate distances from nursery habitats. This highlights the importance of considering the matrix (sand and deep water) as barriers for fish migration.     

Interpreting the sign of coral bleaching as friend vs. foe

Coral bleaching is a major concern to researchers, conservationists and the general public worldwide. To date, much of the high profile attention for bleaching has coincided with major environmental impacts and for many the term coral bleaching is synonymously associated with coral mortality (so‐called ‘lethal’ bleaching episodes). While this synonymous association has undoubtedly been key in raising public support, it carries unfair representation: nonlethal bleaching is, and always has been, a phenomenon that effectively occurs regularly in nature as corals acclimatize to regular periodic changes in growth environment (days, seasons etc). In addition, corals can exhibit sublethal bleaching during extreme environmental conditions whereby mortality does not occur and corals can potentially subsequently recover once ambient environmental conditions return. Perhaps not surprisingly it is the frequency and extent of these non and sublethal processes that yield key evidence as to how coral species and reef systems will likely withstand environmental and thus climatic change. Observations of non and sublethal bleaching (and subsequent recovery) are arguably not as readily reported as those of lethal bleaching since (1) the convenient tools used to quantify bleaching yield major ambiguity (and hence high potential for misidentification) as to the severity of bleaching; and (2) lethal bleaching events inevitably receive higher profile (media) attention and so are more readily reported. Under‐representation of non and sublethal bleaching signs may over‐classify the severity of bleaching, under‐estimate the potential resilience of reefs against environmental change, and thus ultimately limit (if not depreciate) the validity and effectiveness of reef management policies and practices. While bleaching induced coral mortality must remain our key concern it must be better placed within the context of bleaching signs that do not result in a long‐term loss of reef viability.     

BEYOND SUN,SAND, AND STITCHES: ASSIGNING RESPONSIBILITY FOR THE HARMS OF MEDICAL TOURISM

Medical tourism (MT) can be conceptualized as the intentional pursuit of non‐emergency surgical interventions by patients outside their nation of residence. Despite increasing popular interest in MT, the ethical issues associated with the practice have thus far been under‐examined. MT has been associated with a range of both positive and negative effects for medical tourists' home and host countries, and for the medical tourists themselves. Absent from previous explorations of MT is a clear argument of how responsibility for the harms of this practice should be assigned. This paper addresses this gap by describing both backward looking liability and forward looking political responsibility for stakeholders in MT. We use a political responsibility model to develop a decision‐making process for individual medical tourists and conclude that more information on the effects of MT must be developed to help patients engage in ethical MT.     

Seabird nutrient subsidies alter patterns of algal abundance and fish biomass on coral reefs following a bleaching event

Cross‐ecosystem nutrient subsidies play a key role in the structure and dynamics of recipient communities, but human activities are disrupting these links. Because nutrient subsidies may also enhance community stability, the effects of losing these inputs may be exacerbated in the face of increasing climate‐related disturbances. Nutrients from seabirds nesting on oceanic islands enhance the productivity and functioning of adjacent coral reefs, but it is unknown whether these subsidies affect the response of coral reefs to mass bleaching events or whether the benefits of these nutrients persist following bleaching. To answer these questions, we surveyed benthic organisms and fishes around islands with seabirds and nearby islands without seabirds due to the presence of invasive rats. Surveys were conducted in the Chagos Archipelago, Indian Ocean, immediately before the 2015–2016 mass bleaching event and, in 2018, two years following the bleaching event. Regardless of the presence of seabirds, relative coral cover declined by 32%. However, there was a post‐bleaching shift in benthic community structure around islands with seabirds, which did not occur around islands with invasive rats, characterized by increases in two types of calcareous algae (crustose coralline algae [CCA] and Halimeda spp.). All feeding groups of fishes were positively affected by seabirds, but only herbivores and piscivores were unaffected by the bleaching event and sustained the greatest difference in biomass between islands with seabirds versus those with invasive rats. By contrast, corallivores and planktivores, both of which are coral‐dependent, experienced the greatest losses following bleaching. Even though seabird nutrients did not enhance community‐wide resistance to bleaching, they may still promote recovery of these reefs through their positive influence on CCA and herbivorous fishes. More broadly, the maintenance of nutrient subsidies, via strategies including eradication of invasive predators, may be important in shaping the response of ecological communities to global climate change.