Structural change and sustainable development

In this paper, we show that the commonly observed decline in primary (natural resource using) sector output and employment shares, often termed structural change, can be explained as an endogenous response to the presence of nature's constraint. Structural change takes place even if consumer preferences are homothetic, and technological progress does not discriminate against the primary sector. Under certain conditions, structural change allows an open economy to grow with natural resource sustainability. Sustained and environmentally sustainable economic growth is possible even if the natural resource is exploited under open access. Well-defined property rights are neither necessary, nor sufficient for sustainable growth. We show that there is no unique relationship between natural resource endowment and the rate of economic growth over the long run. Resource-rich economies may grow faster or slower than resource-poor ones.     

Hybridization within the species complex of the scleractinan coral Montastraea annularis

The morphologically variable reef coral previously known as Montastraea annularis (Ellis and Solander, 1786) has recently been separated into three species based on differences in morphology, behavior, allele frequencies and some life-history traits of Panamanian specimens. To further investigate the proposed reclassification and its conformity to the biological species concept we conducted reciprocal intra- and inter- specific fertilization experiments with gametes from each of the three species on Florida reefs. With one exception, self-fertilization rates were very low or zero. Within-species crosses resulted in production of planulae, as did all inter-species (hybrid) crosses, but there was much variation in fertilization success within each type of cross. In an experiment with separated gametes, hybrid crosses between M. annularis and M. franksi produced more larvae than within-species crosses for each species. Hybridization crosses between M. faveolata and the other two species produced fewer larvae than did within- M. faveolata crosses in the experiment with separated gametes, but many larvae resulted when the hybridizations were performed by mixing entire gamete bundles. Additional observations showed that M. franksi had 20% larger eggs and fewer eggs per gamete bundle than did the other two species and that it consistently spawned 1 to 1.5 h before the others, a potential temporal barrier to hybridization. These results indicate that there is no inherent pre-zygotic barrier to cross-fertilization among the three morphological species, although post-zygotic survival and fertility remain to be determined. The adherence of the proposed reclassification to the biological species concept requires further examination. Received: 16 April 1997 / Accepted: 17 June 1997     

Intraspecific variation of photosynthesis,respiration and photoprotective carotenoids in<Emphasis Type="Italic"> Gracilaria birdiae</Emphasis> (Gracilariales: Rhodophyta)

In most rhodophytes, the quenchers zeaxanthin or lutein seem to be used for carotenoid photoprotection next to β-carotene. However, our study shows that the tropical intertidal alga Gracilaria birdiae possesses high concentrations of antheraxanthin, as well as lower concentrations of both zeaxanthin and violaxanthin. In higher plants and some algal groups, these three oxygenated carotenoids are known to protect photosystem II in a xanthophyll cycle. We were able to manipulate the carotenoid concentrations in G. birdiae by varying irradiance and temperature in laboratory experiments. Higher concentrations of violaxanthin were found in dark incubations and higher concentrations of zeaxanthin in light incubations, which is an indication that a xanthophyll cycle is active in G. birdiae. Our results also suggest a biosynthesis pathway from lycopene to β,β-carotene and β-cryptoxanthin in the light and synthesis of new lycopene in darkness. This would imply that the production of functional carotenoids is regulated by their need in photoprotection. Despite cultivation in the laboratory under the same conditions for at least 6 years, there were differences in photosynthetic properties and carotenoid dynamics between algae obtained from a population close to the Equator and another population close to the Tropic of Capricorn. These differences most probably have their origin in genetic differentiation mediated by ambient irradiance and temperature regimes. Four phycoerythrin-deficient mutant strains of G. birdiae were studied as well. Photosynthetic properties and carotenoid dynamics were basically the same in the mutants and the wild strains, but two green strains had remarkably low dark respiratory rates. This is an indication of low production of metabolites, which may be a competitive disadvantage.     

Reproductive cycle of two commercial species of sea cucumber (Echinodermata: Holothuroidea) from Caribbean Panama

The reproductive status of the holothuroid species Isostichopus badionotus (Selenka, 1867) and Holothuria mexicana (Ludwig, 1875) was studied over 16 months in Bocas del Toro (Panama), from November 1999 to February 2001. Sexual reproduction was evaluated by the gonad index method, and by histology of gonad development. In addition, population structure was assessed based on sex ratio, minimum reproductive size, and length and weight distributions of males and females. The sex ratio in both species was 1:1, with a unimodal population distribution composed mainly of mature individuals. The minimum reproductive length and weight were 13-20 cm and 150 g, respectively, for both species, although reproductive individuals 10 cm in length were also found. A consistently higher gonad index was observed in H. mexicana, due to a high proportion of mature females and males and high gonad indices in most monthly samples. Gametogenesis and spawning patterns seemed to occur throughout the year, with periods of enhanced activity. Two periods of maximum reproductive activity were tentatively identified: July-November for I. badionotus and February-July for H. mexicana, but neither species had a single, sharply defined annual spawning event. Further work on these exploited holothuroids should examine the relationships between reproduction and environmental factors and between reproductive status and recruitment.     

Ontogenetic changes in the retinal topography of the European hake, Merluccius merluccius: implications for feeding and depth distribution

Changes in the distribution pattern of cells in the ganglion cell layer were studied in the retina of the European hake, Merluccius merluccius (L.), to identify the possible adaptations of visual capabilities to different bathymetric distributions and feeding habits. From early juveniles to adults, the eye diameter increased eightfold; thus, retinal surface increased dramatically with size also. In early juveniles the retinal topography of the cells in the ganglion cell layer showed a concentric arrangement with respect to the centre of the retina. Two specialised areas were found, located at the ventral and dorso-rostral periphery, where the cell density reached 47,900 cells mm^-2, which corresponds to a theoretical visual acuity of 21' (minutes of arc). The visual axes were located upwards and downwards at around 80° from the geometric centre of the retina. In juveniles, the retina underwent important changes as the concentric topographic pattern transformed: the ventral specialised area progressively disappeared, the dorso-rostral area relocated to a rostral position and a new specialised area formed in the temporal retinal region. The visual axes were directed forward and backward. For fish with a total length of 12 cm or more, a horizontal visual streak formed along the rostro-temporal axis of the retina and a new specialised area was formed in the temporo-central region of the visual streak. In adults, acute vision could be identified with the two specialised areas at the temporal and rostral periphery, where the ganglion cell density peak decreased to 3,200-3,600 cells mm^-2 and the resolving power increased to 10'. As visual acuity is partially dependent on the cell types in the ganglion cell layer, cell populations in this layer were distinguished into either ganglion cells or displaced amacrine cells, using morphometric and histological criteria. The proportion of displaced amacrine cells was fairly uniform throughout the retinal surface, always representing between 32% and 39% of all cells in early and advanced juveniles. Only in adults did their density increase to 50%, probably as an adaptation to low light levels, which fish encounter as their distribution increases in depth. A small population of giant ganglion cells was also present in the retina. In the young and adult retinae, they represented 1.2% and 2.7% of the total cell population, respectively. Therefore, it has been shown how in M. merluccius the retinal topography undergoes important changes in relation to varying environmental demands.     

On the gains of committing to inefficiency: Corruption, deforestation and low land productivity in Latin America

We present a new explanation and empirical evidence showing that rural subsidies to large farmers tend to be associated with low land productivity and excessive deforestation. We develop a lobbying model where wealthy farmers trade bribes or political contributions to government politicians in exchange for subsidies; farmers are able to tilt the terms of the bargaining game with policy makers in their favor by pre-committing to an inefficient choice of semi-fixed inputs. Government proneness to accept political contributions or bribes and its willingness to provide subsidies cause farmers to adopt inefficient modes of production as a mechanism to capture such subsidies. Our predictions are consistent with stylized facts on land use in Latin America, and suggest that subsidy schemes have been counterproductive—distorting and constraining development, and triggering excessive depletion of natural resources. We validate some of the predictions of the model through econometric analyses using a new data set for nine countries in Latin America.     

Ultrastructure of the digestive gland of larval and adult stages of the sacoglossan Elysia patagonica

Chloroplast retention in the sacoglossan gastropod Elysia patagonica Muniain and Ortea, 1997 was investigated. Intact chloroplasts of the green algae Bryopsis plumosa (Chlorophyta: Bryopsidacea) were observed by transmission electron microscopy in vacuoles of the digestive cells of benthic adults starved for 20 days. Intact chloroplasts from the microalgal Nannochloropsis sp. (Heterokontophyta: Eustigmatophycea) were found in the digestive cells of veliger larvae feeding for 8 days. Similar digestive cells were observed in adults and planktonic larvae of E. patagonica. The retention ability is confirmed in both stages.     

Spermatozoan ultrastructure and spermatogenesis in aplousobranch ascidians,with some phylogenetic considerations

Spermatozoa and some stages of spermatogenesis were studied for four species of aplousobranch ascidians. Spermatozoa of Clavelina lepadiformis (Müller) (family Clavelinidae) are plesiomorphous in that they have apical acrosomal vesicles and a moderately elongated, cylindrical nucleus. The elongated mitochondrion is twisted ea. 11/2 times around the nucleus. In their ultrastructural morphology, C. lepadiformis sperm conform in some respects to the least-derived ascidian sperm, those of the phlebobranch ascidian Ciona intestinalis L.; however, the sperm of Clavelina lepadiformis have two apomorphies not shared with those of Ciona sp.: (1) the mitochondrion of Clavelina lepadiformis is long and spiralled along the entire nucleus rather than being comparatively compact and not at all helical; (2) the mitochondrial cristae are elongated parallel to the long axis of the nucleus, whereas in Ciona sp. sperm the cristae are unmodified. In Distaplia sp., Aplidium sp. and Synoicum pulmonaria (Ellis and Solander) the spermatozoa are more derived and consist of a proximal cylindrical and a distal corkscrew-like part. The mitochondrion in Distaplia sp. and Aplidium sp. contains electron-dense material and extends in a long thread around the nucleus. In S. pulmonaria the mitochondrion surrounds the anterior part of the nucleus in mature spermatozoa, and an elongated, dense structure displaying fine striation is enclosed in the mitochondrion in late spermatids. The sperm ultrastructural morphology observed in this study is consistent with the majority view that clavelinids are closest to the ancestral ascidian but is also consistent with other conclusions, particularly that the Cionidae are closest to the stem ascidian.     

Heavy metals and organic carbon in sediments from the Tuy River basin,Venezuela

The Tuy River basin, located in north-central Venezuela with an annual average temperature of 27°C and precipitation of 140 cm, was selected to conduct a geochemical study of bottom sediments, with the object of establishing the natural and human influences in the abundance and distribution of Fe, Mn, Cr, Co, Cu, Ni, Pb, Zn and organic carbon. The basin is lithologically divided into two sub-basins, north and south. The north sub-basin drains a iow-grade metasedimentary terrain with a population density of 800 persons km^–2 and approximateiy 600 industrial sites, while the south sub-basin in underlain by metavolcanic and ultramafic rocks, with a population density of less than 10 persons km^–2.Stream bottom sediment samples (150) were collected during the years of 1979–1986 in 16 unpolluted sites and 13 polluted sites. The sediments were air dried at room temperature and sieved through a 120 stainless steel mesh (125 m). Samples of grain size smaller than 125 m were analysed, the heavy metals being determined by atomic absorption spectrometry and the organic carbon (Corg) by dry combustion.The higher concentrations of heavy metals and organic carbon found in the pristine areas were in the south sub-basin, especially in those areas with higher annual precipitation and tropical forest. This indicated that the metavolcanic and ultramafic rocks yield higher concentrations of heavy metals than the metasedimentary rocks. It was also noted that the higher concentrations of Cr and Ni are associated with the ultramafic rocks. The results obtained from the sediment samples collected in the polluted sites showed that the elements Pb, Zn and Corg are enriched up to 4 times as a result of ail the human activities taking place in the basin. Organic carbon is an excellent indicator of domestic wastewater, Pb and Zn are good indicators of the automotive traffic and industrial effluents. The concentrations of each heavy metal did not show any significant correlation with grain size fractions; however, the concentration of organic carbon did show a negative correlation with grain size. The lithological, climatic and vegetation influence in the abundance of heavy metals and organic carbon in stream sediments clearly indicates the necessity of establishing background levels for the area under study when carrying out studies in environmental geochemistry.