The Complexity of Approximately Solving Influence Diagrams

Influence diagrams allow for intuitive and yet precise description of complex situations involving decision making under uncertainty. Unfortunately, most of the problems described by influence diagrams are hard to solve. In this paper we discuss the complexity of approximately solving influence diagrams. We do not assume no-forgetting or regularity, which makes the class of problems we address very broad. Remarkably, we show that when both the treewidth and the cardinality of the variables are bounded the problem admits a fully polynomial-time approximation scheme.

Hector the hermit crab.

Héctor, el cangrejo ermitaño, es muy tímido, odia llamar la atención y evita el contacto con otras criaturas del fondo marino siempre que sea posible. Una noche, mientras duerme, crece una flor en su concha, y de repente Héctor se convierte en el foco de atención de todos. Cree que sus compañeros cangrejos no son amistosos, pero pronto se da cuenta de su error y aprende que hacer amigos no es tan difícil como había imaginado.

Paul the Silentiary’s description of Hagia Sophia in the light of new archaeological evidence

A discussion of the implications of of recent archaeological work at Hagia Sophia in Istanbul for understanding Paul the Silentiary's sixth-century Ekphrasis of the Byzantine church.

Biological features of a puzzling symbiotic association between the hermit crab Dardanus insignis and the porcellanid crab Porcellana Sayana (Crustacea)

The symbiotic lifestyle is widespread among porcellanid crabs, which maintain ecological and co-evolutionary associations with annelid polychaetes, poriferans, cnidarians, echinoderms, gastropod. mollusks, and other crustaceans such as shrimps and hermit crabs, among others. We investigated the ecological association between the hermit crab Dardanus insignis and the porcellanid Porcellana sayana, in southeastern Brazil. Porcellanid crabs, hermit crabs, and available shells were collected monthly from July 2001 to June 2003, with a shrimp boat equipped with two double-rig trawl nets. The majority of P. sayana specimens were collected in shells occupied by D. insignis (96.6%); a few were found in empty shells (3.4%). The catch of both symbionts and hosts increased with increasing depth, with the highest occurrence at 35 m. The F. sayana crabs of various sizes could be found solitary or forming aggregations of up to 14 individuals per host, showing no sex or size segregation. In spite of the high diversity of shell species occupied by the hermit crabs and also available in the field, only a few of them were also utilized by P. sayana. The majority (93%) of shells utilized by P. sayana also hosted other symbiont species, constituting the basis of extensive symbiotic complexes. Thus, the ecological relationship between D. insignis and P. sayana may be classified as a non-obligate and non-specific symbiosis that may also involve other facultative organisms such as sea anemones. (C) 2008 Elsevier B.V. All rights reserved.

Ecological distribution and reproductive period of the hermit crab Loxopagurus loxochelis (Anomura, Diogenidae) on the northern coast of São Paulo State, Brazil

The present study aimed to determine the ecological distribution of Loxopagurus loxochelis as a function of selected environmental factors, as well as its reproductive period based on the combined analysis of the presence of ovigerous females and the development of the gonad. The collections were carried out monthly from January to December 2000 in the region of Ubatuba (SP), using a fishing boat equipped with two double rig nets, in isobaths of 5, 10, 15, 20, 25, 30, 35 and 40 m, at which depths samples of sediment and water were collected. The cephalothoracic shield length and sex of the animals were determined; the abdomen was dissected to verify gonad development stages. A total of 334 individuals was obtained ( 196 males, 48 ovigerous females and 90 non-ovigerous females) with sizes varying from 3.0 to 8.6 mm (5.7 +/- 1.0 mm). A greater abundance of L. loxochelis (95.2%) was observed at depths of 20 and 25 m. These sites mainly revealed a substratum stable with a very fine sand fraction, which facilitates the habit of embedding shown by this hermit. With regard to gonadal analysis, it was possible to classify four gonad development stages for each sex: immature, rudimentary, developing and developed. It was observed that in the winter months about 71% of the females had gonads either developing or developed and 67% were ovigerous. Therefore, the winter period can be considered the peak of reproduction for this species, characterizing a typical seasonal-continuous reproduction.

Breeding season of the hermit crab petrochirus diogenes (anomura : diogenidae) in the north coast of São Paulo state, Brazil

Petrochirus diogenes, one of the largest hermit crabs in the western Atlantic, was studied in the north coast of the State of São Paulo, Brazil, to determine its breeding season through the analysis of gonad development at a macroscopic level. The analysis of 999 crabs collected from 1995 to 1999, based on four categories of gonad development, showed that males present gonads in the advanced stage during all seasons, and although females present developed ovaries all year as well, the peak incidence is in the summer. The peak of recruitment occurs in the winter.

Shell utilization pattern of the hermit crab Clibanarius vittatus (Crustacea, Anomura) in an estuary at São Vicente, State of São Paulo, Brazil

O objetivo do presente estudo foi avaliar a utilização de conchas de gastrópodes por Clibanarius vittatus (Bosc, 1802), na Praia dos Pescadores, em São Vicente (SP), Brasil. Foram realizadas amostras mensais no período de maio de 2001 a abril de 2003, na região intertidal durante a maré baixa. Todos os animais foram pesados e tiveram o comprimento de seu escudo cefalotorácico mensurados, suas conchas identificadas e medidas quanto ao comprimento (mm), abertura (mm) e peso (g). Capturamos 2.344 ermitões (644 machos, 1.594 fêmeas não-ovígeras, 45 fêmeas ovígeras e 61 em intersexo), utilizando 13 espécies de conchas de gastrópodes, das quais Stramonita haemastoma (Linnaeus, 1767), Cymatium parthenopeum (Von Salis, 1793) e Achatina fulica (Bowdich, 1822) representaram mais de 98% das conchas utilizadas. Obteve-se significativa diferença de tamanho entre os sexos, com machos e indivíduos em intersexo atingindo maior porte físico do que fêmeas. Este padrão influenciou fortemente na ocupação e utilização de conchas, principalmente em A. fulica, que foi a maior concha obtida por C. vittatus, utilizada somente por machos e indivíduos em intersexo. Cymatium parthenopeum foi a única espécie de concha em que se obtiveram elevados coeficientes de determinação para todas os parâmetros analisados. Stramonita haemastoma apresentou elevados e significativos coeficientes de determinação, indicando que esta espécie é o principal recurso de concha ocupada na Praia dos Pescadores.