Intraspecific variation in foraging behaviour: Influence of shore height on temporal organization of activity in the chiton Acanthopleura japonica

The chiton Acanthopleura japonica has a wide vertical distribution on moderately exposed Hong Kong shores, and it shows limited seasonal vertical migration. Observations revealed that chitons were active while awash on ebbing or flooding tides during both day- and night-time, but they were inactive in refuges at their resting shore heights when conditions were unfavourable. The most intense activity was always associated with awash periods, but the timing of activity peaks and percentage activity varied between seasons, tidal conditions and the shore height individuals occupied. Activity periods were longer in summer than winter, but varied from day to day, depending on weather conditions such as wind speed and hours of sunshine. Duration of activity also varied with shore height; chitons living in the high-shore remaining active for nearly twice as long as those lower on the shore. Laboratory studies showed that activity was controlled by a circalunidian endogenous rhythm, but can be overridden by exogenous, environmental factors, such as wave splash. Therefore, the temporal organization of activity in A. japonica is controlled by both exogenous and endogenous mechanisms, of which the timing and duration of the awash phase of the tide appears to be the dominant controlling factor. To optimize foraging success, individuals exhibit a continuum of foraging strategies determined by season and the shore height they occupy, resulting in intraspecific variability in the duration and timing of activity. © Inter-Research 2006.link_to_subscribed_fulltex

The effects of acidification on the stable isotope signatures of marine algae and molluscs

Carbon (13C) and nitrogen (15N) stable isotope analysis has become increasingly important in the study of energy flow and tropho-dynamics in many ecosystems. Prior to analysis, samples are often pre-treated with acids to remove inorganic carbonates which may bias the results. The effects of pre-analysis acidification on isotopic values are, however, still poorly understood for marine producers (e.g. algae and cyanobacteria), and consumers (e.g. molluscs), which may confound the comparability of different studies. In this study, such effects (untreated vs. decalcified samples) were examined at two different sampling periods (summer and winter). Acidification did not seem to affect the isotopic composition of consumers, but reduced both δ13C and δ15N of producers. This effect was consistent for the two sampling periods, although both producers and consumers had more enriched δ13C and δ15N values in summer. Acidification had the most distinct, negative effect on the isotopic values of samples which had low carbonate contents. It is, therefore, important to be aware of temporal variations in sample isotopic values and especially the effects of sample treatment when attempting to compare different studies. As an attempt to standardize protocols, it is recommended that only acid-washing carbonate-rich samples is adopted as the most appropriate pre-analysis treatment

Spatio-temporal variations in the diversity and abundance of commercially important Decapoda and Stomatopoda in subtropical Hong Kong waters

In subtropical Hong Kong, western waters (WW) are strongly influenced by the freshwater input from the Pearl River estuary, especially during summer monsoon, whereas eastern waters (EW) are predominantly influenced by oceanic currents throughout the year. Such hydrographical differences may lead to spatio-temporal differences in biodiversity of benthic communities. This study investigated the diversity and abundance of commercially important decapods and stomatopods in EW (i.e. Tolo Harbour and Channel) and WW (i.e. Tuen Mun and Lantau Island) of Hong Kong using monthly trawl surveys (August 2003-May 2005). In total, 22 decapod and nine stomatopod species were recorded. The penaeid Metapenaeopsis sp. and stomatopod Oratosquillina interrupta were the most abundant and dominant crustaceans in EW and WW, respectively. Both univariate and multivariate analyses showed that WW supported significantly higher abundance, biomass and diversity of crustaceans than EW, although there were significant between-site and within-site variations in community structure. Higher abundance and biomass of crustaceans were recorded in summer than winter. Such spatio-temporal variations could be explained by differences in the hydrography, environmental conditions and anthropogenic impacts between the two areas. Temporal patterns in the abundance-biomass comparison curves and negative W-statistics suggest that the communities have been highly disturbed in both areas, probably due to anthropogenic activities such as bottom trawling and marine pollution. © 2006 Elsevier Ltd. All rights reserved.link_to_subscribed_fulltex

Harpiosquilla harpax (Crustacea, Stomatopoda) as a biomonitor of trace metal contamination in benthic sediments in Hong Kong waters

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The source and fate of organic matter and the significance of detrital pathways in a tropical coastal ecosystem

Stable isotope analysis (SIA) and fatty acid profiling were used to elucidate the supply and fate of energy in a tropical coastal ecosystem in Hong Kong, southern China. To investigate seasonal changes in source supply on the diets of different trophic guilds, measurements were taken in three rocky bays before and after the onset of the summer monsoon, when supply of both marine macroalgal and streamborne terrestrial detritus increases. Particulate organic matter, comprising a mixture of marine and terrestrial sources, was the ultimate food source (>60%) for suspension feeders, which were the major prey items of a predatory gastropod (Thais). Increased levels of bacterial fatty acid biomarkers (BaFA) were recorded after the onset of the summer monsoon, indicating amplified dependence on detrital materials in both primary and secondary consumers. The considerable increase in the detritus fraction of sedimentary organic matter at the onset of the summer monsoon was reflected by enhanced levels of BaFA, possibly due to the degeneration of macroalgae. Significant contributions of this marine algal detritus to deposit feeders (Holothuria, 36%) and of terrestrial detritus to an echinoid grazer (Salmacis, 14%) were revealed by SIA mixing models as well as elevated BaFA concentrations. These results indicate a higher dependence on heterotrophic food chains based on decomposing marine algae and terrestrial detritus after the onset of the summer monsoon. Such seasonal variation in the importance of detrital energy sources is, therefore, likely to be important to coastal ecosystem functioning in the monsoonal tropics. © 2008, by the American Society of Limnology and Oceanography, Inc.link_to_subscribed_fulltex

Fluctuating asymmetry of Amphibalanus (Balanus) amphitrite (Cirripedia: Thoracica) in association with shore height and metal pollution

The level of fluctuating asymmetry (FA), which is defined as random deviations from perfect bilateral symmetry in the morphological traits of an organism, increases with increasing developmental instability, and it may be used as an indicator of environmental and/or genetic stresses. This study attempted to relate FA levels in the opercular plates of the barnacle Amphibalanus (Balanus) amphitrite with shore height and body trace metal concentrations. Barnacles were collected from both low and mid shores at six coastal locations with various degrees of marine pollution in Hong Kong. Four opercular traits, namely scutum length, tergum length, scutum width, tergum width were measured in the specimen while concentrations of five common trace metals (Cd, Cu, Cr, Mn and Zn) were determined in their body tissues using inductively coupled plasma-atomic emission spectrophotometry. Among the four traits, only tergum length and scutum width fulfilled the assumption of FA while their measurement errors were low. Across all sites, mid-shore A. amphitrite consistently exhibited a significantly higher FA level in scutum width than that in low-shore barnacles, but FA levels in tergum length were similar between the two shore heights. These results suggested that FA was trait-dependent, and mid-shore barnacles were possibly under high physical stress, such as desiccation and high temperature, leading to high developmental instability. Although no positive association was observed between FA and metal contamination in low-shore A. amphitrite, FA level in tergum length significantly increased with decreasing body concentration of manganese (Mn) in these barnacles. Such a negative relationship may be explained by the mechanism of bioaccumulation and physiological role of Mn in A. amphitrite with respect to the formation of barnacle shell plates. © 2008 Springer Science+Business Media B.V.link_to_subscribed_fulltex