S.G. Cheung

Toxicity of bisphenol A and its bioaccumulation and removal by a marine microalga Stephanodiscus hantzschii

The toxicity of bisphenol A (BPA) to Stephanodiscus hantzschii, a diatom isolated from tidal water of Futian Mangrove Nature Reserve, China, and the bioaccumulation and removal capability of the marine microalga to BPA were investigated in the present study. Toxicity experiments showed that the 96-h EC50 of BPA was 8.65+/-0.26 mg/L, and the cell number and chlorophyll a content of S. hantzschii decreased significantly with increases in BPA at concentrations higher than 3.00 mg/L. S. hantzschii had high removal capability at low BPA concentrations as BPA was bioaccumulated and biodegraded by cells. After 16-day treatment, 88%, 99%, 92%, 61%, 48%, 28% and 26% of BPA were removed by the diatom in the media supplemented with 0.01, 0.10, 1.00, 3.00, 5.00, 7.00 and 9.00 mg/L BPA, respectively. The present study demonstrated that S. hantzschii was a tolerant isolate that could be used to remove BPA from contaminated waters.

Potential use of mangroves as constructed wetland for municipal sewage treatment in Futian, Shenzhen, China.

A pilot-scale mangrove wetland was constructed in Futian, Shenzhen for municipal sewage treatment. Three identical belts (length: 33m, width: 3m, depth: 0.5m) were filled with stone (bottom), gravel and mangrove sand (surface). Seedlings of two native mangrove species (Kandelia candel, Aegiceras corniculatum) and one exotic species (Sonneratia caseolaris) were transplanted to the belts with one species for each belt. The hydraulic loading was 5m(3)d(-1) and hydraulic retention time 3d. High levels of removal of COD, BOD(5), TN, TP and NH(3)-N were obtained. The treatment efficiency of S. caseolaris and A. corniculatum was higher than that of K. candel. Faster plant growth was obtained for S. caseolaris. The substrate in the S. caseolaris belt also showed higher enzyme activities including dehydrogenase, cellulase, phosphatase, urease and beta-glucosidase. The removal rates of organic matter and nutrients were positively correlated with plant growth. The results indicated that mangroves could be used in a constructed wetland for municipal sewage treatment, providing post-treatment to remove coliforms was also included.

Feeding rates and scope for growth of green mussels, Perna viridis (L.) and their relationship with food availability in Kat O, Hong Kong

The feeding and absorption rates (ARs) of the green mussel Perna viridis (L.) were studied from December 1997 to November 1998. During each visit, characteristics of seston including concentrations of total particulate matter (TPM), particulate organic matter (POM), particulate inorganic matter (PIM) and organic fraction of seston (f) were also determined. Higher values of clearance rates (CRs), ARs and absorption efficiencies (AEs) were obtained from November to January and lower values in March and April. CR was a negative power function of TPM and a positive linear function of f. ARs and AEs were significantly correlated with TPM and f (P<0.001), showing that food availability (both seston quantity and quality) was the key factor influencing feeding behaviour of P. viridis. The activity of amylase and cellulase in the digestive gland and crystalline style, and the scope for growth (SFG) were determined in February, May, July and October to represent the situation in winter, spring, summer and autumn, respectively. There was no correlation between digestive enzyme activities and seasons except for amylase activity in the digestive gland. SFG was lowest in February and highest in October, and was correlated with f.

Feeding behaviour of the green mussel, Perna viridis (L.):: Responses to variation in seston quantity and quality

Abstract Mussels, Perna viridis (Linnaeus), were exposed to different food suspensions consisting mixtures of sediment and microalgae (the diatom Skeletonema costatum and the green microalga Dunaliella tertiolecta). The designed ranges for the contents were: total particulate matter (TPM: mg l−1) 4.46–24.19, particulate organic matter (POM: mg l−1) 0.75–7.19, particulate inorganic matter (PIM: mg l−1) 2.38–17.00, and organic content of seston (f) 0.12–0.64. Clearance rates, rates of filtration, pseudofaeces production, ingestion and absorption, were analyzed at these various food quantity and quality. Clearance rates decreased exponentially with the increase of the particulate organic matter. Pseudofaeces production was a positive function of the rate of particle filtration and more pseudofaeces was rejected when filtered matter had a lower organic content. The ingestion rate decreased with the increase of the organic content of seston but the organic ingestion rate increased slowly with the organic content of seston. The pre-ingestive food selection enhanced the rate of particulate organic matter ingestion and the organic enrichment of ingested matter became more pronounced for diets of low food value, of which most filtered matter was rejected as pseudofaeces. The absorption rate was dependent on the content of particulate organic matter and the absorption efficiency increased as the organic content of ingested matter increased. Therefore, feeding processes of green mussels appear well adapted to cope with changes in food quantity and quality. At high particle concentrations but simultaneous low food values in the water column, green mussels have high rates of seston filtration and pseudofaeces production, together with preferential organic ingestion act to compensate for the dilution of organic matter in suspension. The results will be discussed with respect to spatial variations of the food environment and differential growth of P. viridis in Hong Kong waters.

The combined effects of oxygen availability and salinity on physiological responses and scope for growth in the green-lipped mussel Perna viridis.

Mussels were maintained for 4 weeks under different combinations of dissolved oxygen concentration (1.5, 3.0 and 6.0 mg O2 l(-1)) and salinity (15, 20, 25 and 30) in a 3×4 factorial design experiment. Clearance rate (CR), absorption efficiency (AE), respiration rate (RR) and scope for growth (SFG) decreased with decreasing salinity and dissolved oxygen concentration (DO), while excretion rate (ER) increased with decreasing salinity and increasing DO. The O:N ratio was <10 at salinities of 15 and 20, irrespective of DO levels. SFG was negative in most of the treatments, except for those under 6.0 mg O2 l(-1) or at a salinity of 30 when DO was lower. The results may help explain the distribution pattern of Perna viridis in Hong Kong waters and provide guidelines for mussel culture site selection.

Effects of heavy metals on the survival and feeding behaviour of the sandy shore scavenging gastropod Nassarius festivus (Powys).

The effects of Cu, Cd, Zn and Cr on the survival and feeding behaviour of the sandy shore scavenging gastropod Nassarius festivus were compared. The 96-h LC50 for Cu, Cd, Zn and Cr were 0.36, 1.52, 1.76 and 36.9 mg l(-1), respectively. Four sublethal concentrations of each metal plus a control were prepared and the snails were exposed to experimental solutions for 96 h. Feeding behaviour was studied after the snails were starved for five days. As compared with the control, the number of individuals feeding was significantly reduced by exposure to 0.05 mg l(-1) Cu, 0.2 mg l(-1) Zn, 0.5 mg l(-1) Cd and 5 mg l(-1) Cr. The time spent feeding was greater for individuals exposed to greater concentrations of Zn and Cd but no effect was found for Cu and Cr. Chemoreception of food was studied by placing the snails at a fixed distance of 15 cm from the bait. The success rate of reaching the bait was less for individuals exposed to Cr but no effect was found for Zn, Cu or Cd. The time required for an individual to reach the bait decreased as the concentration of Zn increased. In contrast, a longer time was required for individuals exposed to Cr whereas the effect of Cd and Cu was insignificant. The potential of using feeding behaviour and chemoreception in contaminant evaluations is discussed.

Effect of prolonged hypoxia on food consumption, respiration, growth and reproduction in marine scavenging gastropod Nassarius festivus.

The effects of prolonged exposure to reduced oxygen levels (3.0 and 1.5 mg O(2)l(-1)) on marine scavenging gastropods Nassarius festivus were studied for 8 weeks. The percentages of individuals engaged in feeding and amount of food consumed were reduced as oxygen level decreased; absorption efficiency, however, did not vary significantly with oxygen level. Oxygen consumption rates and specific oxygen consumption rates were lower at reduced oxygen levels. Reproduction occurred at all oxygen levels with less egg capsules being produced at lower oxygen levels. Egg size and number of eggs per capsule, however, were not significantly affected by oxygen level. The increase in shell length was 12%, 6% and 5% at 6.0 mgO(2)l(-1) (normoxia), 3.0 mgO(2)l(-1) and 1.5 mgO(2)l(-1), respectively. At the end of the experiment, the amount of energy allocated to growth and reproduction decreased at reduced oxygen levels with values obtained at 3.0 mgO(2)l(-1) and 1.5 mgO(2)l(-1) being 48% and 70% lower than those at 6.0 mgO(2)l(-1). At all oxygen levels, most of the accumulated energy was allocated to shell growth and reproduction, and the amount allocated to somatic growth was relatively insignificant. The reduction in energy allocated to reproduction was greater than that to shell growth as the oxygen level was reduced, indicating a strategic energy allocation of marine scavengers under stressful conditions to enhance survival.

Effects of heavy metals on oxygen consumption and ammonia excretion in green-lipped mussels (Perna viridis)

Physiological responses (oxygen consumption and ammonia excretion) of the green-lipped mussel (Perna viridis) exposed to four concentrations of Cd (0.15, 0.32, 0.70 and 1.50 ppm) and four concentrations of Zn (0.60, 1.29, 2.79 and 6.00 ppm) were monitored for 21 days. Changes occurred in the ammonia excretion rates and O:N ratios with individual mussels experiencing different metal concentrations over time. Oxygen consumption rate decreased significantly with time for Cd-exposed individuals and also decreased significantly with the interaction between the concentration of Zn and time for Zn-exposed individuals. Although low O:N ratios (<30) were obtained in most of the treatments, no predictable correlation was found between concentrations of metals and values of O:N obtained. The value of using O:N ratio as a stress index is questioned.

Horseshoe Crabs in Hong Kong: Current Population Status and Human Exploitation

An updated survey, using both random transect and walk-through search methods, at 17 shores in Hong Kong in summer and winter showed that juvenile horseshoe crabs (Tachypleus tridentatus and Carcinoscorpius rotundicauda) were significantly reduced by over 90% in density since 2002. Of the shores where juveniles were found, the highest density of T. tridenatus was 0.31 and lowest 0.08 ind 100 m–2. Juvenile C. rotundicauda was only found using the walk-through search method, with the highest record of 1.17 and lowest 0.17 ind hr–1 person–1. The mean prosomal width of juvenile T. tridentatus obtained from the walk-through survey varied from 2.6 to 5.5 cm, which corresponded to an age of 4–8 years old. A larger size range for C. rotundicauda was, however, noted, from 2.5 to 9.0 cm.

Feeding rhythms of the green-lipped mussel, Perna viridis (Linnaeus, 1758) (Bivalvia: Mytilidae) during spring and neap tidal cycles.

Food availability and feeding responses of the green mussel Perna viridis were investigated for two complete tidal cycles during both spring and neap tides. Temporal changes in total particulate matter (TPM), particulate inorganic matter (PIM) and particulate organic matter (POM), were smaller during neap than spring tides. Seston characteristics at different times of a tidal cycle were compared for both spring and neap tides. Only during spring tides were TPM and PIM significantly higher at high tides while POM remained relatively constant (P>0.05). The clearance rate of the mussels underwent temporal variations with tides, and was a negative power function of TPM and a positive linear function of f (organic content), during both spring and neap tides. f was the key factor influencing filtration rate, organic ingestion rate, absorption rate and absorption efficiency. All feeding rates increased linearly with increases in organic content. Pseudofaeces were produced only during spring but not neap tides. Feeding rates and absorption efficiency were highest at low and lowest at high tides (P<0.01). There was no significant temporal change in the wet weight and protein content of the crystalline style with the tidal regime. For the digestive gland, alpha-amylase activity was higher at spring than at neap tides, and higher during high tides in a tidal cycle. The digestive gland cellulase activity did not change significantly with the tides. For the crystalline style, both the activity of cellulase and alpha-amylase were not significantly different (P>0.05) between spring and neap tides. Tidal rhythms in feeding and digestion in this species were likely controlled by temporal variations in food availability in the seawater. By adjusting feeding rates and enzymatic activities, absorption in Perna viridis remained constant, irrespective of the changes in food availability.