Characterization of subpopulations and immune-related parameters of hemocytes in the green-lipped mussel Perna viridis.

The green-lipped mussel Perna viridis is distributed widely in the estuarine and coastal areas of the Indo-Pacific region and extensively cultured as an inexpensive protein source. Morphology and immunological activities of hemocytes of P. viridis were investigated using flow cytometry and light and electron microscopy. Three major types of hemocytes were identified in the hemolymph, including dense-granulocyte, semi-granulocyte (small and large size) and hyalinocyte. Other hemocytes, which occurred in low numbers, included granulocytes with different electron-dense/lucent granules and hemoblast-like cells. Based on flow cytometry, two subpopulations were identified. Granulocytes were larger cells, and the more abundant, containing numerous granules in the cytoplasm, and hyalinocytes were the smaller and less abundant with the fewest granules. Flow cytometry revealed that the granulocytes were more active in cell phagocytosis, contained the higher lysosomal content, and showed higher esterase activity and reactive oxygen species (ROS) generation compared with hyalinocytes. Immune functions assessed by the flow cytometry indicated that the granulocytes were the main hemocytes involved in the cellular defence in P. viridis.

Do distribution and expansion of exotic invasive Asteraceae plants relate to leaf construction cost in a man-made wetland?

Exotic species especially Asteraceae plants severely invade wetlands in Shenzhen Bay, an important part of the coast wetland in Guangdong-Hong Kong-Macau Bay Area, China. However, the reasons causing their expansion are unclear. The leaf traits and expansion indices of six invasive Asteraceae plants from the Overseas Chinese Town (OCT) wetland were studied and the results showed that nearly 45% of the total plant species (31 out of 69 species) in the OCT wetland, belonging to 15 families and 27 genera, were exotic invasive species. The expansion indices of six Asteraceae species negatively correlated with their leaf construction cost based on mass (CCM), caloric values and carbon concentration, but their relations with ash content were positive. Multiple linear regression analysis revealed that CCM was the most important factor affecting the expansion of an exotic species, indicating CCM may be an important reason causing the expansion of exotic species in coastal wetlands.

Is Laguncularia racemosa more invasive than Sonneratia apetala in northern Fujian, China in terms of leaf energetic cost?

Laguncularia racemosa and Sonneratia apetala are fast-growing exotic mangrove species in Southern China and widely used for afforestation. However, the invasiveness of the two exotic species is still unclear. We compared structural and physiological traits and energy-use related traits between L. racemosa and S. apetala, and with two natives (Kandelia obovata and Aegiceras corniculatum) in northern Fujian. Results showed that leaf construction cost based on mass (CCM) and caloric values of L. racemosa were significantly lower than S. apetala, and the two natives had highest CCM. Because lower CCM, L. racemosa grew faster with a taller height (4.83 m) and wider ground diameter circumference (40.03 cm) than S. apetala (4.43 m tall and 35.63 cm wide) and the two natives (2.42 m tall and 26.78 cm wide). These findings indicated that L. racemosa could be more invasive than S. apetala in mangrove forests in northern Fujian, China where it still grew well, which deserves more attention.

Effects of the timing of initial feeding on growth and survival of spotted mandarin fish Siniperca scherzeri larvae.

The effects of delayed first feeding on growth and survival of spotted mandarin fish Siniperca scherzeri larvae were examined under controlled conditions. Morphometric characters [yolk-sac volume, oil globule volume, head depth (H(D)), body depth (B(D)), eye diameter (E(D)), musculature height (M(H)), mouth diameter (M(D)) and total length (L(T))], body mass (M), specific growth rate (S(GR)) and survival were evaluated under different first-feeding time (2, 3, 4 and 5 days after hatching). Larvae began to feed exogenously at 2 days after hatching (DAH) and the point of no return (P(NR)) occurred between 5 and 6 DAH at 23 degrees C, range +/-1.0 degrees C. The yolk volume of larvae first-fed at 2 days had a significant difference compared with that of larvae first-fed at 3, 4 and 5 days on 3 and 4 DAH. The larvae first-fed at 2 days achieved comparatively better growth performance than that of 3, 4 and 5 days. On 5 DAH, all morphometric characters had significant differences between 2 and 5 days and 2 and 4 days initial feeding, respectively. Total mortality was recorded on 9 DAH for the larvae first-fed at 5 days. On 12 DAH, significant differences were observed between 2 and 4 days and 3 and 4 days initial feeding for all morphometric characters. From 16 DAH to the end of experiment, all growth variables of the larvae first-fed at 2 days were significantly higher than those in other treatments. The S(GR) (2-9 DAH) first-fed at 2 and 3 days were significantly higher than 4 and 5 day treatments, and the S(GR) (9-16 DAH) first-fed at 2 days was significantly higher than 3 and 4 day treatments. There was no significant difference, however, of S(GR) (16-28 DAH) among treatments. Survival rate was significantly higher at 2 days initial feeding (27.42%) when compared with 3 (15.96%) and 4 days (7.92%) initial feeding at the end of experiment. The present study suggests that the first feeding of S. scherzeri larvae should be initiated at 2 days after hatching for achieving good growth and survival.

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.

Spatio-temporal changes of marine macrobenthic community in sub-tropical waters upon recovery from eutrophication. II. Life-history traits and feeding guilds of polychaete community.

A two-year study was conducted in the vicinity of a harbour in sub-tropical Hong Kong, to examine the progress of recovery of macrobenthic community, based on analyses of both life-history traits and trophic guilds of polychaetes, upon cessation of organic pollution caused by sewage discharge. Seventy seven out of 83 species collected were classified under four ecological groups based on the life-history traits and sensitivity to organic gradients. The mean ATZI marine biotic index (AMBI) derived from these ecological groups showed spatial difference among the five sampling locations. In particular, the presence of different percentages of polychaete species from Groups III (unbalanced community) and IV (polluted community) suggested the presence of pollution stress in certain degree at all sampling locations. However, no significant temporal changes were noted over the study period. From all polychaete species identified, they were classified into 13 feeding guilds. The mean diversity of these feeding guilds at most of the sampling locations was significantly higher than that at one of the inside-harbour locations. The composition of feeding guilds was also significantly different spatially. At one of the inside-harbour locations, the dominant feeding guilds were motile/discretely motile surface deposit feeders with tentaculates or unarmed pharynx, and motile omnivores with jawed pharynx in the first year of study, but were replaced by motile, jawed carnivores in the second year of study. The increased proportion of carnivores over the study period can be seen as a sign of recovery in the community structure since abundance of predators is commonly higher in habitats with better environmental quality. The implications of using life-history traits and feeding guild analyses for benthic community are discussed.

Spatio-temporal changes of marine macrobenthic community in sub-tropical waters upon recovery from eutrophication. I. Sediment quality and community structure.

A two-year study was conducted in the vicinity of a harbour in sub-tropical Hong Kong, to examine the progress of improvement in sediment quality and recovery of macrobenthic community upon cessation of organic pollution caused by sewage discharge. Median sediment particle diameter (Mdphi) and levels of total organic carbon (TOC), total Kjeldahl nitrogen (TKN), ammonia-nitrogen (NH(3)-N) and total phosphorus (TP), as well as macrobenthic species composition and abundance were determined bi-monthly at three inside-harbour and two outside-harbour locations. At the two inside-harbour locations, significantly higher levels of TOC, TKN, NH(3)-N and TP in sediments were observed than the outside-harbour locations. However, no significant temporal change of nutrient levels was detected at all sampling locations during the two-year study, except a significant decrease in TKN and NH(3)-N levels at one outside-harbour location and a decline in NH(3)-N content at another outside-harbour location. Spatially, the highest in mean total species number, individual number, species diversity H' and lowest evenness J was found at one outside-harbour location, whereas the other four locations had relatively similar values. H' was negatively correlated with TOC, TKN, NH(3)-N and TP content in sediments while J was positively correlated with MDphi. Across the study period, the patterns of macrobenthic community were significantly different among all samplings and that the spatial difference of benthic community was best correlated with MDphi, TOC, TKN and water depth. A weak sign of recovery in macrobenthic community structure was detected at inside-harbour locations, with replacement of opportunistic by ubiquitous species over the two-year study. The slow recovery of macrobenthic community was related to sediment characteristics. Results of a larval settlement bioassay using the polychaete Capitella sp. I also demonstrated that the inside-harbour sediments were still unfavourable for colonization and larval settlement of species sensitive to pollution. The slow biodegradation of the organic pollutants and continuous presence of heavy metals in the sediment may hinder settlement and colonization of benthic animals. However, increases of H' and J were observed in a longer time scale when comparing the present data with those obtained four years ago. This suggested that a detectable recovery of benthic community in the harbour may take at least three years and a complete recovery may even take longer duration.

Induction, adaptation and recovery of lysosomal integrity in green-lipped mussel Perna viridis.

Biomarkers are generally applied to detect pollution in environmental monitoring. Such biological responses should accurately reflect the stress over time in a quantitative manner. As such, the initial and maximum responses induced by stress, as well as adaptation and recovery of these biomarkers, need to be fully understood or else erroneous false-negative or false-positive may be arrived. However, most of the biomarker studies only provided information on initially induced responses under different concentrations of toxicants, while biological adaptation and recovery were poorly known. In this study, the time required for induction, adaptation and recovery of lysosomal integrity in green-lipped mussel Perna viridis upon exposure to benzo[a]pyrene was investigated over a period of 62 days. Maximum induction occurred on day 6 when lysosomal integrity was significantly reduced by 51%, and no further change or adaptation was detected thereafter. When mussels were depurated in clean seawater after 18 days of exposure to benzo[a]pyrene, a gradual recovery was observed, with lysosomal integrity returning to its background level and showing a complete recovery after 20 days of depuration. Lysosomal integrity was significantly correlated with the body burden concentrations of benzo[a]pyrene and condition index of the mussels. The relatively fast induction (6 days) and recovery (20 days) without apparent adaptation suggested that lysosomal integrity in P. viridis can serve as a good biomarker in biomonitoring, as its response is not likely to generate both false-negative and false-positive results.

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.

Influences of ammonia-nitrogen and dissolved oxygen on lysosomal integrity in green-lipped mussel Perna viridis: laboratory evaluation and field validation in Victoria Harbour, Hong Kong.

Lysosomal integrity in mussels has been applied as a biomarker to detect the pollution of trace organics and metals in the natural environments. However, few studies have examined the effects of water quality on the response of lysosomal integrity, in particular total ammonia-nitrogen (TAN) and dissolved oxygen (DO). This study demonstrated that high level of TAN (2.0mg/l) and low DO (2.5mg O(2)/l) could significantly reduce the lysosomal integrity in green-lipped mussel Perna viridis, respectively by 33% and 38%, whereas the mussel lysosomal integrity decreased by 70% in the combined treatment of TAN and low DO under laboratory conditions after one week. The mussel lysosomal integrity of all treatment groups could return to the control level after a three week recovery period. In the field validation in Victoria Harbour, Hong Kong during an one-year study period, lysosomal integrity in P. viridis identified the cleanest site east to the harbour, where the lowest TAN and highest DO concentrations were found. While lysosomal integrity in mussels seemed not affected by seasonal changes, approximately 40% of the variation of this biomarker could be attributable to the changes in TAN and DO in seawater. In conclusion, the response of the mussel lysosomal integrity can be confounded by both TAN and DO prevailing in the natural environments and thus caution must be exercised in relating the observed changes in lysosomal integrity to any specific pollutant in coastal water quality monitoring studies.

The combined effects of elevated pCO2 and food availability on Tigriopus japonicus Mori larval development, reproduction, and superoxide dismutase activity.

Previous studies have shown that ocean acidification has little effect on adult Tigriopus japonicus copepods, and mainly impairs the early development and reproduction of females. This study investigated the possible interactive effect between CO2-induced seawater acidification and food availability on larval development and reproductive output in T. japonicus. Copepods were exposed to either pH8.1 or pH7.3 under different food concentrations (0.5×104-80.0×104cells/mL). Both the development of nauplii and copepodites was delayed at pH7.3 with a greater effect at lower food concentrations. The reproductive output followed a bell-shaped curve with the highest reproductive output at food concentrations between 30×104 and 40×104cells/mL. As an indicator of oxidative stress, the activity of superoxide dismutase increased at lower pH, with a greater increase at lower food concentrations. Therefore, the effect of elevated pCO2 on T. japonicus was food dependent.

Predator-prey interaction between muricid gastropods and mussels under ocean acidification.

Predation of the muricid gastropod Thais clavigera on two-sized groups of the mussel Brachidontes variabilis was studied under three pCO2 levels, 380, 950, and 1250µatm. At 950µatm pCO2 level, the prey handling time decreased significantly and large-sized B. variabilis were preferred by T. clavigera. However, the prey consumption rate was independent of pCO2 levels, although the prey searching time increased significantly at elevated pCO2. These findings indicated that the predator-prey interaction between T. clavigera and B. variabilis was altered under ocean acidification, which will have a long-term impact on the population dynamics of the interacting species.

Microplastic ingestion reduces energy intake in the clam Atactodea striata.

The effects of microplastic concentrations (10itemsl-1 and 1000itemsl-1) on the physiological responses of Atactodea striata (clearance rate, absorption efficiency, respiration rate) were investigated. The fates of ingested microplastics and the efficiency of depuration in removing ingested microplastics were also studied. A. striata ingested microplastics and the clearance rate was reduced at high concentration of microplastics. Since the respiration rate and absorption efficiency remained unchanged in exposed A. striata, reduction in the clearance rate would reduce the energy intake. Ingestion and retention of microplastics in the body were further limited by the production of pseudofaeces and faeces, and depuration in clean water, resulting in a very small amount of microplastics stored in the body of the clam.

Size effects of suspended particles on gill damage in green-lipped mussel Perna viridis.

The green-lipped mussels Perna viridis were exposed to <500 microm suspended solids (SS) with concentrations of 0 (control), 250, 500, 750 and 1000 mg/L for 14 days, followed by transferring to clean, filtered seawater for 28 days. Results of scanning microscopy showed significantly higher damages to the cilia on the frontal surface of the gill filaments than that on the abfrontal surface in both demibranchs. Percent ciliary depletion varied with SS concentrations and time. No sign of recovery of the gill filaments was observed after the mussels were transferred to clean seawater. In a second experiment, mussels were exposed to SS with size range from <63, >125-<250 and >250-<500 microm at 600 mg/L, together with a control (0 mg/L) for 14 days, followed by transferring to clean, filtered seawater for 28 days. Results of scanning microscopy showed significant ciliary damages in both the ascending and descending lamellae under the three particle size groups as compared with the control. Percent depletion of frontal cilia was most serious for the >250-<500 microm size group and least for the <63 microm size group. However, percent depletion of abfrontal cilia was most serious for the >125-<250 microm size group and least for the <63 microm size group. No recovery of ciliary damages was observed. The effects of particle size of suspended sediments on the morphological damages of gill filaments in the green-lipped mussels were discussed.

Intermittent exposure to reduced oxygen levels affects prey size selection and consumption in swimming crab Thalamita danae Stimpson.

Portunid crabs Thalamita danae (carapace width: 46-56 mm) were exposed to low oxygen level (4.0 mg O2 l(-1)) and hypoxia (1.5 mg O2 l(-1)) for 6 h each day with three size classes (large: 15.0-19.9 mm, medium: 10.0-14.9 mm, small: 5.0-9.9 mm) of mussels Brachidontes variabilis offered as food. Consumption rate, prey size preference, and prey handling including breaking time, handling time, eating time and prey value, were studied during the time the crabs were exposed to reduced oxygen levels and results were compared with the crabs maintained at high oxygen level (8.0 mg O2 l(-1)) throughout the experiment. Consumption of mussels from all size classes was significantly higher at high oxygen level than at reduced oxygen levels. No mussel size preference was observed for crabs exposed to 4.0 or 8.0 mg O2 l(-1) but those crabs exposed to 1.5 mg O2 l(-1) preferred medium mussels. Both breaking time and handling time increased with mussel size but did not vary with oxygen level. Prey value of each mussel consumed (mg dry wt eaten crab(-1) s(-1)) was calculated by dividing the estimated dry weight of the mussel by the observed handling time. Mean prey value varied significantly with mussel size, with values obtained for large mussels being higher than small mussels at 4.0 and 8.0 mg O2 l(-1); the effect of oxygen level, however, was insignificant. In view of portunid crabs as major predators of mussels, results may help explain dominance of mussels in eutrophic harbours in Hong Kong.

Burrowing responses of the short-neck clam Ruditapes philippinarum to sediment contaminants.

The burrowing responses of a common tropical bivalve, the short-neck clam Ruditapes philippinarum, to cadmium (Cd)-spiked sediment, variations of sediment grain size and natural sediments collected from 15 locations in Hong Kong's inshore waters were investigated through a series of laboratory tests. Results showed that the burrowing response exhibited a negative relationship with an increase in Cd concentration in the spiked sediments. The level of Cd was also found to be directly proportional to the percent mortality of the clam. However, there was no significant difference (p > 0.05) in the time elapsed for the clam to burrow into sediments with different grain size composition. The elapsed time for 50% of the test clams to burrow into the sediment (ET50) over a period of 48 h was calculated for the sediment samples collected from the 15 locations. Results of ANOVA showed significant difference (p < 0.05) among the sediment samples. Tukey's multiple comparison test revealed two groups of sediments: group 1 containing 3 sediment samples collected from Victoria Harbour and group 2 containing 12 samples obtained from other coastal areas of Hong Kong. The ET50 value for group 1 sediments was found to be greater than 2,880 min whilst the ET50 for group 2 sediments had a mean of 173.9 min. Agglomerative classification of the sediment samples, based on metal content (As, Cd, Cr, Cu, Ni, Pb, Zn), also showed two similar groups, suggesting that the ET50 values were correlated with the metal level of the sediment samples. Group 1 sediments collected from Victoria Harbour had greater concentrations of Cd, Cr, Cu, Ni and Zn as compared to that in group 2. The present study demonstrated that high metal content in the sediment tends to inhibit the burrowing responses of the clam and that there is potential to develop the burrowing responses of R philippinarum as a sublethal sediment toxicity test.

Responses of the green-lipped mussel Perna viridis (L.) to suspended solids.

Laboratory experiments were conducted to investigate the lethal and sublethal effects of suspended solids on the survival and physiological, behavourial and morphological changes of the green-lipped mussel Perna viridis collected from Tolo Harbour, Hong Kong. Results showed that P. viridis survived in all test conditions of suspended solids from 0 to 1,200 mg/l over a period of 96 h. Physiological responses of the green-lipped mussel under 14-d exposure of suspended solids from 0 to 600 mg/l, followed by 14-d recovery in natural seawater, revealed no significant changes (p > 0.05) in oxygen consumption and dry gonosomatic index for treatments in different concentrations of suspended solids and exposure time. Changes in clearance rate were only found to be significant (p < 0.001) with exposure time. Responses in behavourial and morphological changes of the green-lipped mussel were also studied under similar experimental treatments and exposure time. Byssus production was significantly (p < 0.001) related to exposure time. Gill damage, however, was significantly greater in treatments (p < 0.001). Present findings suggested that P. viridis could tolerate a high level of suspended solids in the laboratory. There were dose-dependent effects of suspended solids on morphology of gill filaments. Implications of survival and responses of the green-lipped mussel to suspended solids in the marine environment are discussed.

Comparisons of the metabolic responses of two subtidal nassariid gastropods to hypoxia and re-oxygenation.

Changes in the levels of carbohydrate, lipid, protein and anaerobic metabolites (succinate, lactate, acetate, fumarate and propionate), upon exposure to hypoxia (1.5 mg O2 l(-1)) and after reoxygenation in subtidal gastropods Nassarius siquijorensis and N. conoidalis, were compared. A significant decrease of the glycogen content was observed under hypoxia in N. conoidalis but not in N. siquijorensis. A greater increase in the concentrations of anaerobic metabolites was observed in N. conoidalis under hypoxia, and their levels did not return to baseline after returning to normoxia for 24h. In contrast, a lower rate of accumulation of the metabolites was observed in N. siquijorensis, and complete recovery from anaerobic metabolism was observed after reoxygenation. The results lend further support to the role of hypoxia in governing the different distributional patterns between the two subtidal gastropods in Hong Kong waters.

Isolation and mRNA expression of hypoxia-inducible factor α (HIF-α) in two sublittoral nassariid gastropods: Nassarius siquijorensis and Nassarius conoidalis.

Hypoxia-inducible factor (HIF) is commonly found in vertebrates as an adaptation against hypoxia. In this study, partial sequences of HIF were first reported for subtidal nassariid gastropods, Nassarius siquijorensis and Nassarius conoidalis. The phylogenetic tree of HIF-α among various animal groups was inferred. Adults of both N. siquijorensis and N. conoidalis were exposed to 24 h and 1 week hypoxia (1.5 mg O2 l(-1)) and the change of HIF-α mRNA level was determined in gill, foot muscle and hepatopancreas by using quantitative real-time PCR. The expression profile of HIF-α was also investigated in veliger larvae after 1 and 24 h hypoxia exposure. The HIF-α mRNA from gills of N. siquijorensis was 2.22-fold elevated after 24 h of exposure but returned back to the baseline level after 1 week of exposure. In contrast to N. siquijorensis, both short-term and long-term exposure to hypoxia significantly enhanced the mRNA level of HIF-α in the gill of N. conoidalis. Both 24 h and 1 week of exposure resulted in a significantly higher degree of expression of mRNA of HIF-α in the foot muscle of N. siquijorensis and N. conoidalis. For hepatopancreas, the mRNA level of HIF-α remained unchanged in N. siquijorensis after both 24 h and 1 week of exposure to hypoxia but a significant enhancement was observed in N. conoidalis after 1 week of exposure to hypoxia. The mRNA level of HIF-α significantly reduced after 24 h of exposure to hypoxia in the larvae of N. siquijorensis. The different expression patterns of HIF-α mRNA between the two species reflected their different adaptation abilities to hypoxia, which will further explain the differences in the distribution of these species in Hong Kong waters with N. siquijorensis being predominant in regions suffered from summer hypoxia.

Acute hypoxic exposure affects gamete quality and subsequent fertilization success and embryonic development in a serpulid polychaete.

Hypoxia likely compromises the reproductive success of those marine organisms carrying out external fertilization because their gametes and embryos are inevitably exposed to the external environment. Hydroides elegans, a dominant serpulid polychaete in Hong Kong waters, can spawn throughout the year but the number of recruits drops during summer when hypoxia commonly occurs. This study attempted to explain such observation by investigating the gamete quality, including sperm motility, egg size, complexity and viability, after 1-h hypoxic exposure (1 mg O2 l(-1)). In addition, how gamete quality affects fertilization success and embryonic development was examined. After 1-h hypoxic exposure, sperm motility was significantly reduced, compromising fertilization success. Although the eggs remained viable, more malformed embryos and retarded embryonic development were observed. We interpreted that the harmful effect of hypoxia on embryonic development was attributed to the teratogenicity and induced oxidative stress, ultimately causing the reduction in recruitment during summer.