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 1250 μatm. At 950 μatm 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.

The effects of microplastic concentrations (10 items l− 1 and 1000 items l− 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.

Hypoxia due to the over enrichment of waters by nutrients is becoming a global problem. In mussels, enhanced byssus thread production is an important adaptation to the presence of crustacean predators and to energetic hydrodynamic regimes. Thread production is an energy-consuming process, so this study used the green mussel Pernaviridis (L.) to examine the response to predator exposure combined with hypoxia. Hypoxia is common in sheltered bays in Hong Kong, and the mussels were collected in one such bay, Lok Wo Sha (latitude/longitude: 22º18′ N/114º10′ E) in January, 2009. The predator used in the experiments was the swimming crab Thalamita danae. Oxygen concentrations used in the 48-h experiments ranged from hypoxic to normoxic (1.5 ± 0.3 mg l−1, 3.0 ± 0.3 mg l−1 and 6.0 ± 0.3 mg l−1). Fewer byssus threads which were also shorter and thinner were produced at reduced oxygen levels, no matter if the predator was present or not; the frequency the mussels shed stalks was also lower. Mussels exposed to the predator, however, have enhanced byssus thread production at all oxygen levels when compared with the control. This has highlighted the significance of anti-predator responses for the survival of individuals even under a stressful environment in which energy supply is limited by aerobic metabolism. Interactive effects between oxygen level and predator exposure were observed for the byssus thread production (frequency of shed stalks, mean byssus thread length, cumulative byssus thread volume), with values obtained at 1.5 and 3.0 mg O2 l−1 being statistically indistinguishable for the control group without predator but not for the predator group. The lack of differences in the byssus thread production at lower oxygen levels in the absence of predator may indicate the minimum amount of byssus that is required for settlement on a substrate.

Effects of low dissolved oxygen on early development and swimming behaviour of veliger larvae of the scavenging gastropod Nassarius festivus were studied. Embryonic development was significantly delayed when dissolved oxygen level was reduced to 3.0 mg O2 l−1 and no embryo hatched successfully at 0.5 mg O2 l−1. Veliger larvae hatched at 4.5 mg O2 l−1 had significantly smaller velar lobe, shell length and shell width. Median 48-h LC50 value of the veliger larvae was estimated at 1.25 mg O2 l−1 with lower swimming speed (swimming velocity and dispersal velocity) being recorded for the survivors exposed to reduced oxygen levels. The percentage of veliger larvae that developed into crawling juveniles was significantly reduced and metamorphosis was delayed at 4.5 mg O2 l−1 whereas all larvae at 3.5 mg O2 l−1 died before they underwent metamorphosis. Juveniles developed at 4.5 mg O2 l−1 were also smaller than those at 6.0 mg O2 l−1. Results indicated that dissolved oxygen levels well above hypoxia levels (2.8 mg O2 l−1) have already had significant impact on the hatching success and larval development in gastropods, which may lead to long-term decreases in population growth.

This study investigated whether a mussel predator (the swimming crab Thalamita danae) and a scavenger (the gastropod Babylonia lutosa) could induce anti-predator response in the green-lipped mussel Perna viridis by chemical means. The crabs and gastropods had been either starved or recently fed with the mussels. We examined the number, diameter, length and volume of byssus threads produced by the mussels in order to compare the intensity of anti-predator responses when they were exposed to different stimuli. Our results showed that the mussels produced a significantly larger volume of byssus threads when they were exposed to a crab that had recently consumed conspecifics than the mussels in the control group. The starved crab had a weak effect on increasing the number, length, diameter and volume of byssus threads. Furthermore, the scavenging gastropods failed to increase byssus thread production in the mussels, no matter they had been starved or consumed conspecifics. Indeed, byssus thread production is energetically costly. It would be maladaptive for the mussels to increase byssus thread production in response to a low predation risk presented by a scavenger.Research Highlights► Anti-predator response was induced in prey when a predator fed on prey conspecifics. ► Anti-predator response was not elicited when the prey was consumed by a scavenger. ► Predator is essential in transforming digested products of prey into diet cues.

Green-lipped mussels Perna viridis, collected from Peng Chau, Hong Kong were allotted into two treatment groups, each containing three experimental tanks. The first treatment group comprised of mussels fed with the diatom Thalassiosira pseudonana only, whereas the second treatment group contained mussels fed with the marine rotifer Brachionus plicatilis, which was in turn fed with diatom T. pseudonana. The mussels were fed two times each day over the experimental period of 14 days. On Days 4, 7 and 14, three mussels were collected from each tank of each treatment group and treated as a single replicate. Fatty acid profiles of diatoms, marine rotifers and the three organs (digestive gland, mantle margin and adductor muscle) of the two mussel groups were analyzed. Results showed that monosaturated fatty acid (MUFA) 16:1n7 was conserved along the food chain among diatoms, marine rotifers and green-lipped mussels. This suggested that 16:1n7 or the ratio of 16:1n7 to saturated fatty acid (SFA) 16:0 can be a trophic marker for diatom T. pseudonana and elevated amounts of 16:1n7 in mussels can reflect the dominance of diatoms in its diet. The present results also showed that rotifers could accumulate MUFA 18:1n7 and PUFA 20:4n6 which were transferred up to mussels, especially 18:1n7, as zooplankton have the ability to synthesize or actively accumulate certain fatty acids that they need for growth or reproduction. There was an increase in the amount of 18:1n7 in the digestive gland of mussels fed with rotifers but the level of this fatty acid remained unchanged in those fed with diatoms, further confirming that 18:1n7 can be used as a marker for the presence of rotifers in trophic relationship studies. The relatively faster responses in the digestive gland of mussels to acquire the fatty acid signatures from their food suggested that the fatty acid profiles in the digestive gland can be a good marker to show a short-term fluctuation of food conditions in the marine environment.

The present study investigated the growth, secondary production and gonad development of two co-existing amphioxus species Branchiostoma belcheri and B. malayanum in subtropical Hong Kong from June 2005 to June 2006. Based on the modal progression analysis, amphioxus populations were decomposed into separate cohorts. The von Bertalanffy growth models were also estimated according to the size incremental data. From the growth models, the size ranges of one, two and three-year-old B. belcheri were estimated to be 5–28 mm, 28–38 mm and 38–45 mm BL, respectively; while the one and two-year-old B. malayanum were estimated to be 7–30 mm and 30–35 mm BL, respectively. The secondary production was calculated at 1.15 g m− 2 yr− 1 DW or 0.63 g m− 2 yr− 1 AFDW for B. belcheri with density 424 ind m− 2, and 0.51 g m− 2 yr− 1 DW or 0.40 g m− 2 yr− 1 AFDW for B. malayanum with density 121 ind m− 2. The production to biomass ratio (P/B) was 1.13 for B. belcheri and 0.98 for B. malayanum. Changes in the gonad length index indicated that B. belcheri spawned mainly in June and July, while B. malayanum mainly in April and August. As compared with B. belcheri, B. malayanum was characterized by rapid growth, shorter life span, early maturity and lower population density. Such differences in population dynamics may allow both species to share a similar habitat and co-exist in subtropical waters of Hong Kong.