Chaoshu Zeng

Toxic Effects of Ammonia, Nitrite, and Nitrate to Decapod Crustaceans: A Review on Factors Influencing their Toxicity, Physiological Consequences, and Coping Mechanisms

Nitrogenous wastes including ammonia-N, nitrite-N, and nitrate-N are increasingly becoming a global issue in aquatic ecosystems due to escalating anthropogenic activities and are a ubiquitous concern in aquaculture. These pollutants are interrelated via the nitrification cycle, with the direct metabolic product ammonia-N generally being the most toxic with high species specificity. Furthermore, while environmental factors influencing nitrogenous waste toxicity are similar, the causative underlying mechanisms are often substantially different. In this review, we focus on decapod crustaceans due to their high commercial value and likelihood of encountering these pollutants in their benthic or near-benthic habitat. While a large body of publications exists in this area, to date a comprehensive literature review on relative toxicities of all three nitrogenous wastes, physiological consequences, and adaptive mechanisms of crustaceans is lacking. Understanding these processes will likely have implications for environmental/fisheries management and the aquaculture industry. Additionally, there are strong indications that theoretical “safe” values, traditionally used for predicting toxicity thresholds, substantially underestimate the impact of nitrogenous waste on the growth and physiological condition of crustaceans. These consequences will be emphasized along with various methods of uptake, elimination, and detoxification that ultimately explain differences in nitrogenous waste toxicity to decapod crustaceans.

Development of Captive Breeding Techniques for Marine Ornamental Fish: A Review

The increasingly popular aquarium hobby is fueling the rapid growth of the aquatic ornamental industry, particularly the trade of marine ornamental species. However, currently there is a heavy reliance on wild caught marine ornamentals to satisfy consumer demand. As public awareness of the plight of marine ecosystems grows, the often destructive and unmanaged exploitation of coral reefs for the marine ornamental trade has raised concerns. Therefore, there is consensus that urgent actions need to be taken to limit destructive exploitation of coral reefs, and to ensure the sustainability of the marine ornamental industry. An obvious and vital action of such efforts would be the development of captive breeding techniques for marine ornamental species, in particular, coral reef fish. Research efforts towards captive breeding of marine ornamental species, also known as marine ornamental aquaculture, stand to supplement or replace the supply of wild caught specimens for the marine ornamental trade, and potentially help boost reef recovery efforts through restocking. However, unfortunately, the marine ornamental aquaculture sector is still in its infancy, receiving limited research attention, and, in turn, has experienced very slow development compared to the technical and industrial advances made in foodfish aquaculture. While it is true that at present, multiple bottlenecks have severely limited the progress of marine ornamental aquaculture, through careful appraisal and adaptation of culture techniques developed for foodfish, and by addressing the specific needs of marine ornamental aquaculture, significant progress could be made for the marine ornamental aquaculture industry. With this objective in mind, this review attempts to summarize the major bottlenecks facing the marine ornamental aquaculture industry, and to highlight weaknesses in the current state of research. Major areas in need of increased research efforts include broodstock management, such as the development of specific broodstock diets and broodstock husbandry, spawning induction via hormone technologies that are tailored to the size and sensitivity of small broodstock ornamentals, and comprehensive, species-specific larval rearing techniques, including system design and larval culture conditions as well as larval feeds and nutrition.

Macrobrachium rosenbergii nodavirus disease (white tail disease) in Australia

The index case of white tail disease (WTD) is presented in adult broodstock prawns Macrobrachium rosenbergii from the Flinders River in western Queensland, Australia, in mid-2007. Histological examination revealed extensive myonecrosis with massive infiltration of myonuclei and some haemocytes. Juveniles from the same broodstock but not from 3 other families displayed white muscle lesions. Low-grade chronic mortalities approaching 100% over 1 yr occurred. Reverse transcriptase polymerase chain reactions (RT-PCR) were attempted for both M. rosenbergii nodavirus (MrNV) with 2 sets of primers and for the satellite virus, extrasmall virus (XSV). All 3 PCRs generated amplicons of the expected sizes. Basic local alignment search tool (BLAST) analyses of the 3 consensus sequences identified a 91% match with MrNV viral capsid protein gene, 96% match with MrNV RNA-directed RNA polymerase gene, and a 99% match with M. rosenbergii XSV capsid protein gene. The clinical signs, histopathological lesions and RT-PCR amplicons could be reproduced in M. rosenbergii inoculated with cell-free extracts fulfilling Rivers postulates. We conclude that this is an endemic strain of MrNV as the sequences are dissimilar to strains of MrNV circulating around Asia and the Americas. This case only poorly meets the Office International des Epizooties (OIE) case definition for WTD due to the age of the prawns involved and the nature of the inclusion bodies. Perhaps the OIE case definition needs broadening.

Synchronization of endogenous tidal vertical migration rhythms in laboratory-hatched larvae of the crab Carcinus maenas

Abstract Previous studies showed that freshly collected zoea-1 larvae of the shore crab Carcinus maenas from coastal waters possessed endogenous circatidal vertical migration rhythms. Present experiments demonstrate that zoea-1 larvae released by crabs which extruded eggs in the field but were then kept in laboratory constant conditions since collection, also exhibited endogenous circatidal vertical migration rhythms. Such rhythms were shown by newly hatched larvae from nearly all of 50 females which released larvae in the laboratory, some of which had carried eggs in the laboratory for up to 3–4 months. Thus, since entrainment by tidal variables could not have occurred during embryonic development in these cases, other synchronising factors must set the phase of the larval circatidal rhythms. Present results suggest that the hatching process itself is the main synchroniser; upward swimming occurred just after hatching and at 12.4 h intervals thereafter. This is consistent with previous observations of peak hatching in Carcinus primarily around nocturnal high tide, with larvae in the field exhibiting tidal vertical migration rhythms of ebb-phased upward swimming associated with dispersal during ebbing tides. The results appear to be the first to characterize a synchroniser for tidal swimming rhythms of planktonic larvae and to provide experimental evidence that the hatching process can serve as the synchroniser for a larval swimming rhythm.

Acute toxicity of sodium nitrate, potassium nitrate, and potassium chloride and their effects on the hemolymph composition and gill structure of early juvenile blue swimmer crabs(Portunus pelagicus Linnaeus, 1758) (Decapoda, Brachyura, Portunidae).

Various nutrients, including K+ and NO3-, are increasingly being discharged into aquatic systems via anthropogenic sources, which may impact marine organisms. The present study was conducted on blue swimmer crab (Portunus pelagicus) early juveniles to determine the acute toxicity of NaNO3, KNO3, and KCl; if a toxicity interaction exists between K+ and NO3-; the hemolymph Na+, K+, and Ca2+ changes; and the gill histopathological alterations following exposure to elevated NaNO3, KNO3, and KCl levels. A total of 20 replicate crabs were exposed to each of the five NaNO3, KNO3, and KCl concentrations for 96 h. After 96 h, the surviving crabs were sampled for hemolymph Na+, K+, and Ca2+ levels and fixed for histological examination of the anterior gills. The 96-h median lethal concentration of NaNO3-N, KNO3-N, KNO3-K, and KCl-K was 3,452, 112, 312, and 356 mg/L, respectively, for early P. pelagicus juveniles. The toxicity of NaNO3-N was significantly less (p < 0.01) than that of KNO3-N. Furthermore, at the same K+ levels, KNO3-K was significantly (p < 0.05) more toxic than KCl-K, indicating a toxicity interaction between K+ and NO3-. Following exposure to elevated KNO3 and KCl levels, the crabs had significantly higher (p < 0.01) hemolymph K+ levels compared to the control. Conversely, following exposure to elevated NaNO3 concentrations, the crabs had significantly higher (p < 0.01) hemolymph Na+ levels but significantly lower (p < 0.01) hemolymph K+ levels. Despite the markedly different hemolymph ionic changes following NaNO3 and KNO3/KCl exposure, the histopathological changes to the anterior gill lamellae of the crabs appeared to be similar, including lamellae swelling, epithelial thickening, pillar cell disruption, necrosis, and distortion.

Heritability of circatidal vertical migration rhythms in zoea larvae of the crab Carcinus maenas (L.)

Previous studies showed that in the crab Carcinus maenas, endogenous circatidal vertical migration rhythms are exhibited by zoea-1 collected from the field and by larvae newly hatched in the laboratory from field collected ovigerous females. In the present studies, non-ovigerous females were brought into laboratory and kept in non-tidal conditions until eggs were produced. About 100 such females spawned in the laboratory during 1992–1994. Observations were then made on swimming behaviour patterns of groups of larvae hatched, either naturally or from detached eggs, from more than 20 of these laboratory-spawned females, some of which had been kept in the laboratory for nearly a year and one of which spawned twice with an interval of 8 months. Most groups of larvae hatched under such circumstances exhibited vertical migration rhythms of circatidal periodicity despite never having experienced tidal conditions in their life time, even as embryos. Such behaviour is particularly surprising in larvae hatched from detached egg masses. The results indicate that the periodicity of larval circatidal rhythms is inherited and raise the question as to how such rhythms are synchronised. In exceptional cases larvae hatched from detached eggs of two laboratory-spawned females, and which had been incubated under light-dark cycles, showed weak circadian swimming rhythms. In contrast, exposure of detached eggs from one of those females to successive 2 h episodes of water agitation every 12 h, resulted in expression of circatidal rhythmicity in the newly hatched larvae. This simulated tidal agitation appears to mimic the mechanical stimulation produced by females during the hatching process which has been shown previously to set phase of the larval circatidal swimming rhythms. Synchronisation of circatidal migration rhythms in larvae hatched from detached eggs remains speculative but could occur by mutual entrainment. The study appears to be the first to demonstrate artificial entrainment of a circatidal rhythm in a planktonic organism.

Occurrence of additional Zoea-VI larvae in the mud crab, Scylla paramamosain (Estampador), reared in the laboratory

Mud crabs, Scylla spp. , are commercially important in many Indo-Pacific countries. The larval development of mud crabs has been reported previously as five zoeal and one megalopal stages. This paper reports larval rearing experiments that revealed variability in larval developmental stages in the mud crab Scylla paramamosain, one of four mud crab species. In addition to normal five zoeal stages, an alternative pathway of developing through six zoeal stages was observed for the crab. There were evidences suggested that the appearance of the additional Zoea-VI larvae was associated with unfavourable dietary conditions, including poor quality of diet, inadequate quantity of dietary supply and a period of starvation for newly hatched larvae. Based on exuviae and larval specimens, the morphology of the additional Zoea-VI larvae was described.

Rhythms of larval release in the shore crab Carcinus maenas (Decapoda: Brachyura)

The process of larval release in field collected ovigerous Carcinus maenas was monitored in the laboratory using a time-lapse video recorder. Under constant light (L:L) and simulated natural light/dark cycles (L:D), larval release normally occurred in two or more main events at about daily and/or tidal intervals. Since larval release in the crab was expressed with circadian and circatidal periodicity in continuous light and in the absence of tidal cues, it suggests involvement of endogenous timing. Crabs showing daily larval release rhythms released larvae at various times of the day in L:L. In contrast, under simulated L:D cycles, 37 out of 38 crabs released larvae during the dark phase, suggesting nocturnal release of larvae in the crab under natural conditions. Larval release from freshly collected females which shed larvae within two days of collection occurred predominantly around the times of expected nocturnal high tide. When both local semidiurnal high tides occurred in daylight during long summer days, larval release appeared to start 2–3 h earlier than the expected morning high tide, before the onset of daylight. Larval release at the time around high tide, linked to a previously described larval tidal migration rhythm of ebb-phased upward swimming, is likely to have been selected for by enhancing the larval offshore dispersal process. Nocturnal larval release is probably adaptive in the avoidance of visual predators by ovigerous females as they release larvae.

Survival, osmoregulation and ammonia-N excretion of blue swimmer crab, Portunus pelagicus, juveniles exposed to different ammonia-N and salinity combinations

Ammonia-N toxicity to early Portunus pelagicus juveniles at different salinities was investigated along with changes to haemolymph osmolality, Na(+), K(+), Ca(2+) and ammonia-N levels, ammonia-N excretion and gill Na(+)/K(+)-ATPase activity. Experimental crabs were acclimated to salinities 15, 30 and 45 per thousand for one week and 25 replicate crabs were subsequently exposed to 0, 20, 40, 60, 80, 100 and 120 mg L(-1) ammonia-N for 96-h, respectively. High ammonia-N concentrations were used to determine LC(50) values while physiological measurements were conducted at lower concentrations. When crabs were exposed to ammonia-N, anterior gill Na(+)/K(+)-ATPase activity significantly increased (p<0.05) at all salinities, while this only occurred on the posterior gills at 30 per thousand. For crabs exposed to 20 and 40 mg L(-1) ammonia-N, both posterior gill Na(+)/K(+)-ATPase activity and ammonia-N excretion were significantly higher at 15 per thousand than those at 45 per thousand. Despite this trend, the 96-h LC(50) value at 15 per thousand (43.4 mg L(-1)) was significantly lower (p<0.05) than at both 30 per thousand and 45 per thousand (65.8 and 75.2 mg L(-1), respectively). This may be due to significantly higher (p<0.05) haemolymph ammonia-N levels of crabs at low salinities and may similarly explain the general ammonia-N toxicity pattern to other crustacean species.

Ecdysone receptor in the mud crab Scylla paramamosain: a possible role in promoting ovarian development

In arthropods, it is known that ecdysteroids regulate molting, limb regeneration, and reproduction through activation of the ecdysone receptor (EcR). However, the ecdysteroid signaling pathway for promotion of ovarian development in crustaceans is still unclear. In this study, three cDNA isoforms of EcR were cloned from the mud crab Scylla paramamosain. qRT-PCR revealed that the SpEcR mRNA was abundant in the eyestalk, ovary and epidermis. During ovarian development, the SpEcR transcripts increased from stage I (undeveloped stage) and reached a peak at stage IV (late vitellogenic stage) before dropping to a lower level at stage V (mature stage). Meanwhile, levels of 20-hydroxyecdysone (20E) in the hemolymph, detected by HPLC-MS, displayed a similar pattern of increase with ovarian development. Results from in situ hybridization indicated that SpEcR mRNA was present in the follicular cells during vitellogenesis. Results from in vivo experiments revealed that 20E at 0.2 μg/g body weight significantly stimulated the expression of SpEcR and vitellogenin (SpVg) in female crabs during the early vitellogenic stage but not during the previtellogenic stage. This was confirmed by results from in vitro experiments which indicated that SpEcR and SpVg expression levels were significantly upregulated in early vitellogenic ovarian explants incubated with 5.0 μM 20E at 3 and 6 h but not in previtellogenic ovarian explants. Finally, results from in vitro gene silencing experiments indicated that the expression of SpEcR and SpVg in the ovary was significantly inhibited by SpEcR dsRNA. All these results together indicated that in S. paramamosain, 20E, and SpEcR, located in the follicular cells, play important roles in the promotion of ovarian development via regulating the expression of SpVg.