The importance of the gut microbiome to health and wellbeing of humans and livestock is well understood, but this topic has only recently gained attention in aquaculture, particularly in prawn farming. This project reviewed the current knowledge of the gut microbiome of farmed prawns and visited farms to establish how work in this area may benefit the local industry. The review of recent literature found that: • recent studies suggest that the gut microbiome probably plays as vital a role in farmed prawns as it does in other animals, with evidence emerging that a ‘normal’ microbiome exists. • the prawn gut microbiome is however influenced both by feed and the environment- it is seeded by bacteria ingested with food and pond water. Consultation with Queensland prawn farmers found that: • there is widespread acceptance of the likely importance of the gut microbiome • pond management strategies probably affect the bacterial microbiome of ponds, though the impact on the gut microbiome is unknown. There is an opportunity to develop a collaborative RD&E project focussed on improving productivity of prawn farms using practices that manipulate the gut microbiome.

During spermatogenesis, giant tiger shrimp (Penaeus monodon) from Queensland, eastern Australia had a high proportion of testicular spermatids that appeared hollow' because their nuclei were not visible with the haematoxylin and eosin stain. When examined by transmission electron microscopy, the nuclei of hollow spermatids contained highly decondensed chromatin, with large areas missing fibrillar chromatin. Together with hollow spermatids, testicular pale enlarged (PE) spermatids with weakly staining and marginated chromatin were observed. Degenerate-eosinophilic-clumped (DEC) spermatids that appeared as aggregated clumps were also present in testes tubules. Among 171 sub-adult and adult P.monodon examined from several origins, 43% displayed evidence of hollow spermatids in the testes, 33% displayed PE spermatids and 15% displayed DEC spermatids. These abnormal sperm were also found at lower prevalence in the vas deferens and spermatophore. We propose Hollow Sperm Syndrome (HSS)' to describe this abnormal sperm condition as these morphological aberrations have yet to be described in penaeid shrimp. No specific cause of HSS was confirmed by examining either tank or pond cultured shrimp exposed to various stocking densities, temperatures, salinities, dietary and seasonal factors. Compared with wild broodstock, HSS occurred at higher prevalence and severity among sub-adults originating from farms, research ponds and tanks. Further studies are required to establish what physiological, hormonal or metabolic processes may cause HSS and whether it compromises the fertility of male P.monodon.

This chapter describes the current status of mud crab farming in Asia; the suitability of the mud crab biology and lifecycle to aquaculture; an update on recent developments in research and examines a number of significant areas requiring further development.

The significance of cannibalism in the diet of juvenile pond-cultured blue swimmer crabs (Portunus pelagicus (L.)) was investigated using dual stable isotope analysis of carbon and nitrogen. In a laboratory feeding experiment, δ15N demonstrated a constant trophic shift (Δδ15N≈+1.6‰), and therefore seemed to be a reliable indicator for assessing trophic position for P. pelagicus. This agrees with previously reported trends. Difference in growth rate did not seem to influence δ15N values. In contrast, δ13C did not display consistent shifts between trophic levels (range of Δδ13C: +1 to +1.7‰). The results from the pond experiment showed that larger individuals had a more enriched δ15N than smaller individuals, which, when compared to the results from the laboratory experiment, indicates that larger individuals were at a higher trophic level. This is most likely due to cannibalism prevailing in the pond rather than a direct result of faster growth rate. Cannibalistic behaviour might further increase growth, resulting in the observed positive correlation between size and δ15N.

The problem of cannibalism in communally reared crabs can be eliminated by separating the growing crabs into holding compartments. There is currently no information on optimal compartment size for growing crabs individually. 136 second instar crablets (Portunus sanguinolentus) (C2 ca. 7-10 mm carapace width (CW)) were grown for 90 days in 10 different-sized opaque and transparent walled acrylic compartments. The base area for each compartment ranged from small (32 mm x 32 mm) to large (176 mm x 176 mm). Effects of holding space and wall transparency on survival, CW, moult increment, intermoult period and average weekly gain (AWG) were examined. Most crabs reached instars C9-C10 (50-70 mm CW) by the end of experiment.The final survival rate in the smallest compartment was 25% mainly due to moult-related mortality predominantly occurring at the C9 instar. However, crabs in these smaller compartments had earlier produced significantly larger moult increments from instar to instar than those in the larger compartments (P < 0.05). Crabs in the smaller compartments (<65 mm x 65 mm) also showed significantly longer moult periods (P < 0.05). The net result was that AWG in CW was 5.22 mm week(-1) I for the largest compartment and 5.15 mm week(-1) in smallest and did not differ significantly between compartment size groups (P = 0.916). Wall transparency had no impact on survival (P = 0.530) but a slight impact on AWG (P = 0.014).Survival rate was the best indicator of minimum acceptable compartment size (similar to 43 mm x 43 mm) for C10 crablets because below this size death occurred before growth rate was significantly affected. For further growth, it would be necessary to transfer the crablets to larger compartments. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.

Given the rationale that pond systems are likely the most cost-effective system for large-scale production of P. monodon broodstock, this project aimed to determine whether pond-rearing poses a significant risk for broodstock production. The gross reproductive development of males reared in low-density broodstock ponds was found comparable to sibling males reared in controlled-environment tanks. Furthermore,none of the environmental ‘stressors’ and dietary manipulations examined impacted on male reproductive tract development. Thus, within the boundaries of the parameters tested, we can state that rearing of male broodstock in low-density ponds does not pose inherent risks of gross reproductive tract impairment. Given the typically lower costs of constructing and managing broodstock in large-scale pond systems, as compared to smaller raceway and tank systems, the incorporation of a pond-rearing phase in broodstock production could clearly increase cost-effectiveness of broodstock production at a commercial scale.

Intensive nursery systems are designed to culture mud crab postlarvae through a critical phase in preparation for stocking into grow out systems. This study investigated the influence of stocking density and provision of artificial habitat on the yield of a cage culture system. For each of three batches of postlarvae, survival, growth and claw loss were assessed after each of three nursery phases ending at crab instars C1/C2, C4/C5 and C7/C8. Survival through the first phase was highly variable among batches with a maximum survival of 80% from megalops to a mean crab instar of 1.5. Stocking density between 625 and 2300 m(-2) did not influence survival or growth in this first phase. Stocking densities tested in phases 2 and 3 were 62.5, 125 and 250 m(-2). At the end of phases 2 and 3, there were five instar stages present, representing a more than 20-fold size disparity within the populations. Survival became increasingly density-sensitive following the first phase, with higher densities resulting in significantly lower survival (phase 2: 63% vs. 79%; phase 3: 57% vs. 64%). The addition of artificial habitat in the form of pleated netting significantly improved survival at all densities. The mean instar attained by the end of phase 2 was significantly larger at a lower stocking density and without artificial habitat. No significant effect of density or habitat on harvest size was detected in phase 3. The highest incidence of claw loss was 36% but was reduced by lowering stocking densities and addition of habitat. For intensive commercial production, yield can be significantly increased by addition of a simple net structure but rapidly decreases the longer crablets remain in the nursery.

Limb-loss in crustaceans can reduce moult increment and delay or advance the timing of moulting, both aspects that are likely to impact upon soft-shell crab production. Pond-reared blue swimmer crabs Portunus pelagicus were harvested and maintained in a crab shedding system. The wet weight, carapace width (CW) and the occurrence of limb-loss were assessed before stocking in the shedding system and after each of the next three moults. Many of the crabs were initially missing one or two limbs and these did not grow as much as the crabs that were intact at the start of the trial. Despite its strong correlation with wet weight, CW changes proved to be misleading. Limb-loss reduced the %CW increment but not the per cent weight increment (where the later is calculated from the actual pre-moult weight). Pre-moult weight explained much of the variation in post-moult weight, with crabs moulting to approximately double their weight. Limb-loss reduced 'growth' and production from the pond because it reduced pre-moult weight but limb-loss did not alter the weight change on shedding a given weight of crabs, although some of that change now included regeneration of limbs. One can hypothesize that much of the size variation seen in pond-reared crabs may be due to accumulated effects of repeated limb-loss, rather than genetic variation.

Factors influencing the rate of cannibalism in juvenile blue-swimmer crabs Portunus pelagicus were investigated under controlled conditions using time-lapse video recordings. This study was undertaken to improve blue-swimmer crab culture and experimentally addressed (1) prey vulnerability (2) cannibal–victim interactions, and (3) activity patterns of juveniles in varying degrees of refuge. Crabs used in the study were aged 15 weeks and sorted into two size classes; small (≤60mm carapace width (CW)) and large (≥65mm CW) of a similar sex ratio. Vulnerability and thus survival was influenced by body size variation, moult stage and refuge availability. Crabs with carapace width ≤ 60mm were more vulnerable than larger individuals, as indicated by significant differences in survival rates. As predicted, juveniles in transition stages associated with ecdysis were especially vulnerable. Premoult (redliner) crabs appeared to be in a high state of agitation as evidenced by the frequency of agonistic encounters and this may be a contributing factor to the high mortality observed at this critical premoult stage. Increases in refuge density increased survival of juveniles proportionally, indicating that the quantity of shelter is important for reducing cannibalism in this species. Cannibal–victim interactions were frequently asymmetrical in terms of size and moult stage. Cannibals were significantly heavier than victims, and were predominantly at intermoult stage. Sexual biases among cannibals and victims were not found in this study. Activity patterns of juveniles were influenced by the experimental conditions. Crabs provided with high refuge showed reduced aggressive activity and increased time spent resting, but unchanged locomotion or feeding activity. Regular grading as well as the presence of suitable shelter for newly moulted crabs is recommended for improving culture of P. pelagicus. Research into inducing synchronous moulting may also yield promising results.

Commercial farming of the mud crab Scylla serrata is a significant industry throughout South East Asia. The limited scientific knowledge of mud crab nutritional requirements and digestive processes, however, is recognised as a major constraint to the future growth of this industry. To better understand the mechanisms of digestion in the mud crab we have analysed the diversity of digestive enzymes from the midgut (MG) gland. Significant protease, amylase, cellulase and xylanase activities were detected in soluble extracts from this organ. Temperature profiles for all enzymes were basically similar with optimal activities observed at 50 °C. Examination of pH tolerances revealed optimal activities for protease and amylase at pH 7 while maximum cellulase and xylanase activities were observed at pH 5.5. Under optimum conditions, protease and amylase activities were approximately two orders of magnitude greater than those seen for either cellulase or xylanase. Interestingly, MG extracts were able to liberate glucose from either starch or carboxymethyl (CM)-cellulose suggesting that a range of carbohydrates may be utilised as energy sources. The effects of dietary carbohydrates on feed digestibility, digestive enzyme levels and growth were also studied by inclusion of additional starch or CM-cellulose at the expense of casein in formulated diets. It was shown that amylase, cellulase and xylanase activities in extracts from the midgut gland were highest in mud crabs fed diets containing 47% carbohydrate. Based on these findings, we suggest that the ability of the mud crab to modulate digestive enzyme activities may represent a mechanism to maximise access to essential nutrients when the dietary profile changes.

Techniques for the mass propagation of the mud crab Scylla spp. in hatcheries are in demand to supply the requirements of an expanding growout industry. Culture of the crab larvae currently utilizes an Artemia -based diet through most of the hatchery cycle. A series of experiments was designed to test the performance of the nauplii of different commercially available Artemia cyst products, sourced from various geographic regions, and nauplii enriched with a commercial lipid emulsion. The influence of microalgae addition to the larval culture medium to allow continuous enrichment of Artemia was also tested. Each of the test diets was analyzed biochemically and the resulting profile, combined with larval performance data was used to determine the relationships between larval growth and mortality and nutritional components. Nauplii of eight Artemia cyst products were tested, but only two promoted growth and survival through the first metamorphosis from the zoea to the megalopa stage. Crab larval growth and survival were similar among the Artemia types fed up to the onset of metamorphosis, at around days 12 to 16. From this point onwards, the ‘good’ and the ‘poor’ Artemia types diverged. Larvae fed with nauplii of the ‘poor’ Artemia types failed to complete the metamorphic molt. To obtain maximum larval performance when using an Artemia nauplii diet, mud crab hatcheries need to be selective of cyst source. Enriching Artemia nauplii elevated total lipid levels and those of a number of fatty acids, including docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). However, it did not influence the ability of larvae to complete development. The nutritional component that best correlated with larval survival was phospholipid. It is suggested that further work is required to elucidate the role of phospholipid in the diet of mud crab larvae.