Induction of moulting in hatchery‐reared mangrove crab Scylla serrata juveniles through temperature manipulation or autotomy

The effects of temperature and autotomy of chelipeds on survival, growth and moulting of mangrove crab (Scylla serrata) juveniles were investigated under laboratory conditions for 60 days. Hatchery‐produced crabs with 2.0–2.3 cm internal carapace width (1.7–2.2 g body weight) at intermoult stage were exposed to one of four temperature treatments (constant 29, 32 or 35°C, or ambient [24–31°C]) or subjected to cheliped autotomy. All crabs held at 35°C had 100% mortality due to incomplete moulting during first moult. The mean survival of crabs at termination was 58%, 64% and 50% for ambient temperature, 29 and 32°C respectively. Specific growth rate (SGR) of crabs in the ambient and 29°C were comparable but significantly lower than those at 32°C. The moult interval of the crabs was significantly shorter in treatments with constant water temperature of 29 and 32°C compared with ambient temperature. The survival of crabs with intact chelipeds was comparable with those with one or two autotomized chelipeds. Crabs with intact or one autotomized chelipeds had significantly higher SGR than crabs with both chelipeds autotomized in the first moult. On the second moult, however, high SGR was observed in crabs with two chelipeds autotomized. The moult interval was significantly shorter in the autotomized crabs compared with crabs with intact chelipeds. The results suggest that the optimum water temperature for rearing S. serrata juveniles ranges from 29 to 32°C. Likewise, autotomy of chelipeds can promote moulting without adversely affecting survival of crabs.     

Limb-loss in pond-reared blue swimmer crabs Portunus pelagicus (L.): effect on growth in an indoor shedding system

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.     

Effects of ammonia and nitrite on survival, growth and moulting in juvenile tiger crab, Orithyia sinica (Linnaeus)

The effects of ammonia and nitrite on survival, growth and moulting were investigated in juvenile tiger crab, Orithyia sinica (carapace length 3.91±0.15 mm, carapace width 3.84±0.23 mm, n=440), after 30 days exposure to ammonia‐N (0, 20, 50, 100 and 150 mg L^?1) and nitrite‐N (0, 50, 100, 150, 200 and 250 mg L^?1) using a continuous flow system. Survival rates of tiger crab exposed to ammonia and nitrite decreased linearly with the exposure time and concentration. The growth rate of tiger crab exposed to 50, 100 and 150 mg L^?1 ammonia was significantly lower than that of control crabs. The growth rate of tiger crab exposed to nitrite decreased at 150, 200 and 250 mg L^?1 nitrite. During the ammonia and nitrite exposure, the intermoult period of the juveniles of tiger crab O. sinica was shortened between the first and second moult, and the number of moulting of crabs exposed to a higher concentration were significantly higher than that of control crabs.     

Innate immune response and gene expression of Scylla paramamosain under Vibrio parahaemolyticus infection

An epidemic of ‘milky disease’ in the mud crab (Scylla paramamosain) generally breaks out in the fall when the crab is near maturity, resulting in large economic losses in crab farming. Vibrio parahaemolyticus has been proven to be one of the major pathogens. In this study, the mud crabs were challenged with V. parahaemolyticus, and their innate immune responses were investigated in terms of total haemocyte counts (THCs), haemocytic enzyme activities and gene expression levels during a 114‐h period. The THCs of the mud crabs decreased significantly after 42 h of exposure. The activities of the haemocytic enzymes, including acid phosphatase‐alkaline phosphatase, phenoloxidase, superoxide dismutase (SOD), peroxidase and nitric oxidase synthethase, were significantly enhanced during the challenge course. The gene expression levels also significantly increased for all tested genes (proPO, Cu/Zn‐SOD, Prx, LYS, CRU and ALF) with the exception of CAT down‐regulated expression. The results may imply that the immune responses of the mud crab could be activated by the pathogens, and the data here will provide many clues for further systematic investigation of ‘milky disease’ caused by V. parahaemolyticus and the disease prevention in mud crab S. paramamosain.     

Phenotypic differences between hatchery-reared and wild mud crabs, <Emphasis Type="Italic">Scylla Serrata</Emphasis>, and the effects of conditioning

Hatchery-reared animals for stock enhancement should be competent to survive and grow at rates equivalent to those of wild conspecifics. However, morphological differences are often observed, and pre-conditioning steps may be required to improve the fitness of hatchery-reared juveniles prior to release. In the present study, hatchery-reared Scylla serrata juveniles were reared either individually (HR-solitary) or groups in tanks (HR-communal), the latter group being exposed to intraspecific competition and foraging for food. After 21 days, both groups were compared to similar size wild-caught juveniles in terms of morphometric measurements of carapace spination, abnormalities and carapace colouration. There were some limited significant differences between HR-communal crabs and HR-solitary crabs in terms of length of 8th and 9th lateral spines and in body-weight-carapace width ratio, but both treatments differed from wild crabs, which were heavier and had longer carapace spines for their size. In contrast, both HR treatments exhibited common abnormalities including deformities in the shape of the abdomen, in particular occurrence of an asymmetrical telson or a deeply folded telson. In all cases, abnormalities persisted through moulting. Initially, carapace colour differed in all measures of colour between HR and wild crabs. However, these differences reduced after a period of 4–8 days of conditioning on coloured tank backgrounds or dark sand or mud backgrounds, without moulting. Similarly, hatchery-reared crabs exhibited very limited burying behaviour on first exposure to sediment, but this increased to levels observed in wild crabs within 2–4 days. Thus, short-term conditioning of hatchery-reared crabs on dark sediments may be effective in increasing predator avoidance and survivorship in released animals, and present results suggest that this can be achieved after relatively short periods of 1 week or less.     

Effect of dietary protein level on fattening and mineral profiles of mud crab,Scylla serrata,in individual cages under mangrove ecosystem

A fattening demonstration was carried out in Sorlagondi (15.8640⁰N, 80.9671⁰E) mangrove ecosystem in Diviseema region, Krishna district, Andhra Pradesh, India, by adapting a holistic approach of “Integrated Mangrove Fishery Farming System.” The local Yanadi tribal communities engaged in mangrove afforestation programmes were provided with 144 individual crab fattening cages (60 × 40 × 30 cm). Two formulated feeds were prepared with two levels of protein (32% and 36%) and were tested on fattening of the marketable size (200–1,000 g) of mud crabs, Scylla serrata. Significantly (p < 0.05) higher weight gain, feed and protein efficiency measures were observed in crabs fed with CP‐36% diet across all size groups. This diet also reduced the time (days) taken for fattening crabs irrespective of the size. Moisture content was very high in soft crabs (>800 g/kg) compared with hard crabs (679.70–688.65 g/kg), whereas the reverse trend was observed for protein, ether extract and ash content. Both diet and size of the crab have not shown much influence on proximate composition. However, cultured hard crabs had higher ether extract (20.10 g/kg) than wild hard crabs (17.06 g/kg). The changes in concentration of mineral values on wet basis indicated the increase in concentration in hard crabs compared with soft crabs, whereas when the percentage change, values on dry matter basis indicated the reduction in concentration of mineral contents of K, Na, P, Cu, Fe and Zn. This higher concentration of minerals in soft crabs could possibly be attributed to the reabsorption of minerals into soft muscle or to the body fluids.     

Stepwise function of natural growth for Scylla serrata in East Africa: a valuable tool for assessing growth of mud crabs in aquaculture

Predicting growth is critical in aquaculture, but models of growth are largely missing for mud crab species. Here, we present the first model of natural growth in juvenile and adult mud crabs Scylla serrata from East Africa using a stepwise growth function based on data on intermoult periods and growth at moult from field mark‐recapture, pond and laboratory studies. The results showed a sigmoid growth pattern in carapace width and suggest that S. serrata in East Africa will reach 300 g and sexual maturity ~9.9 months after settlement, and a commercial size of 500 g after 12.4 months. Analyses of the literature identified several issues with the common praxis to compare standard growth measures between aquaculture studies with different initial size or growing periods. Using the new growth function to estimate the proportional difference between modelled and obtained growth as an alternative method, we show that growth rates of S. serrata cultured in cage systems, which are dominant in East Africa, was <40% of the estimated natural growth and growth obtained in pond systems. The analysis also indicated that growth rates of S. serrata in Southeast Asia was over 50% higher compared with similar culture systems in East Africa, and that different species of mud crabs had large differences in growth rates. This study shows that growth in the present mud crab aquaculture systems in East Africa is below their expected potential. Further work is needed to identify the factors behind this observation.     

Domestication of the mud crab <Emphasis Type="Italic">Scylla serrata</Emphasis>

The significant decrease in wild mud crab population highlights the need to manage the resources and domesticate crabs. This paper presents the initial results of the domestication of mud crab Scylla serrata aimed at producing good-quality captive broodstock. The analysis of the genetic structure of the base population was done as a prerequisite for domestication. Adult S. serrata from the northern to southern parts of the Philippines (Cagayan, Camarines, Samar, and Surigao) were obtained for genetic diversity analysis and domestication. Analysis of molecular variance showed that differences in the genetic variability between the four populations were not significant. Moreover, no significant deviation from Hardy–Weinberg Equilibrium was observed in each sample population and even in pooled populations. Body weight was positively correlated with the carapace width. Second spawning occurred 41–46 days after the first spawning and 34 days from second to third spawning. However, there was a decrease in the number of zoea in repeat spawnings. Twenty-four first-generation (F₁) families were produced from the four sites. The duration from spawning of the base population (P₀) to attainment of broodstock size F₁ was 10–14 months. Four second-generation (F₂) families were produced after 11–12 months. Up to the F₂, crabs tested negative for six viruses: white spot syndrome virus, infectious hypodermal and hematopoietic necrosis virus, gill-associated virus, yellow head virus, Taura syndrome virus, and infectious myonecrosis virus. The reproductive performance of P₀ was comparable to the succeeding generations. Several families were obtained from one population in a year. However, due to the cannibalistic behavior of crabs, more space is required for the nursery and grow-out phase. The domestication of S. serrata is the first study done on any mud crab species in the Indo-west Pacific region. The initial results would serve as guide to understand and eliminate the barriers to mud crab domestication. The breeding technology developed from this study will support the production of good-quality seedstock for farming.     

Integrated rearing system approach in the farming of mud crab,shrimp, fish,oyster and periphyton in bracksihwater pond

Diversification of production systems has been identified as one of the major strategies for the long‐term sustainability of brackishwater aquaculture. On the context of diversification, a 180‐day culture experiment was carried out where four farming systems were tested: monoculture of mud crab, Scylla serrata (T1), polyculture of mud crab, with mullet Mugil cephalus and shrimp Penaeus indicus (T2), polyculture of mud crab with mullet, shrimp and oyster Crassostrea cuttackensis (T3), polyculture of mud crab with mullet, shrimp, oyster along with periphyton net (T4) covering 10% of the water spread area. There was no significant difference in the growth performances of mud crab, shrimp and oyster among the treatment but a significantly higher (p < .05) final average body weight (ABW) of M.cephalus was recorded in T4 (121.19 ± 2.09 g). Production considering only the fed animals (excluding oyster) was significantly (p < .05) higher in T4 (2,115 kg/ha) compared to other treatments. The overall water quality parameters do not vary significantly (p > .05) among the treatment ponds. Although there was no significant difference in total ammonia nitrogen (TAN) and nitrite‐N, highest value was recorded in T2 (56.23 ± 39.47 µg/L and 185.53 ± 110.71 µg/L) and T1 (55.78 ± 32.26 µg/L and 180.83 ± 103.65 µg/L). The results from the current experiment based on water quality and growth of animals showed that there is no negative interaction between the mud crab and other animals stocked in the polyculture pond which indicated that polyculture provides an efficient and sustainable utilization of resources.     

Impact of salinity changes on growth,oxygen consumption and expression pattern of selected candidate genes in the orange mud crab (Scylla olivacea)

Change in environmental salinity level is a major limiting factor for the aquaculture productivity because it imposes severe stress on organisms that in turn retards growth. The orange mud crab (Scylla olivacea) is an important coastal aquaculture species (farming is practised in 10‰–20‰ salinity levels) in Bangladesh. The present study was conducted to investigate the changes in growth, O₂ consumption and mRNA expression levels of five selected genes in the orange mud crab (S. olivacea) exposed to three different experimental salinity levels (0‰, 10‰ and 20‰) for three months. Crabs reared at 10‰ and 20‰, showed significantly higher (p < .05) growth performance and expression of growth regulatory genes (Actin and α‐amylase). The highest levels (p < .05) of O₂ consumption and expression of ion regulatory genes (Na⁺‐K⁺‐ATPase, V‐type H⁺‐ATPase and Diuretic Hormone) were obtained at 0‰. Moderate levels of growth and expression of selected candidate genes were observed at 10‰ treatment while the highest levels of growth and gene expression were obtained at 20‰ (control salinity). Strong interactions were observed between growth performance and expression of growth genes (R² = 0.81–0.91), and rate of O₂ consumption and expression of ion regulatory genes (R² = 0.83–0.93), implying that the selected genes are important candidates for growth and ionic balance in S. olivacea. Growth performance was found to be very low at 0‰ initially, after 30 days crabs showed better growth performance at this salinity level. It is thus inferred that orange mud crab individuals might require 3–5 days for acclimation to salinity stress but it can take at least 30 days for acclimation to regular growth. Results indicate that with proper acclimation, the orange mud crab (Scylla olivacea) can be farmed at low salinity conditions and possibly in freshwater condition.     

Size at sexual maturity and body size composition of mud crabs <Emphasis Type="Italic">Scylla</Emphasis> spp. caught in Don Sak,Bandon Bay,Gulf of Thailand

Size at sexual maturity and body size composition of mud crabs Scylla spp. were examined as the basis for settling a minimum landing size as a fishing regulation in Don Sak, Bandon Bay, Gulf of Thailand, which has suffered serious mangrove habitat degradation. Mud crabs were caught using baited traps and gill nets. Hooked metal rods were also used to lever the crabs out of their burrows inside the mangroves. Two mud crab species, S. paramamosain and S. olivacea, occur in the bay; S. paramamosain is the dominant species, accounting for 87% of the samples. The size at which 50% of the S. paramamosain females reached maturity (SM₅₀) was estimated as an external carapace width (ECW) of 112.0 mm based on the morphology of the abdomen. Allometric changes in the crushing chelae height to ECW ratio suggested that the SM₅₀ of S. paramamosain males occurred at 106.4 mm ECW. The body size composition revealed that mainly immature mud crabs were exploited in Don Sak. To maintain a sustainable fishery for mud crabs, fishing regulations—including a minimum landing size based on the SM₅₀ estimates—are essential, as is habitat restoration.     

Community livelihood and patterns of natural resources uses in the shrimp‐farm impacted Mekong Delta

This case study looks at changing livelihood strategies of the coastal population in Soc Trang Province in the Mekong Delta, Vietnam, and their impacts on natural resources. It provides an opportunity not only to document the impact of shrimp farming on coastal livelihood but also to better understand the link between brackish water aquaculture development and natural resource use. The approach includes a socio‐economic survey in six villages of the province focusing on risk strategies and livelihood diversification. Shrimp farming was found to be less risky and more profitable for households and private companies with a higher investment capacity than for poorer households. Households facing a high risk in shrimp farming diversified their aquaculture production, with other high‐value species like mud crab and elongated goby as a coping mechanism. The use of natural resources' collection is shifting from home consumption towards market‐oriented sales of juvenile mud crabs, clams or fish (elongated goby) to supply seed for brackish water aquaculture developments.     

Dietary phospholipids requirement of the early juvenile (C1) swimming crab,Portunus trituberculatus

A 30‐day experiment was conducted to investigate the effect of phospholipids (PLs) on growth performance, survival, fatty acid profile and gene expression of the early juvenile (C1) swimming crab (Portunus trituberculatus). Five semi‐purified diets were formulated with graded PLs levels (0%, 1%, 2%, 4% and 8%). Each diet was fed to four replicates of crabs (30 crabs per replicate, initial weight: 8.4 ± 0.1 mg). In this study, crabs fed diets with 2%, 4% and 8% PLs had significantly higher survival rates than crabs fed with 0% and 1% PLs. Crabs fed diets with 0% and 1% PLs had significant lower weight gain (WG) than crabs fed with 8% PLs. But crabs fed with diets containing 2%, 4% and 8% PLs showed no significant difference in WG. Besides, the content of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in whole body was higher than that in diets, but the monounsaturated fatty acids (MUFA) content showed an opposite trend. The EPA and DHA content in whole body increased with the increase in dietary PLs. Moreover, the moulting number increased with the increasing dietary PLs levels, the lowest moulting number was observed in crabs fed diets without supplementation of PLs. The ecdysteroid receptor (EcR) was found to be significantly up‐regulated by PLs levels. This study demonstrated that 2% PLs could meet the requirement of early juvenile (C1) swimming crab based on the growth and survival. And 4%–8% PLs supplemented in diets could reduce the leaching of feed.     

Development of nursery culture techniques for the mud crab Scylla paramamosain (Estampador)

Cannibalism is one of the main causes of mortality in the culture of the mud crab Scylla paramamosain, particularly in the early post‐larval and juvenile stages when the densities of hatchery‐reared crabs may be very high before stocking into ponds or release into the wild for stock enhancement. In a series of experiments investigating cannibalism mitigation, the influence of stocking density, the effectiveness of sand substrate, brick and shell shelters and feed type were compared in culture of crabs from instar 1 for short nursery periods of 15–30 days. Inclusion of brick and shell shelters significantly increased survival over sand substrate alone. However, inclusion of shelters did not affect growth rates. In scaled‐up nursery production in lined‐ponds, with shelters, live Artemia biomass and fresh chopped shrimp or tilapia were found to be equally effective feeds for juvenile crabs stocked at a density of 70 m⁻² from instar 1 and grown for 30 days [52–66% survival, 21.6–24.6 mm carapace width (CW)]. In an extended nursery period for a further 30 days, crabs of 22 mm CW, stocked at 30 m⁻² in the same ponds, attained a final size of 34.5–36.2 mm CW with a survival of 64.3–67.0% using the same feeds.     

Effects of dietary phospholipid and cholesterol levels on growth and fatty acid composition of juvenile swimming crab,Portunus trituberculatus

The objective of this study was to evaluate the effects of different phospholipids (PL) and cholesterol (CH) levels on the growth, moulting and fatty acid composition of juvenile swimming crab, Portunus trituberculatus. Six diets were designed to contain three PL levels (0, 10 and 20 g/kg) and two CH levels (2 and 8 g/kg). Juvenile swimming crabs (3.48 ± 0.02 g/crab) were reared for 8 weeks. The weight gain (WG) was significantly (p < .05) increased by supplementation of 8 g CH/kg of diet. However, no significant interaction between dietary PL and CH levels was found on the growth performance (p > .05). The moulting frequency (MF), protein efficiency ratio (PER) and feed efficiency ratio (FCR) were not significantly (p > .05) affected by the dietary treatments. The serum total cholesterol (TCH) significantly (p > .05) increased with increasing dietary PL level. The C20:4n‐6 and C20:5n‐3 content of the whole body of crabs increased with the addition of PL to the diet containing 2 g/kg CH of diet. An interaction was observed between PL and CH on certain saturated and unsaturated fatty acid concentrations of body.     

Effects of diet and sediment type on survival, growth and moulting of juvenile tiger crab, Orithyia sinica L.

This study investigated the effects of different types of diet and sediment on the growth parameters of juvenile tiger crab Orithyia sinica under laboratory conditions, focusing on survival, growth and moulting. In the first set of experiments, juvenile tiger crabs were reared with five different types of sediment, i.e., soft sand, coarse sand, soft sand mixed with coarse sand, mud or bare bottom. Soft sand resulted in not only the highest survival rate of 78% but also the best rate of growth and feed intake. Crabs reared with a mud substrate showed the worst results in these parameters. The intermoult period, however, was not affected by sediment types. In the second set of experiments, crabs were fed different types of diet to determine their effects on the same growth parameters. Diets were composed of different combinations with Manila clam (Ruditapes philippinarum), brine shrimp (Artemia nauplii), microencapsulated feeds and marine diatom (Chaetoceros gracilis). Survival and growth in crabs fed Manila clam as a diet component were significantly higher than in those on other diets. The highest survival and growth were achieved with a diet that included all four components. These results provide valuable information on the appropriate bottom substrates and diet types necessary for artificial mass culture of the tiger crab.     

Effect of dietary lipid levels on growth performance,fatty acid profile and enzymatic activity of juvenile swimming crab,Portunus trituberculatus

The main objective of this investigation was to study the lipid requirements of the early juvenile (C1) swimming crab (Portunus trituberculatus) based on growth performance, survival, moulting and fatty acid profile. Four test diets were formulated with graded lipid levels (3.63%, 6.70%, 10.72% and 13.91%). Each diet was fed to 4 replicates of crabs (30 crabs per replicate initial weight (8.4 ± 0.1 mg). In this study, crabs fed diets with 13.91% lipid had significantly (p < 0.05) higher survival than crabs fed with 3.63% lipid, but no significant (p > 0.05) improvement of survival was observed when dietary lipid increase from 6.70% to 13.91%. Crabs fed diets with 10.72% and 13.91% lipid had significant higher weight gain(WG) than crabs fed with 3.63% lipid. While crabs fed with diets containing 6.70%, 10.72% and 13.91% lipid showed no significant (p > 0.05) difference in weight gain(WG). Moreover, the lowest moulting number was observed in crabs fed diets with 3.63% lipid, but there was no significant difference (p > 0.05) among other groups. The content of LC‐PUFA and DHA in the crabs fed diets with 3.63% lipid was significant (p < 0.05) lower compared to other groups. But there was no significant (p > 0.05) difference in EPA and ARA content among all groups. The activity of lipase increased as dietary lipid level increased (from 6.70% to 10.72%). However, beyond 10.72%, a significant (p < 0.05) decreased in lipase activity was observed. The regression analysis of weight gain data indicated that crab fed diet containing 10.47% lipid level is considered as optimum lipid level for its maximum growth and moulting process.     

Effects of dietary fish oil substitution with blending vegetable oils on growth performance,antioxidant enzyme activities and tissue fatty acid composition of juvenile swimming crab,Portunus trituberculatus

An 8‐week feeding trial was conducted to evaluate the effects of replacement of fish oil (FO) with blending vegetable oils (VOs) on growth performance, antioxidant enzyme activities and fatty acid composition in tissue of swimming crab Portunustrituberculatus. Five isonitrogenous and isolipidic diets were formulated to contain VOs (colza oil: palm oil: linseed oil = 4:2:1) to replace 0 (the control diet), 250, 500, 750 and 1000 g/kg of FO (defined D0, D25, D50, D75, D100). Three hundred juvenile swimming crabs (initial weight 2.34 ± 0.08 g) were randomly stocked and sorted into 300 individual rectangle plastic baskets in three cement pools. Each treatment has three replicates, one replicate has 20 swimming crabs, and each diet fed 60 crabs distributed in 60 baskets. The results indicated that crabs fed the control diet showed significantly higher survival, final body weight, per cent weight gain (PWG), specific growth rate and moulting frequency, crude protein and crude lipid contents in muscles than those fed the D75 and D100 VO diets (p < .05). Crabs fed the D25 VO diet showed significantly higher concentration of triglyceride, low‐density lipoprotein cholesterol and total protein, activities of superoxide dismutase, catalase and glutathione peroxidase (GSH‐Px) in haemolymph than those fed the control diet (p < .05). Fatty acid composition in hepatopancreas was positively correlated with dietary composition. In summary, based on the PWG, the optimal replacement of FO with VOs was estimated to be 250 g/kg. These findings demonstrated that swimming crabs make better use of FO than VOs.     

Maximum handling size,prey size and type selection by snakehead (Channa argus) feeding on juvenile Chinese mitten crab (Eriocheir sinensis)

Laboratory predation trials were conducted to investigate maximum handling size, prey size and type selection by small (35–37 cm in total length, L_T), medium (43–46 cm L_T) and larger (58–60 cm L_T) snakehead (Channa argus) when feeding on a wide size (9–34 mm in carapace width, C_W) range of juvenile Chinese mitten crab (Eriocheir sinensis). The maximum handling size of predator feeding on crabs monotonically increased with predator L_T and mouth gape width, G_W. Snakehead with at least 16.0 cm L_T or 27.7 mm G_W would be capability to consume the smallest size group of crabs, whereas the predator with 72.6 cm L_T or 63.4 mm G_W would be capability to consume all size groups of crabs in these trails. Prey‐size selection trials showed that snakehead has a high preference to the small‐sized crabs, and lower preference to the medium or large size crabs. The preference index was significantly affected by prey size and prey size × predator size interaction, however, was not affected by predator size. In prey type experiments, snakehead consumed lower proportions of juvenile crabs when fed on the crab and crucian carp than when fed on only the crab, which suggests alternative fish prey may reduce predation risk of the crab by snakehead in nature. These results could be useful for improving the fishery management and release strategies for the crab.