The combined effects of temperature and salinity on larval survival and development of the mud crab, Scylla serrata, were investigated in the laboratory. Newly hatched larvae were reared under 20 °C temperature and salinity combinations (i.e. combinations of four temperatures 25, 28, 31, 34 °C with five salinities 15, 20, 25, 30, 35 g L⁻¹). The results showed that temperature and salinity as well as the interaction of the two parameters significantly affected the survival of zoeal larvae. Salinity at 15 g L⁻¹ resulted in no larval survival to the first crab stage, suggesting that the lower salinity tolerance limit for mud crab larvae lies somewhere between salinity 15 and 20 g L⁻¹. However, within the salinity range of 20–35 g L⁻¹, no significant effects on survival of zoeal larvae were detected (P>0.05). The combined effects of temperature and salinity on larval survival were also evident as at low salinities, both high and low temperature led to mass mortality of newly hatched larvae (e.g. 34 °C/15 g L⁻¹, 34 °C/20 g L⁻¹ and 25 °C/15 g L⁻¹ combinations). In contrast, the low temperature and high salinity combination of 25 °C/35 g L⁻¹ resulted in one of the highest survival to the megalopal stage. It was also shown that at optimal 28 °C, larvae could withstand broader salinity conditions. Temperature, salinity and their interaction also significantly affected larval development. At 34 °C, the mean larval development time to megalopa under different salinity conditions ranged from 13.5 to 18.5 days. It increased to between 20.6 and 22.6 days at 25 °C. The effects of salinity on larval development were demonstrated by the fact that for all the temperatures tested, the fastest mean development to megalopa was always recorded at the salinity of 25 g L⁻¹. However, a different trend of salinity effects was shown for megalopae as their duration consistently increased with an increase in salinity from 20 to 35 g L⁻¹. In summary, S. serrata larvae tolerate a broad range of salinity and temperature conditions. Rearing temperature 25–30 °C and salinity 20–35 g L⁻¹ generally result in reasonable survival. However, from an aquaculture point of view, a higher temperature range of 28–30 °C and a salinity range of 20–30 g L⁻¹ are recommended as it shortens the culture cycle.

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 ofEcRwere cloned from the mud crabScylla paramamosain. qRT-PCR revealed that theSpEcRmRNA was abundant in the eyestalk, ovary and epidermis. During ovarian development, theSpEcRtranscripts 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 fromin situhybridization indicated thatSpEcRmRNA was present in the follicular cells during vitellogenesis. Results fromin vivoexperiments revealed that 20E at 0.2 μg/g body weight significantly stimulated the expression ofSpEcRandvitellogenin(SpVg) in female crabs during the early vitellogenic stage but not during the previtellogenic stage. This was confirmed by results fromin vitroexperiments which indicated thatSpEcRandSpVgexpression 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 fromin vitrogene silencing experiments indicated that the expression ofSpEcRandSpVgin the ovary was significantly inhibited bySpEcRdsRNA. All these results together indicated that inS. paramamosain, 20E, andSpEcR, located in the follicular cells, play important roles in the promotion of ovarian development via regulating the expression ofSpVg.

Aquatic animals can often undergo substantial physiological responses to salinity; however, associated lipid/fatty acid alterations to their various tissues have received little attention. To investigate this, we measured the growth of mud crab, Scylla serrata, juveniles over two moults (duration of 23–60 days) at salinities of 4, 12, 20, 28, 36 and 44‰ (30 replicates/treatment). After the second moult, 6-day post moult crabs were sampled for hepatosomatic index (HSI), haemolymph osmolality, Na⁺, K⁺ and Ca²⁺ levels, anterior/posterior gill Na⁺/K⁺-ATPase activity as well as the lipid/fatty acid composition of the anterior and posterior gills, hepatopancreas and muscle. High salinities of 36 and 44‰ significantly lowered crab growth and HSI (p<0.01). S. serrata strongly hyper-regulated haemolymph osmolality and ions likely due to significantly enhanced posterior gill Na⁺/K⁺-ATPase activity. At decreasing salinities, eicosapentaenoic acid, arachidonic acid, n-3 and long chain polyunsaturated fatty acid (PUFA) significantly increased (p<0.05), likely to maintain Na⁺/K⁺-ATPase activity via increased membrane fluidity. Muscle and hepatopancreas n-3/n-6 PUFA ratios significantly decreased (p<0.05) at increasing salinities indicating possible fatty acid metabolic modifications. Results indicate that S. serrata juveniles are well adapted to low salinities, with higher salinities likely reducing their metabolism, reflected by lower growth, HSI and higher posterior gill and hepatopancreatic triacylglycerol.

Salinity plays a key role affecting ovarian development, osmoregulation and metabolism of female Chinese mitten crab, Eriocheir sinensis during reproductive migration. In this study, female E. sinensis after their puberty molt were subjected to four salinities of 0, 6, 12, and 18‰ for 40 days to investigate the salinity effects on their ovarian development as well as a range of important physiological parameters. Elevated salinity accelerated the ovarian development with ovigerous crabs found at salinity treatments of 12 and 18‰ despite no copulation had occurred. Meanwhile the survival rate of female crabs showed a decreasing trend with increasing salinity. Higher salinity also led to increased hemolymph Na⁺, K⁺, Ca²⁺, Cl⁻, and Mg²⁺ concentrations. The 6‰ treatment had the highest contents of hemolymph total and major free amino acids while the Na⁺/K⁺ -ATPase activity in the posterior gills was the lowest among treatments. Total n-3 polyunsaturated fatty acids (∑n-3PUFA) and n-3/n-6 PUFA ratio in the anterior gills showed a decreasing trend with salinity while 18‰ had the highest ∑PUFA and ∑n-6PUFA. The ∑n-3PUFA content and n-3/n-6 PUFA ratio of the posterior gills showed a fluctuating pattern and the highest value was detected at 0‰, while an increasing trend was found for the ∑n-6PUFA with increasing salinity. The hemolymph glucose showed a decreasing trend with increasing salinity and the highest total cholesterol in hemolymph was detected at 12‰. The 18‰ treatment had the highest levels of hemolymph γ-glutamyltransferase, alkaline phosphatase and acid phosphatase, as well as glucose, urea and acid phosphatase in hepatopancreas while the highest hemolymph superoxide dismutase and malondialdehyde were detected at 0‰. Overall, the results showed that salinity increase from freshwater to brackish conditions led to lower metabolism, accelerated ovarian development, and the appearance of ovigerous crabs without copulation in female E. sinensis post puberty molt.

Hatchery culture of mud crabs has not yet achieved commercial viability despite decades of research efforts. Further research is therefore needed to better understand larval culture requirements of the crab. Based on anecdotal observations, an experiment was carried out to test whether the background colour of the culture vessel affected larval culture success. Newly hatched larvae of Scylla serrata were reared in culture vessels of five colours, i.e., black, dark green, maroon, sky blue and white. Larval survival and development were monitored daily until all of them either moulted to the first crab stage or died. The results showed clear effects of background colour on larval survival. A general tendency of higher larval survival in darker-coloured backgrounds was evident. In particular, overall zoeal survival for larvae reared in black vessels was significantly higher than those reared in white ones. Background colour also appeared to affect larval development. Larvae reared in darker backgrounds generally had shorter development times and more synchronized moulting. A significant delay in zoeal development was observed in larvae reared in white vessels. Dark backgrounds possibly facilitated more efficient feeding, reduced settlement of larvae at the bottom of the vessels as well as minimized stress. This result appears to be the first to demonstrate that background colour can significantly affect larval survival and development of a crustacean species.\u