Thermal Tolerance and Physiological Changes in Mud Crab, Scylla paramamosain Crablet at Different Water Temperatures

Feeding rhythm of the zoea larvae of Scylla paramamosain: The dynamic feeding rhythm is not completely synchronized with photoperiod

The feeding rhythm is one of the key factors determining the success of artificial breeding of S. paramamosain. To understand the feeding rhythm of the different zoea larva developmental stages of S. paramamosain, the feeding rate, digestive enzyme activity, and expression of metabolism-related genes were investigated in the present study. The results showed that the S. paramamosain feeding rate has strong diurnal feeding rhythm, being significantly higher at 10:00-14:00 from stages ZI to ZIV. While the feeding rate peaked at 14:00 on Days 10 and 11, the peak shifted to 18:00 on Day 12. The activity of digestive enzymes amylase, pepsin and lipase decreased at night but increased in the daytime, showing a single-phase rhythm similar to that of the feeding rate, suggesting that the digestive enzyme activity was closely associated with the feeding rate during the larval development. Compared to pepsin and lipase, the activity of amylase was the most consistent with feeding rate. In particular, amylase activity peaked at 18:00 on Day 12. Due to its synchronicity with feeding activity, the activity of amylase could provide a potential reference for determining the best feeding time during zoea stages in S. paramamosain breeding. Moreover, the relative mRNA expression of metabolism-related genes SpCHH and SpFAS at most tested points was lower from 10:00 to 14:00, but higher at 18:00 to 6:00 of the next day. On the other hand, the expression patterns of SpHSL and SpTryp were converse to those of SpCHH and SpFAS. Our findings revealed that the S. paramamosain zoea has an obvious feeding rhythm, and the most suitable feeding time was 10:00-18:00 depending on different stages. The feeding rhythm is a critical aspect in aquaculture, influencing a series of physiological functions in aquatic animals. This study provides insights into the feeding rhythm during the zoea development of S. paramamosain, making a significant contribution to optimizing feeding strategy, improving aquafeed utilization, and reducing the impact of residual feed on water environment.

Optimal temperature and thermal tolerance of postlarvae of the freshwater prawn Cryphiops (Cryphiops) caementarius acclimated to different temperatures

In this study, the optimum temperature and thermal tolerance of postlarvae of the commercially important freshwater prawn Cryphiops (Cryphiops) caementarius were determined after acclimation to six different rearing temperatures (19 °C, 22 °C, 24 °C, 26 °C, 28 °C, and 30 °C) during a 45 day-culture period. Best growth parameter values were obtained within the temperature range of 24 °C to 28 °C, where the optimum temperature for growth was found to be at 26 °C (weight gain 81.70%; specific growth rate 1.33 %/day) but had not significant effect (p > 0.05) on survival (64%-71%) of postlarvae. Increasing the acclimation temperature significantly (p < 0.05) increased both the critical thermal maximum (CTMax: from 33.82 °C to 38.48 °C) and minimum (CTMin: from 9.27 °C to 14.58 °C). The thermal tolerance interval increased (p < 0.05) from 24.55 °C to 25.48 °C in postlarvae acclimated at 28 °C but decreased (p < 0.05) to 23.90 °C in those acclimated at 30 °C. The acclimation response rate was lower for CTMax and higher for CTMin. The current (12.48 °C) and future (9.48 °C) thermal safety margins were like those reported for other tropical crustaceans. A thermal tolerance polygon over the range of 19-30 °C resulted in a calculated area of 242.25 °C2. The presented results can be used for aquaculture activities and also to help to protect this species against expected climate warming impacts.

Thermal tolerance of the male freshwater prawn Cryphiops caementarius exposed to different acclimation temperatures

The variation in water temperature influences metabolic and biochemical processes in ectothermic organisms, affecting development, behavior, and thermal responses. We conducted laboratory experiments based on different acclimation temperatures to determine the thermal tolerance in male specimens of the freshwater prawn Cryphiops caementarius. During 30 days, male prawns were exposed to acclimation temperatures of 19 °C (control), 24 °C, and 28 °C treatments. The Critical Thermal Maxima (CTMax) values at these acclimation temperatures were 33.42 °C, 34.92 °C, and 36.80 °C; whereas values for the Critical Thermal Minimum (CTMin) were 9.38 °C, 10.57 °C, and 13.88 °C. All acclimation temperature treatments had a positive effect (P < 0.05) on CTMax and CTMin, with high and significant correlations (CTMax: r = 0.992, P < 0.01; CTMin: r = 0.946, P < 0.01). The area of the thermal tolerance polygon over the three acclimation temperatures was 211.32 °C² and the acclimation response rate values were high (CTMax from 0.30 to 0.47; CTMin from 0.24 to 0.83) but similar to those from other tropical crustacean species. These results demonstrate that adult males of the freshwater prawn C. caementarius can tolerate extreme water temperatures through a thermal plasticity response, which could be advantageous during a global warming scenario.

Sex-specific thermal tolerance limits in the ditch shrimp Palaemon varians: Eco-evolutionary implications under a warming ocean

As global temperatures continue to rise due to global change, marine heatwaves are also becoming more frequent and intense, impacting marine biodiversity patterns worldwide. Organisms inhabiting shallow water environments, such as the commercially relevant ditch shrimp Palaemon varians, are expected to be the most affected by rising temperatures. Thus, addressing species' thermal ecology and climate extinction-risk is crucial to foster climate-smart conservation strategies for shallow water ecosystems. Here, we estimated sex-specific upper thermal tolerance limits for P. varians via the Critical Thermal Maximum method (CTmax), using loss of equilibrium as endpoint. We further calculated thermal safety margins for males and females and tested for correlations between upper thermal limits and shrimps' body size. To determine sex-biased variation in P. varians' traits (CTmax, weight and length), we compared trait variation between females and males through the coefficient of variation ratio (lnCVR). Females displayed an average CTmax value 1.8% lower than males (CTmax_females = 37.0 °C vs CTmax_males = 37.7 °C). This finding may be related to the larger body size exhibited by females (156% heavier and 39% larger than males), as both length and weight had a significant effect on CTmax. The high energetic investment of females in offspring may also contribute to the differences recorded in thermal tolerance. Overall, organisms with a smaller body-size displayed a greater tolerance to elevated temperature, thus suggesting that smaller individuals may be positively selected in warmer environments. This selection may result in a reduction of size-at-maturity and shifts in sex ratio, given the sexual dimorphism in body size of shrimps. The thermal safety margin of P. varians was narrow (∼2.2 °C for males and ∼1.5 °C for females), revealing the vulnerability of this species to ocean warming and heatwaves.

A Review of the Nursery Culture of Mud Crabs, Genus Scylla: Current Progress and Future Directions

Simple Summary

Nursery culture knowledge is important for successful commercial seed production especially for mud crab, genus Scylla, a highly valued delicacy. The aim of this review paper is to summarise the status and information on the current nursery culture stage of the mud crab. Various aspects of mud crab hatchery described in this paper are expected to facilitate practitioners and lay people to easily understand a mud crab nursery. This review also provides guidelines for researchers in conducting future research and development on a mud crab nursery in order to increase the production of mud crab crablets for the farming industry.

Abstract

The nursery stages of mud crab, genus Scylla, proceed from the megalopa stage to crablet instar stages. We review the definition and several of the key stages in mud crab nursery activities. The practice of the direct stocking of megalopa into ponds is not recommended due to their sensitivity. Instead, nursery rearing is needed to grow-out mud crabs of a larger size before pond stocking. Individual nursery rearing results in a higher survival rate at the expense of growth and a more complicated maintenance process compared with communal rearing. The nursery of mud crabs can be done both indoors or outdoors with adequate shelter and feed required to obtain a good survival percentage and growth performance. Artemia nauplii are still irreplaceable as nursery feed, particularly at the megalopa stage, while the survival rate may be improved if live feed is combined with artificial feed such as microbound diet formulations. Water quality parameters, identical to those proposed in tiger shrimp cultures, can be implemented in mud crab rearing. The transportation of crablets between different locations can be done with or without water. The provision of monosex seeds from mud crab hatcheries is expected to become commonplace, increasing seed price and thus improving the income of farmers. Numerous aspects of a mud crab nursery including nutrition; feeding strategies; understanding their behaviour, i.e., cannibalism; control of environmental factors and practical rearing techniques still need further improvement.