The current study aimed to determine the acute and sub-chronic toxicity of ammonia to juvenile surf clams (Mactra chinensis Philippi). Acute toxicity tests were conducted with seven concentrations of ammonium chloride using a 96 h static-renewal approach. Sub-chronic ammonia exposure tests (20 d exposures) were conducted with 6 concentrations at 20 °C. The 96 h median lethal concentration (96 h LC₅₀) was 11.1 (10.0; 12.0) mg/L total ammonia nitrogen (TAN) and 0.56 (0.50; 0.60) mg/L unionized ammonia (NH₃). The relative growth rate was significantly reduced at concentrations higher than 1.6 mg/L TAN (0.075 mg/L NH₃) in the 20 d tests. The estimated maximum acceptable toxicant concentration (MATC) based on the reduced growth of juvenile M. chinensis was between 0.8 and1.6 mg/L TAN (0.038–0.075 mg/L NH₃). Histopathological changes were evaluated in the surviving clams after 20 days of exposure. Exposure to 14.1 mg/L TAN (0.661 mg/L NH₃) resulted in changes in the mantle, foot and digestive diverticulum. We also examined the antioxidant enzyme activities of superoxide dismutase (SOD) and catalase (CAT) in 10 d and 20 d at 6 different levels (plus a control) of ammonia from 0.8 mg/L to 14.1 mg/L TAN. Ammonia exposure at 0.8 mg/L TAN (0.038 mg/L NH₃) significantly affected SOD and CAT activities. The level of enzymic activity decreased with the increasing concentration of TAN. The results improved our understanding of oxidative damage under ammonia exposure and provided data for the aquaculture of sunray surf clams.

Tiger barb (Puntigrus tetrazona) is an economically important ornamental cyprinid widely cultured in the global aquarium trade; however, comprehensive information on its early development and reproductive performance remains limited. This study characterized embryonic development, larval and juvenile ontogeny, and reproductive performance under controlled hatchery conditions. Fertilized eggs were continuously observed from fertilization to hatching using stereomicroscopy, while larval development was monitored from hatching to 30 days post-hatching (dph). Reproductive performance was evaluated across five consecutive spawning cycles by determining estimated mean egg production per stocked female, fertilization rate, hatching rate, and larval survival. At 26.5–27.5 °C, embryonic development progressed through cleavage, blastula, gastrula, neurula, and organogenesis, and hatching commenced at 25 h 10 min post-fertilization; approximately 50% of viable embryos had hatched by 27 h 17 min, and hatching was completed by approximately 35 h 28 min. Newly hatched larvae measured 3.88 ± 0.56 mm in total length, and complete yolk-sac absorption occurred at approximately 7 dph. Characteristic vertical body pigmentation appeared at 15 dph, whereas juvenile characteristics were evident by approximately 20 dph, while complete scale coverage and adult-like coloration were observed by 30 dph. Reproductive performance declined significantly with successive spawning events, with fecundity decreasing from 182.33 ± 1.57 to 64.33 ± 3.52 eggs female⁻¹ between the first and fifth spawnings, accompanied by reductions in fertilization, hatching, and larval survival rates. These findings provide a morphological developmental reference and practical hatchery benchmarks that can support broodstock management, larval rearing, and seed production of tiger barb in ornamental aquaculture.

Nitrite is a common pollutant in marine environments and can cause mortality in crustaceans and bivalves. The purpose of the current study is to understand nitrate’s toxicity to juvenile clams due to its potential impact on aquaculture and marine ecosystems. Juvenile sunray surf clams (Mactra chinensis Philippi) (1.00 ± 0.10 cm shell length, 0.75 ± 0.04 cm shell height) were exposed to varying concentrations of nitrite for 96 h and 20 days, respectively. The LC₅₀ for survival at 96 h was 37 mg/L NO₂-N. Histological evaluations were made on juvenile clams exposed at 30 mg/L after 20 d of exposure. Epithelial cells and digestive diverticulum are the best sub-lethal effect indicators. Shell length and antioxidant enzyme activities were measured at the beginning of the experiment and then observed 10 and 20 days after exposure. A logarithmic relationship was obtained between the relative growth rate (based on the shell length) of juvenile M. chinensis and the nitrite concentration. Compared to the control, activity suppression of superoxide dismutase and catalase activity was detected from the concentration of 1 mg/L NO₂-N. It is recommended that nitrite concentrations remain below 1 mg/L to prevent stress during the early developmental stages of clams.