Oviducal gland transcriptomics of Octopus maya through physiological stages and the negative effects of temperature on fertilization.

Background: Elevated temperatures reduce fertilization and egg-laying rates in the octopus species. However, the molecular mechanisms that control the onset of fertilization and egg-laying in the octopus' oviducal gland are still unclear; and the effect of temperature on the expression of key reproductive genes is unknown. This study aims to better understand the molecular bases of octopus fertilization and egg-laying, and how they are affected by elevated temperatures. Method: RNA-seq of oviducal glands was performed for samples before, during, and after fertilization and their transcriptomic profiles were compared. Also, at the fertilization stage, the optimal and thermal-stress conditions were contrasted. Expression levels of key reproductive genes were validated via RT-qPCR. Results: In mated females before egg-laying, genes required for the synthesis of spermine, spermidine, which may prevent premature fertilization, and the myomodulin neuropeptide were upregulated. Among the genes with higher expression at the fertilization stage, we found those encoding the receptors of serotonin, dopamine, and progesterone; genes involved in the assembly and motility of the sperm flagellum; genes that participate in the interaction between male and female gametes; and genes associated with the synthesis of eggshell mucoproteins. At temperatures above the optimal range for reproduction, mated females reduced the fertilization rate. This response coincided with the upregulation of myomodulin and APGW-amide neuropeptides. Also, genes associated with fertilization like LGALS3, VWC2, and Pcsk1 were downregulated at elevated temperatures. Similarly, in senescent females, genes involved in fertilization were downregulated but those involved in the metabolism of steroid hormones like SRD5A1 were highly expressed.

Effects of recent thermal history on thermal behaviour, thermal tolerance and oxygen uptake of Yellowtail Kingfish (Seriola lalandi) juveniles.

This study determined the physiological and metabolic responses of cultivated Yellowtail Kingfish (Seriola lalandi) juveniles in accordance with their recent thermal history. The fish were acclimated at 20, 23, 26, 29 and 32 °C for 21 days to determine the final preferred temperature, thermal tolerance and the effect of acclimation temperatures on their oxygen uptake and aerobic scope. The final preferred temperature of juveniles was established at 26 °C. The critical thermal maximum (CTmax) ranged from 34.2 to 36.9 °C, while the critical thermal minimum (CTmin) ranged from 10.9 to 17.3 °C, depending on acclimation temperature. With the CTmax and CTmin values, the thermal window was determined to have an area of 258°C2, which is characteristic of subtropical organisms. Although, the metabolic rate was relatively constant (ranging 390.6-449.8 mg O2 kg-0.8 h-1) between 20 and 26 °C (Q10 = 1.6, 1.0), an increase to 544.8 mg O2 kg-0.8 h-1 at 29 °C (Q10 = 1.9) and decrease of 478.4 mg O2 kg-0.8 h-1 at 32 °C (Q10 = 0.6) were observed. The maximum value obtained for aerobic scope was 310.9 mg O2 kg-0.8 h-1 at 26 °C. These results suggest that the acclimation temperature of 26 °C is an optimum thermal condition for a physiological and metabolic performance of yellowtail kingfish juveniles. On the contrary, the response observed during the evaluation of critical temperatures, oxygen uptake and aerobic scope indicated that yellowtail kingfish in the juvenile state could be vulnerable when it experiences for long periods (e.g., >21 days) temperatures above 29 °C. According to our results, the thermoregulatory behaviour of yellowtail kingfish in the juvenile stages could be one of the most important mechanisms to maintain its optimal physiological performance by actively selecting a stable thermal environment close to 26 °C. In addition, it was determined the limits of the pejus state of juvenile yellowtail kingfish at 29 °C, where an increase of oxygen uptake to maintain the aerobic energy metabolism was observed, this could certainly affect the growth of juveniles in culture systems if they do not return in a thermal range of 23-26 °C. These results can contribute to infer the different effects of acclimation temperature on the growth, thermal tolerance and respiratory capacity of S. lalandi juveniles on aquaculture systems.