Days Post Hatching Research Articles

Effect of dietary l-glutamine supplementation on the intestinal physiology and growth during Solea senegalensis larval development

The maturation of the intestinal digestive and absorptive functions might limit the amount of absorbed nutrients to fulfil the high requirements of the fast-growing marine fish larva. Glutamine (Gln) has been described to improve intestinal epithelium functions, due to its involvement in energy metabolism and protein synthesis. The purpose of this study was to evaluate dietary 0.2% Gln supplementation on aspects of intestinal physiology, protein metabolism and growth-related genes expression in Senegalese sole larvae. Experiment was carried out between 12 and 33 days post hatching (DPH) and fish were divided into two experimental groups, one fed Artemia spp. (CTRL) and the other fed Artemia spp. supplemented with Gln (GLN). GLN diet had two times more Gln than the CTRL diet. Samples were collected at 15, 19, 26 and 33 DPH for biometry, histology, and digestive enzymes activity, and at 33 DPH for gene expression, protein metabolism and AA content determination. Growth was significantly higher for Senegalese sole fed GLN diet, supported by differences on protein metabolism and growth-related gene expression. Slight differences were observed between treatments regarding the intestinal physiology. Overall, GLN diet seems to be directed to enhance protein metabolism leading to higher larval growth.

Temperature-Induced Sex Differentiation in River Prawn (Macrobrachium nipponense): Mechanisms and Effects.

Macrobrachium nipponense is gonochoristic and sexually dimorphic. The male prawn grows faster and usually has a larger size than the female. Therefore, a higher male proportion in stock usually results in higher yield. To investigate the impact of temperature on sexual differentiation in M. nipponense, two temperature treatments (26 °C and 31 °C) were conducted. The results showed that compared to the 31 °C treatment (3.20 ± 0.12), the 26 °C treatment displayed a lower female/male ratio (2.20 ± 0.11), which implied that a lower temperature could induce masculinization in M. nipponense. The temperature-sensitive sex differentiation phase was 25-35 days post hatching (DPH) at 26 °C while 15-20 DPH at 31 °C. Transcriptome and qPCR analysis revealed that a lower temperature up-regulated the expression of genes related to androgen secretion, and down-regulated the expressions of genes related to oogonia differentiation. Thirty-one temperature-regulated sex-differentiation genes were identified and the molecular mechanism of temperature-regulated sex differentiation was suggested. The finding of this study indicates that temperature regulation can be proposed as an innovative strategy for improving the culture yield of M. nipponense.

Mechanistic insights into the early life stage microbiota of silver pompano (Trachinotus blochii).

Deep investigations of host-associated microbiota can illuminate microbe-based solutions to improve production in an unprecedented manner. The poor larval survival represents the critical bottleneck in sustainable marine aquaculture practices. However, little is known about the microbiota profiles and their governing eco-evolutionary processes of the early life stages of marine teleost, impeding the development of suitable beneficial microbial management strategies. The study provides first-hand mechanistic insights into microbiota and its governing eco-evolutionary processes in early life stages of a tropical marine teleost model, Trachinotus blochii. The microbiota profiles and their dynamics from the first day of hatching till the end of metamorphosis and that of fingerling's gut during the routine hatchery production were studied using 16S rRNA amplicon-based high-throughput sequencing. Further, the relative contributions of various external factors (rearing water, live feed, microalgae, and formulated feed) to the microbiota profiles at different ontogenies was also analyzed. A less diverse but abundant core microbial community (~58% and 54% in the whole microbiota and gut microbiota, respectively) was observed throughout the early life stages, supporting 'core microbiota' hypothesis. Surprisingly, there were two well-differentiated clusters in the whole microbiota profiles, ≤10 DPH (days post-hatching) and > 10 DPH samples. The levels of microbial taxonomic signatures of stress indicated increased stress in the early stages, a possible explanation for increased mortality during early life stages. Further, the results suggested an adaptive mechanism for establishing beneficial strains along the ontogenetic progression. Moreover, the highly transient microbiota in the early life stages became stable along the ontogenetic progression, hypothesizing that the earlier life stages will be the best window to influence the microbiota. The egg microbiota also crucially affected the microbial community. Noteworthily, both water and the feed microbiota significantly contributed to the early microbiota, with the feed microbiota having a more significant contribution to fish microbiota. The results illustrated that rotifer enrichment would be the optimal medium for the early larval microbiota manipulations. The present study highlighted the crucial foundations for the microbial ecology of T. blochii during early life stages with implications to develop suitable beneficial microbial management strategies for sustainable mariculture production.

Broodstock development, induced spawning and larval rearing of the bilih, Mystacoleucus padangensis (Bleeker, 1852), a vulnerable species, and its potential as a new aquaculture candidate.

Background: Mystacoleucus padangensis living in Lake Singkarak, Indonesia, has high potential market demand but is threatened by overfishing and has not been successfully cultured. This study describes the first broodstock development, induced breeding, and larval rearing of M. padangensis. Methods: A total of 1,000 female and 1,000 male broodfish were collected from the wild and reared in two concrete ponds (128 m 2) at the Centre for Biodiversity Conservation, P.T. Semen Padang, Indonesia. The broodfish were fed commercial feed to satiation at 09:00 and 17:00 h. The females (average weight 7.56 ± 0.85 g) and males (4.86 ± 1.20 g) were selected at a ratio of 1:4 (female:male), and gonad maturation was induced with a single dose of GnRH analogue (Ovaprim) of 0.1 ml/fish. At 16 h after hormone injection, eggs were collected individually into a plastic vessel. Spermatozoa were collected with sterile syringes. Eggs were fertilized using the "dry" method, and 0.5 ml samples (equal to 100 eggs) were taken. The eggs were incubated in a plastic strainer with a water volume of 1.57 litres and placed in a tarpaulin pond with a volume of 150.72 litres. Results: The overall hatching rate was 78.93 ± 4.13%. The newly hatched larvae were 3900.81 µm long, with a yolk sac of 82881.480 µm 2. The mouth opened at 72 days post hatching (DPH) with a gape measuring approximately 61.880 µm. The protocol of larval feeding started with artificial feed, followed by Artemia nauplii up to 30 DPH. Weaning of larvae started at 4 DPH. Larvae started metamorphosis by 15 DPH and ended by 22 DPH when the larvae reached 7430.27 µm. Larval rearing resulted in an average survival rate of 28.4 ± 3.04%. Conclusions: Its successful spawning induction and high larval hatching and survival rates make M. padangensis an excellent aquaculture candidate.

Year-Round Spawning, Filial Cannibalism, and Embryonic and Larval Development of the Coral Reef Fish Orchid Dottyback, Pseudochromis fridmani

Aquaculture of marine ornamental fish could potentially reduce the fishing pressure on wild stocks by the global aquarium trade, but its expansion is often constrained by the limited understanding on the biology and early life history of candidate species. The orchid dottyback Pseudochromis fridmani is a valuable and popular marine ornamental fish, but scientific reports on its baseline biology, especially the egg filial cannibalistic behaviour, are scarce. The present study documented key aspects of reproductive biology and early ontogeny of P. fridmani in captivity, including reproductive behaviour, patterns of spawning and filial cannibalism throughout a 12-month period by seven pairs of brood fish, as well as the embryonic and larval development. The results showed that the captive spawning of the broodstock pairs generally occurred every 5–11 days, most commonly every 6–8 days, throughout a year. Despite feeding the brood fish to satiation throughout the data collection period, the average monthly rate of egg filial cannibalism reached 55 ± 37%, but the cannibalism frequency appeared to be pair-specific. The egg incubation duration was approximately 96 h at 27 ± 1 °C, and the development of embryos from cleavage, blastula, gastrula, and segmentation to the pharyngula stage is herein described in detail. The newly hatched P. fridmani larvae possessed important structures and organs for first feeding, including pigmented eyes, developed jaws, and a straight-tube gut. The 0 to 14 days post-hatching (DPH) period appeared to be an important larval stage, as P. fridmani larvae were observed to complete major changes in morphology, gut development, and phototactic behaviour by 14 DPH. Under the culture conditions of this study, the earliest transition to the juvenile stage was observed on 31 DPH, and the majority of fish became juveniles by 56 DPH. The results of this study inform baseline aquaculture production protocols and direct future research, particularly to reduce filial cannibalism through broodstock management and to improve larval culture through supporting the early ontogenetic development of P. fridmani. Additionally, these findings form a foundation for further studying the biology and ecology of P. fridmani in the wild.

Allometric Growth Patterns and Ontogenetic Development during Early Larval Stages of Schizothorax waltoni Regan and Percocypris retrodorslis in Southwest China

Due to a sharp decline in resources, Schizothorax waltoni Regan and Percocypris retrodorslis have been listed as wildlife under second-class protection in China. Under culture conditions, the early development and allometric growth patterns of S. waltoni and P. retrodorslis were researched from the hatching stage to 60 DPHs (days post-hatching), and a sampling of ten to fifteen larvae was made every day, followed by measurements with Axio Vision 4.8 software (Carl Zeiss AG, Jena, Germany). Morphological indicators included the anal fin length, the body depth, the body depth at the anus level, the caudal fin length, the dorsal fin length, the eye diameter, the head length, the head depth, the pectoral fin length, the tail length, the trunk length, the snout length, the total length, and the ventral fin length. Based on the morphology development of S. waltoni Regan and P. retrodorslis, four periods of larval growth were identified: pre-flexion larvae at 0–14 DPHs and 0–16 DPHs; flexion larvae at 14–23 DPHs and 16–26 DPHs, post-flexion larvae at 23–50 DPHs and 26–52 DPHs, and the juvenile stage. In newly hatched larvae, most organs and body parts were not differentiated, and they successively developed within 26 DPHs. The depletion of the yolk sac was observed at 23 DPHs and 25 DPHs. Allometric growth mainly occurs in the head and tail regions, indicating that body parts related to feeding and swimming behaviors were more important than the other parts. In addition, the growth pattern shows that the development of organs gives priority to the functions of gill respiration, sensation, exogenous feeding, and swimming. The inflection points of body part growth patterns only appeared before 40 DAH, so future studies should concentrate focus on developing the best feeding from the first feeding to 40 DAH. These outcomes were discussed with regard to the ontogeny of the functional morphology in relation to ecology and aquaculture. It is expected that this research can provide valuable perspectives in species conservation.

Effects of feed transition on digestive tract digestive enzyme, morphology and intestinal community in cuttlefish (Sepia pharaonis)

Sepia pharaonis is an excellent candidate for aquaculture in China. However, the low survival rate during early feed transition is a bottleneck restricting industrial development. Understanding the changes in digestive physiology and intestinal microflora during feed transition should enable us to meet their nutritional needs to improve production. In this study, we investigate the digestive enzyme of S. pharaonis and undertake histological observations of the digestive gland and intestine. The intestinal microflora 16S rRNA genes were also analyzed using high-throughput sequencing of the pre, mid, and post-feed transition stages (20, 40, and 60 days post-hatching (DPH), respectively). The digestive enzymes from the digestive gland (trypsin and chymotrypsin) rapidly decrease at 40 DPH when compared to their levels at 20 DPH, but mostly recovered by 60 DPH. The alkaline phosphatase and lipase increased sharply by 40 DPH, then peaked at 60 DPH. The intestinal digestive enzymes followed similar trends during feed transition, except for lipase activity, which decreased after 20 DPH and remained low, even at 60 DPH. Feed transition affects the morphogenesis of the digestive tract and feed transition stress leads to the impairment of the digestive gland and intestinal morphology, which reduces the digestive capacity, but almost totally recovers by 60 DPH. Moreover, the comparison of the intestinal microbial composition during feed transition revealed that the dominant phylum Bacteroidetes gradually increased to a peak at 40 DPH and then decreased until 60 DPH. The microbial composition changed with the most abundant genus Pseudomonas being replaced by Acinetobacter. The phylum and family level investigation suggested the microbiota in the rearing water had limited influence on the intestinal microbiota. The intestinal microbiota diversity increased during feed transition. This study improves our understanding of changes and adaptations in cuttlefish during feed transition.

Effect of temperature on food intake and digestion of Artemia in Lophiosilurus alexandri larvae

ABSTRACT The objective of this study was to elucidate the effect of temperature on food intake and time for digestion in larvae of Lophiosilurus alexandri. An experiment was performed with four constant temperatures (23°C, 26°C, 29°C, and 32°C), four ages (8, 13, 18, and 23 days post-hatching (DPH)) and eight sampling times (5, 15, 30, 45, 60, 120, 180, and 300 minutes after feeding). The effect of temperature was more evident at 18 and 23 DPH. Due to the increase in age and temperature, there was a decrease in the time to reach the highest rate of digested nauplii. In addition, the larvae were estimated to ingest up to 0.30 ± 0.10 MJ/larva at 32°C in the first meal of the day. Temperatures of 29°C and 32°C provided the most satisfactory results for intake, digestion rate, and intake as percentage of live weight in one meal.

Debaryomyces hansenii CBS 8339 promotes larval development in Seriola rivoliana

The present study aimed to know the effect of the administration of Debaryomyces hansenii yeast on growth, development and skeletal deformities, through the quantification of molecular and morphophysiological indicators in longfin yellowtail larvae. Larvae from the yeast group were fed with the rotifers and Artemia metanaupli half enriched with D. hansenii (50% of live prey) and half enriched with Origreen® (50% of live prey) and were compared to control (100% Origreen®) from 5 to 30 days post-hatching (DPH). Each treatment had two methodological replicates. Studies on bone mineralization, skeletal deformities, intestinal histological analysis, digestive enzyme activity, differential gene expression (RT-qPCR) and transcriptome analysis (RNA-Seq) were performed in whole larvae. Results showed that larvae fed D. hansenii encapsulated within enriched live preys had higher survival and growth, higher intestinal mucin secretion, as well as higher activity of alkaline phosphatase, pepsin and α-amylase. The degree of bone mineralization in the cranial and caudal fin complex was higher in larvae fed D. hansenii and a lower incidence of deformities at the vertebral column was also observed. In general, the gene expression throughout the development fluctuated between the control group and the yeast treatment; however, at 30 DPH, a higher expression of the bone morphogenetic protein type 2 (bmp2), collagen type 1α1 (col1α1) and proliferating cell nuclear antigen (pcna) genes was detected in larvae fed with yeast. The transcriptomic analyses using RNA-Seq revealed that the main genes related to bone mineralization degree and digestive tract maturation, were overrepresented in the D. hansenii treatment, 1,25-dihydroxyvitamin D (3) 24-hydroxylase (cyp24), cytochrome P450 family 27 subfamily A (cyp27a), protein 5 related to the low-density lipoprotein receptor (lrp5), myocyte-specific enhancing factor 2C (mef2), enterokinase (entk), pepsin and alkaline phosphatase. Based on the results, S. rivoliana larvae supplemented with the yeast D. hansenii presented higher growth and survival, a higher degree of maturation of the digestive tract, a higher degree of bone mineralization and a reduction in skeletal deformities, for which the continuous use of yeast is recommended as a food supplement to larvae from 5 days post-hatching onwards.

Morphophysiological responses of Octopus tehuelchus juveniles during the transition period between endogenous and exogenous feeding

In cephalopod hatchlings there is a transitional period considered critical, in which the digestive system undergoes morphophysiological changes associated with the passage from the consumption of vitelline reserves to an independent feeding. During this period, the characterization of the digestive dynamics and growth is key information to define their nutritional requirements under culture. Octopus tehuelchus, which has become increasingly interesting for cultivation, is a species endemic to South America with holobenthic development and large juveniles. In this work, the enzymatic and cytological changes of the digestive gland (DG) along with the variation in body weight and size of O. tehuelchus hatchlings were evaluated when reared at 16 °C under two feeding treatments: fed with the isopod Exosphaeroma sp. and starved. In fed juveniles, the predatory activity began a few hours after hatching; and up to 6 days post hatching (DPH) there was neither somatic growth nor cellular differentiation of the DG, but there was an activation of acid enzymes. From 6 to 10 DPH, there was an increase in the size and weight of juveniles, and the maturation of the DG began with the appearance of heterolysosomes and heterophagosomes. Furthermore, lipases activity reached its maximum level and there was a considerable increase in both alkaline phosphatases and peptidases. From 15 to 25 DPH, the DG became fully mature and the lipase activity reached minimum levels, while the alkaline peptidases continued increasing. At the end of the experiment, at 25 DPH, fed juveniles doubled their weight. In the starved juveniles, neither somatic growth nor cytological differentiations of the DG were registered. Besides, in this treatment, acid phosphatases and lipase activities (i.e. enzymes related to yolk consumption) remained at basal levels or decreased over time, even with yolk platelets still present, suggesting that the consumption of yolk reserves would be delayed, regulating the survival time. In fed juveniles, the ingestion of isopods seems to act as a modulating factor of the digestive activity, triggering an increment in the activity of lipases, acid phosphatases, as well as alkaline peptidases which are related to the digestion of exogenous food. It is concluded that morphological and physiological changes during the digestive maturation are related to the ingestion of Exosphaeroma sp. In this sense, this isopod species would be a suitable food which promotes the rapid growth of O. tehuelchus during their early post-hatching stage under culture.

Effects of dietary protein level on the growth, reproductive performance, and larval quality of female yellow catfish (Pelteobagrus fulvidraco) broodstock

This study was conducted to evaluate the effects of dietary protein levels on the growth, spawning, and larval quality of female yellow catfish (Pelteobagrus fulvidraco) broodstock through a six-week feeding trial. Five isoenergetic diets were formulated, and the crude protein levels were 34.4 (DP34), 38.8 (DP39), 44.2 (DP44), 49.3 (DP49), and 52.7% (DP53). The results showed that the growth and reproductive performance, including the specific growth rate (SGR), feed efficiency (FE), gonadosomatic index (GSI), and hatching rate of yellow catfish broodstock in the DP44, DP49, and DP53 groups were significantly higher than those of other groups (P < 0.05). The optimal dietary protein levels for yellow catfish broodstock were 45.8% and 44.1% based on SGR and GSI. The lowest condition factor (CF), GSI, and smallest egg size were found in the DP34 group, while the total egg production was the highest in the DP53 group. Vitellogenin (VTG) contents in eggs and plasma were enhanced significantly with the increasing dietary protein levels from 34% to 49%, but decreased precipitously in the DP53 group (P < 0.05). The optical densities of androgen receptor (AR) and estrogen receptor (ER) in the DP34 group were significantly lower than those in other groups. The mRNA expression levels of CYP19a, AR, and ERβ were significantly increased in the DP49 group (P < 0.05). For larval quality, the deformity rate decreased significantly with the increasing dietary protein levels (P < 0.05). The larval survival rate at 9 DPH (days post-hatching) as a function of dietary protein level was analyzed by second-order polynomial regression. The results showed that a level of 47.7% was optimal for improving larval quality. Thus, a suitable range of dietary protein of yellow catfish broodstock for growth and reproduction is 44.1–47.7%.

Digestive system ontogeny and the effects of weaning time on larval survival, growth and pigmentation development of orchid dottyback Pseudochromis fridmani

Although live prey remain essential for most marine finfish larviculture, the larvae or young juveniles are ultimately weaned onto formulated feeds at foodfish and ornamental fish hatcheries. Understanding the development of larval digestive capabilities is crucial for optimizing weaning success. The present study used the digestive system ontogeny to interpret the effects of different weaning times on larval rearing of orchid dottyback Pseudochromis fridmani, a marine ornamental fish species. The study documented the ontogenetic digestive system development of P. fridmani larvae from hatching to 40 days post-hatching (DPH). The histological analysis showed that as a demersal spawning species, the newly hatched P. fridmani larvae possessed advanced gut development which allowed the onset of exogenous feeding soon after hatching. Most major larval gut morphological changes were completed by 13 DPH; however, the stomach and gastric glands were first observed between 32 and 36 DPH. At this stage, P. fridmani larvae were expected to acquire substantially improved digestive capabilities, and were likely capable of digesting formulated feed. In the subsequent experiment, P. fridmani larvae were weaned onto formulated feed from 20, 23, 26, 29, or 32 DPH over a three-day period of co-feeding with enriched Artemia; the control group were exclusively fed enriched Artemia until the end of experiment. The timing of weaning significantly affected the larval growth and pigmentation development rather than survival (mean: 44–64%). P. fridmani larvae weaned later tended to grow larger, and the control group grew significantly larger in length than all weaning treatments (p < 0.05). Interestingly, at the end of the experiment (40 DPH), none of larvae from the control obtained adult-type pigmentation despite their larger size. In contrast, the introduction of formulated feed appeared to stimulate pigmentation development in P. fridmani individuals with a clear trend of higher pigmentation completion by 40 DPH (mean: 62–97%) with earlier weaning. Overall, this study demonstrated that P. fridmani larvae could be weaned onto formulated feed from as early as 20 DPH, before the stomach and gastric glands were fully developed; indeed, the timing of adult-type pigmentation change in P. fridmani was also advanced by the introduction of the weaning diet.

Transition to Artemia feeding phase for orchid dottyback Pseudochromis fridmani larvae: Establishing suitable prey shift time and strategy

Marine finfish hatcheries often replace small live prey with the larger traditional prey, brine shrimp Artemia, to feed the growing fish larvae. However, research on fish larviculture, including marine ornamental fish, often overlooks the importance of fine-tuning such prey transition. This study investigated the suitable time and approach for the larvae of an ornamental species, orchid dottyback Pseudochromis fridmani, to transit to the Artemia feeding phase. In experiment I, the suitable prey shift time window was estimated by abruptly feeding P. fridmani larvae with newly hatched Artemia nauplii (AN) from 5, 8, 11 or 14 days post-hatching (DPH) until 19 DPH. Although P. fridmani could ingest AN from 5 DPH, the final larval survival of 11-DPH and 14-DPH treatments were significantly higher, and it might relate to the enhanced digestive capability of older larvae. In experiment II, when prey shift began on 9, 11, or 13 DPH, the larvae were either abruptly shifted (AS) to AN feeding, or given a three-day gradual transition (GT) period of co-feeding original prey (rotifers and copepods) with AN before completing the prey switch. By 22 DPH, the prey shift strategy of GT significantly improved larval survival compared to the AS treatments; prey shift time treatments of 11-DPH and 13-DPH also reached significantly higher survival than 9-DPH treatments. However, survival of all individual treatments continued to decrease in the final stage of experiment. To further improve the prey transition success, experiment III investigated the effects of Artemia enrichment and prey shift time, by abruptly feeding larvae with either AN or enriched Artemia metanauplii (EA) from 9, 11, or 13 DPH onwards. Although the larvae fed AN were significantly larger than those fed EA, the absolute differences were small. There was a significant interaction between Artemia type and prey shift time on larval survival. The EA-11-DPH and EA-13-DPH treatments maintained steady survival post prey transition, and reached the highest mean survival of 58 ± 10%, and 55 ± 12% by 20 DPH, respectively. Moreover, while the larvae fed AN abnormally reduced prey intake over time, most larvae in EA-11-DPH and EA-13-DPH treatments still maintained high Artemia ingestion. These results suggest that dietary highly unsaturated fatty acids in enriched Artemia were crucial for larval survival of P. fridmani. Overall, this study shows that P. fridmani larvae could skip AN and directly feed on EA from 11 DPH without compromising survival or feeding performance.

The effects of live prey and greenwater on the early larval rearing of orchid dottyback Pseudochromis fridmani

Captive breeding of marine ornamental fish is considered as a sustainable alternative to the current practice of fishing the wild stocks to supply the marine aquarium trade. However, efficient larviculture remains the biggest bottleneck, as many marine ornamentals suffered total mortality or only have extremely low survival at early larval stage. This study investigated the optimal conditions for the rearing of early larvae of a valuable ornamental fish, the orchid dottyback Pseudochromis fridmani, in terms of density and combination of live prey, as well as the application of the “greenwater” technique. In the first experiment, larvae were reared in clearwater and fed rotifers at 2, 5, 15, and 40 mL−1. Although there was no significant difference in larval survival among treatments on 8 days post-hatching (DPH), the larvae fed higher rotifer densities (15 and 40 rotifers mL−1) had significantly better growth. In the second experiment, by 8 DPH, a significant interaction between rearing condition (clearwater vs. greenwater) and live prey combination (rotifers only vs co-feed rotifers and copepods) on larval survival was detected, and the mean survival of larvae co-fed rotifers and copepods in clearwater (18%) was significantly lower than the other treatments (45–64%). Moreover, larvae reared in greenwater, or co-fed rotifers and copepods, grew significantly larger than those reared in clearwater, or fed rotifers only, respectively. In the third experiment, the larval survival was higher cell density of Nannochloropsis paste (NAN) used to make greenwater (0–3.40 × 106 NAN cells mL−1). The larval survival of the highest algae density treatment was the highest on 8 DPH (72 ± 8%), and it was significantly higher than those reared in the treatments of 1.13 × 106 or 0 NAN cells mL−1, but not significantly different from that of 2.27 × 106 cells mL−1 treatment. Overall, the results of this study suggest that co-feeding early P. fridmani larvae with rotifers and copepods, with the complementary use of greenwater comprising of ≥ 2.27 × 106 NAN cells mL−1, can enhance larval survival, growth and development. However, when the algal cell density is below this threshold, using such a co-feeding strategy may compromise the survival of P. fridmani larvae. The results of this study highlighted the importance of optimizing the application of live prey and greenwater for improving the survival of marine fish larvae.

Effects of photoperiod on growth, digestive, metabolic and non-special immunity enzymes of Takifugu rubripes larvae

Photoperiod is one of the important environmental factors, which have important effects on the early growth and development, survival, metabolism and immune system of larvae. In the present study, Takifugu rubripes larvae were exposed to four different LED photoperiods: 8 L:16 D, 16 L:8 D, 20 L:4 D and 24 L:0 D from −1 to 30 days post hatching (DPH). We measured growth, activity of pepsin (PPS), lipase (LPS), alanine aminotransferase (ALT), aspartate aminotransferase (AST), and non-specific immune enzyme activity of alkaline phosphatase (AKP), and acid phosphate (ACP), superoxide dismutase (SOD), Catalase (CAT), glutathione peroxide (GSH-PX), total antioxidant capacity (T-AOC) at 1, 5, 10, 15, 20, and 30 DPH. The results showed that there was no difference in the effects of photoperiod on the growth within 18 DHP, while 20 L:4 D had a positive effect on the growth of the larvae after 19 DPH. The effect of photoperiod on digestive enzymes did not show obvious regularity. Compared with photoperiod, the changes of some digestive and metabolic enzymes were mainly affected by exogenous food (LPS, ALT, AST changed obviously after feeding Artemia). On the 30 DPH, the AKP of 8 L: 16 D and 24 L: 0 D groups was significantly higher than that of the other two groups, which indicated that 8 L: 16 D and 24 L: 0 D lighting might cause stress to fish body and lead to the increase of AKP. GSH-PX in newly hatched fish at 24 L:0 D was significantly higher than that in other groups. Photoperiod had no significant effect on the enzyme activity of PPS, LDH, ACP, CAT and no obvious pattern was found. Overall, our results suggested that 20 L:4 D promoted larval growth after 19 DPH and shorter photoperiods can be used to save electricity before 18 DPH of Takifugu rubripes larva.

The potential use of the euryhaline rotifer Proales similis for larval rearing of the freshwater pike silverside Chirostoma estor estor

Successful larval rearing of many fish species depends on the adequate selection of live food. Rotifers of the genus Brachionus are the most widely used in feeding protocols for freshwater and marine aquaculture. In this work, the utility of the euryhaline rotifer Proales similis as potential prey for the freshwater pike silverside Chirostoma estor estor larval rearing was investigated. The rotifers P. similis, Brachionus ibericus and B. plicatilis (body length: 84 ± 1.24, 193 ± 2.90 and 206 ± 4.20 μm, respectively) were used to evaluate C. estor estor larvae prey selectivity during the first 3 to 18 days post-hatching (DPH). Functional responses were tested at prey densities of 0.5, 1, 2, 4, 8 and 16 ind. mL−1. Larval survival was evaluated using four treatments: P. similis (T1), B. ibericus (T2), B. plicatilis (T3) and a mixed diet of all three taxa (T4). Results showed that P. similis was the C. estor estor most selected prey during the 3 to 7 DPH when compared to B. ibericus and B. plicatilis. From 8 to 18 DPH, the selectivity for P. similis shifted to increase selectivity for B. ibericus and B. plicatilis. At 4 to 12 DPH, functional responses indicated a greater P. similis consumption by larvae at high density (16 ind. mL−1). Larval survival was significantly higher in T1 than other treatments (T2 – T4). These findings provide evidence that P. similis can be included during the C. estor estor larval rearing. Proales similis could be therefore used at least during the 3 to 7 DPH, but the feeding protocols should be then complemented with brachionid rotifers. The present data are valuable for aquaculture practices and help approach the management and conservation issues of this endemic endangered fish species of the Central Plateau of Mexico.

Early Auditory Experience Modifies Neuronal Firing Properties in the Zebra Finch Auditory Cortex.

Songbirds learn to sing much as humans learn to speak. In zebra finches, one of the premier songbird models, males learn to sing for later courtship through a multistep learning process during the developmental period. They first listen to and memorize the song of a tutor (normally their father) during the sensory learning period. Then, in the subsequent sensory-motor learning phase (with large overlap), they match their vocalizations to the memorized tutor song via auditory feedback and develop their own unique songs, which they maintain throughout their lives. Previous studies have suggested that memories of tutor songs are shaped in the caudomedial nidopallium (NCM) of the brain, which is analogous to the mammalian higher auditory cortex. Isolation during development, which extends the sensory learning period in males, alters song preference in adult females, and NCM inactivation decreases song preference. However, the development of neurophysiological properties of neurons in this area and the effect of isolation on these neurons have not yet been explained. Here, we performed whole-cell patch-clamp recording on NCM neurons from juvenile zebra finches during the sensory learning period, 20, 40, or 60 days post-hatching (DPH) and examined their neurophysiological properties. In contrast to previous reports in adult NCM neurons, the majority of NCM neurons of juvenile zebra finches showed spontaneous firing with or without burst firing patterns, and the percentage of neurons that fired increased in the middle of the sensory learning period (40 DPH) and then decreased at the end (60 DPH) in both males and females. We further found that auditory isolation from tutor songs alters developmental changes in the proportions of firing neurons both in males and females, and also changes those of burst neurons differently between males that sing and females that do not. Taken together, these findings suggest that NCM neurons develop their neurophysiological properties depending on auditory experiences during the sensory song learning period, which underlies memory formation for song learning in males and song discrimination in females.

Pepsinogen expression during larval development of a Persian Gulf Sparid, Sobaity

The present study aimed to detect the pepsinogen gene expression and pepsin enzyme activity, followed by ontogenetic development of gastrointestinal tube with emphasis on the functional stomach in Sobaity larvae (Sparidentex hasta) from hatching to 40 days post- hatching (DPH) reared at 21 °C. The exocrine pancreas including zymogens granules was detected coincident with the gradual yolk-sac absorption (1–3 DPH). The mouth and anus opened at 4 DPH simultaneously with the early convolution of the gut and its segmentation in anterior and posterior intestine, and the buccopharyngeal cavity. The most obvious differentiation in the digestive tract was the formation of an incipient stomach at 12 DPH, surrounded by striated muscle fibers and the development of the gastric glands. The functional stomach appeared 20 DPH onward, where it increased in size and the gastric folds elongated. The stomach was fully developed at 25 DPH with the transition from striated to smooth muscle cells and appearance of gastric glands in the posterior region of the stomach. Pepsinogen was not expressed before 12 DPH and significantly detected coincident with an increase in growth performance and also before the onset of weaning. It increased exponentially parallel to the larval stomach development and concomitant to the change in the diet from live to microbound food. The pepsin enzyme activity, even at a low level, was observed two days after pepsinogen gene expression (14 DPH), increasing sharply after the onset of weaning. Moreover, according to a phylogenetic tree based on the amino acid, the Sobaity’ sequence of pepsinogen was included in the subtype A2, when expressed during late larval development. These results enhanced our understanding of ontogenetic development of pepsinogen in this species, as the pepsinogen expressed earlier than when is expected, and suggest that farmers can initiate weaning at an earlier larval age than is currently practiced, regarding the fact that Sobaity larvae developed a functional stomach 25 DPH onwards.

Survival, growth, ingestion rate and foraging behavior of larval green mandarin fish (Synchiropus splendidus) fed copepods only versus co-fed copepods with rotifers

The importance of copepods as the diet for rearing early larvae of the green mandarin fish Synchiropus splendidus has been demonstrated in our previous studies, however in those studies, copepods were always co-fed with rotifers in the attempt to compensate for low copepod feeding density due to substantially low copepod culture productivity as compared to rotifers. Hence to what extent the roles of rotifers played in the larval culture is unclear. In this study, survival and growth of newly hatched S. splendidus larvae were compared when they were fed copepods (Parvocalanus crassirostris) at different densities either alone or co-fed with ss-type rotifers (Brachionus rotundiformis) until 11 days post-hatching (DPH). It showed that co-feeding rotifers at all copepod densities tested (0.1, 0.5 and 1 copepod mL−1) did not produce clear beneficial effects, suggesting co-feeding rotifers is unnecessary for early larval rearing. A subsequent experiment compared larval ingestion rates on copepods and rotifers under diffident co-feeding conditions, the ingestion rates and electivity index calculated showed that copepods were always positively selected over the rotifers by all ages of larvae (4, 6, 8 and 10 DPH). Furthermore, analyzing larval feeding behavior sequence (i.e. targeting, attack, capture, rejection following capture, and ingestion) on the copepods and rotifers when the two prey were offered to larvae of different ages (6, 8, 10 and 12 DPH) at a same density (1 copepod +1 rotifer mL−1), it was clear larvae of all ages attacked copepods more frequently. Moreover, copepods were never observed being rejected after being captured by the larvae whereas rejection was common for rotifers. Interestingly, the ratios of attacking leading to successful capture were significantly higher on rotifers than on copepods by larvae of all ages. However, this was more than offset by substantial lower ratios of targeting leading to attacks and higher rejection ratios following capture (up to 60%) on rotifers, resulting in significantly lower rotifer ingestion rates compared to copepods by larvae of all ages. Our results provided insights into copepods' superior performance over rotifers for early larvae of S. splendidus, and the results suggest that co-feeding copepods with rotifers is not necessary for early larval rearing.

Swimming Abilities of Temperate Pelagic Fish Larvae Prove that They May Control Their Dispersion in Coastal Areas

The Sense Acuity and Behavioral (SAAB) Hypothesis proposes that the swimming capabilities and sensorial acuity of temperate fish larvae allows them to find and swim towards coastal nursery areas, which are crucial for their recruitment. To gather further evidence to support this theory, it is necessary to understand how horizontal swimming capability varies along fish larvae ontogeny. Therefore, we studied the swimming capability of white seabream Diplodus sargus (Linnaeus, 1758) larvae along ontogeny, and their relationship with physiological condition. Thus, critical swimming speed (Ucrit) and the distance swam (km) during endurance tests were determined for fish larvae from 15 to 55 days post-hatching (DPH), and their physiological condition (RNA, DNA and protein contents) was assessed. The critical swimming speed of white seabream larvae increased along ontogeny from 1.1 cm s−1 (15 DPH) to 23 cm s−1 (50 and 55 DPH), and the distance swam by larvae in the endurance experiments increased from 0.01 km (15 DPH) to 86.5 km (45 DPH). This finding supports one of the premises of the SAAB hypothesis, which proposes that fish larvae can influence their transport and distribution in coastal areas due to their swimming capabilities. The relationship between larvae’s physiological condition and swimming capabilities were not evident in this study. Overall, this study provides critical information for understanding the link between population dynamics and connectivity with the management and conservation of fish stocks.