Ontogeny of the digestive tract in butter catfish Ompok bimaculatus (Bloch) larvae

The ontogeny of the digestive tract was studied histologically in butter catfish Ompok bimaculatus from hatching to 30?days post-hatching (dph). At hatching, the digestive tract of butter catfish consisted of a straight tube with a smooth lumen dorsally attached to the yolk sac. Between 1 and 2 dph, the mouth opened, oral valves were visible and canine-like teeth and taste buds were detected. During this period, intestine was differentiated into the anterior and posterior intestine, and the digestive accessory glands were also developed. Exogenous feeding started at 2?dph, and there was a 2-day mixed endogenous–exogenous feeding period. Most of the yolk sac reserves were consumed between 2 and 3 dph, and by 5?dph, the yolk sac was completely depleted and no longer visible in histological sections. Between 3 and 4 dph, several vacuoles (neutral lipids) were observed in the intestine and also in hepatocytes, indicating a functional absorption of nutrients from food. At 8 dph, differentiation of gastric glands was noticed, and by 9–11?dph, there were abundant gastric tubular glands arranged along numerous longitudinal folds. During the same period, pyloric sphincter appeared as an epithelial fold that separated the stomach from the anterior intestine. From 12 dph to the end of the study at 30?dph, no noticeable histological modifications were observed. The development of gastric glands is considered as the last major events in digestive tract development and their presence designates the end of larval period and the onset of the juvenile period. Hence, it is suggested that, butter catfish larvae have a morphologically complete digestive tract by 12?dph. These findings on the development of the digestive system in butter catfish may lead to a better understanding of the ontogeny and would be useful to improve the larval rearing techniques of this promising catfish species for freshwater aquaculture diversification.     

美洲黑石斑鱼(Centropristis striata)消化系统胚后发育的组织学观察

通过形态学与连续组织切片的方法,对美洲黑石斑鱼(Centropristis striata) 1-34 d仔鱼消化系统的胚后发育进行系统观察,分析描述鱼体消化道(食道、胃与肠道)以及消化腺(肝脏与胰腺)的发育过程.对1-15 d仔鱼连续取样,每次取样30尾,15 d后隔天取样,每次取样15尾.结果显示,在水温为(24±1)℃、盐度为30-32的条件下,初孵仔鱼卵黄囊体积很大,消化管为封闭的管状结构.美洲黑石斑鱼孵出3d后,口裂形成、开始摄食,肛门与外界连通,消化道逐渐分化形成食道、胃及肠道,肝脏、胆囊和胰腺也逐渐形成.7d后,卵黄囊与油球基本消失,食道、胃部以及肠道黏膜褶皱开始形成,消化道黏膜上皮细胞逐渐分化,肝脏出现脂肪颗粒,仔鱼具备了基本的摄食能力.11d时,仔鱼食道可见黏液细胞,随日龄的增加上皮组织中黏液细胞数量迅速增多,褶皱日益丰富;胃部分化形成贲门部、胃本体与幽门部,胃壁褶皱不断增多、伸长;肝脏血窦与中央静脉明显.20d时,鱼体胃腺形成,说明胃部消化、吸收蛋白质的能力增强;肠道次级黏膜褶皱出现,肠圈与褶皱更加复杂化;胰脏分布有大量酶原颗粒.32 d时,仔鱼消化道组织结构分明,自腔面向内依次为黏膜层、黏膜下层、肌层与浆膜层,消化道与消化腺结构和功能逐步完善.仔鱼3-7 d为内源性营养向外源性营养过渡期,应及时提供充足适口的生物饵料,仔鱼20 d后可以逐渐驯化投喂微型配合饲料.     

Morphological development of post hatch larval goldlined seabream Rhabdosargus sarba (Forskål, 1775)

Ultrastructural changes in Rhabdosargus sarba larva in early life history were investigated. At hatching, the digestive system was histologically undifferentiated. The digestive tract was a straight tube attached to the dorsal end of the yolk‐sac and was not connected to either the mouth or the anus. The layer of gut epithelium at some regions of the luminal surface was straight and microvilli were not present. These straight borders were not observed at 1 day post hatching (DPH) onwards as microvilli increased in number on the luminal surface and became more regular. At 2 DPH, the digestive system was well differentiated and the separation of the mid‐ and hindgut by the intestino‐rectal valve became more advanced. At 0 DPH, the eye was spherical and the retina had a zonation with undifferentiated cells. The eye also lacked differentiated photoreceptors (PR). The retinal PRs increased in length and in number as the yolk‐sac was absorbed. By 2 DPH, the eye was fully pigmented, suggesting that the larval vision system was functional. The larvae had a pure cone retina at the onset of exogenous feeding. Morphological and functional differentiation of the digestive tract and the eye of the larvae preceded the completion of yolk and oil globule absorption. The oil globule was exhausted at 4 DPH and at 2 DPH, the yolk‐sac was completely absorbed. Food particles were observed at 3 DPH. Food particle ingestion and absorption of the yolk‐sac were observed as vision became fully functional.     

Ontogenetic Development of the Digestive System in Agastric Chinese Sucker,Myxocyprinus asiaticus,Larvae

This study was conducted to determine the ontogenetic development of the digestive tract and its accessory structures (liver, pancreas, and gall bladder) in agastric larval Chinese sucker Myxocyprinus asiaticus with the histological and ultrastructural approaches from hatching to 56 days after hatching (DAH). On the basis of its feeding mode, and analyzing the main histological features of the digestive system, larval development in Chinese sucker was divided into three stages from hatching: stage 1 (endotrophic period): 1–6 DAH; stage 2 (endoexotrophic period): 7–14 DAH; stage 3 (exclusively exotrophic period): from 15 DAH onward. At hatching, the digestive tract of the larvae consisted of an undifferentiated straight tube. At 4 DAH, the mouth opened, and the digestive tract was differentiated into buccopharyngeal cavity, esophagus and intestine. At 7 DAH, fish started to feed exogenously. Yolk sac was completely exhausted at 15 DAH. Until 56 DAH, the digestive tract of the larvae displayed regularly arranged microvilli, abundant vacuoles, and protein inclusion bodies. The pancreas, liver, and gall bladder were functional from 6 DAH, which enabled larvae to ingest, digest, and assimilate the first exogenous food. In comparison with teleosts that have a stomach, the development of the digestive tract of the agastric Chinese sucker seemed relatively slow .     

Ontogeny of the digestive system in hatchery produced Beluga (Huso huso Linnaeus, 1758); a comparative study between Beluga and genus Acipenser

The study on histological characteristics of the digestive system of Beluga (Huso huso) was conducted from hatching until 50 days posthatching at 16.5 °C. Development of the digestive system in this species followed the general pattern described for other Acipenserids, although there were differences in the timing of organ development among species. At hatching, the mouth was opened and digestive system was represented by a gastric cavity filled with yolk and lined by endodermal cells, and a partially differentiated hindgut. Gastric glands started to differentiate at 46.5 degree‐ days posthatching (ddph), the earliest appearance time among sturgeon fishes studied to date. At the onset of exogenous feeding (144.9 ddph), yolk sac reserves were not completely depleted in the stomach, suggesting a period of mixed nutrition. The complete development of the digestive system was not accomplished until 235.2 ddph when it showed all histomorphological features typical of juvenile specimens. According to histological results, it seems advisable to start co‐feeding H.huso larvae with inert diets at the onset of exogenous feeding, because exocrine pancreas and glandular stomach are fully differentiated, although the complete substitution of live prey by inert feed is not recommended until 235 ddph.     

Morphological and histological changes in digestive tract development during starvation in the miiuy croaker

A histological method was used to describe the ontogenetic development of the digestive tract of laboratory-reared miiuy croaker (Miichthys miiuy) and to evaluate the effects of short-term food deprivation on the morphology and histology of the digestive tract. Larvae and juveniles were maintained at 24 °C in a thermostatically controlled system. Three starvation experiments were conducted during different developmental stages: 1–7 days after hatching (dah; prior to benthic swimming); 26–35 dah (during settling); and 42–53 dah (after benthic swimming). According to the structural changes in the ontogenetic development of the digestive tract, three stages were observed. The first stage was from hatching to 3 dah; the digestive tract was undifferentiated in newly hatched larvae and then showed remarkable morphological changes and differentiation. During this period, larvae depended on endogenous nutrition. The second stage (4–20 dah) was a critical period in which larvae transitioned from endogenous feeding to exogenous feeding and the digestive tract fully differentiated into the buccopharynx, oesophagus, stomach, anterior intestine and posterior intestine. Goblet cells and vacuoles appeared in the digestive tract, and pharyngeal teeth and taste buds developed. During the third stage (20–36 dah), the gastric glands developed and the stomach differentiated into the fundic, cardiac and pyloric regions. At 25 dah, pyloric caeca developed and mucosal folds and spiral valves were clearly distinguishable. After 30 dah, the digestive tract did not undergo any noticeable differentiation, indicating the complete development of the digestive system. The wet weight and SGR (specific growth rate) of miiuy croaker larvae and juveniles greatly decreased when they were deprived of food, and compensatory growth was observed in re-feeding juveniles. The livers of starved larvae and juveniles were atrophied and dark coloured, the intestines were transparent and thin, and the stomach cubages were reduced. The histological effects of starvation were mainly evident in the degeneration of cells in digestive organs, as seen in the shrinkage and separation of cells and the loss of intercellular substances in the liver, pancreas, intestine and stomach. These changes became more severe with increased duration of starvation. In addition, the histological structure of the digestive tracts of starved larvae and juveniles partly recovered after re-feeding, and the effects of starvation on miiuy croaker were age dependent.     

Routine metabolism of larval green sturgeon (Acipenser medirostris Ayres)

Routine metabolic rates of green sturgeon (Acipenser medirostris) from hatching to 31 days post hatch (dph) were determined under normoxic conditions. During the endogenous feeding stage that comprised period from hatching to 15 dph, the oxygen consumption rate (MO₂, μg O₂ larva⁻¹ h⁻¹) increased 5-fold before yolk reserves became exhausted and MO₂ rates steady. The allometric relationships between MO₂ and body mass had mass exponents greater than 1.0 (b=1.64±0.21) and equal to 1.0 (b=1.04±0.07) during the endogenous and exogenous feeding phases, respectively. The magnitude and changes of MO₂ rates in green sturgeon larvae reflected their early ontogeny, especially during the endogenous feeding phase when the increase in metabolic rates was associated with organogenesis, acquisition of organ functions, and the conversion of yolk sac into new tissues. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ontogeny of the digestive tract in shi drum (Umbrina cirrosa L.) reared using the mesocosm larval rearing system

Histological changes of the digestive tract were studied in shi drum (Umbrina cirrosa) from hatching until 41 days post hatching (dph), when the fry had a mean (±S.D.) total length (TL) of 32 ± 2 mm and wet weight (WW) of 0.42 ± 0.07 g. Larvae were reared using the mesocosm technique, the most natural among commercially employed rearing methods for marine larvae. Shi drum opened their mouth at 2 dph (2.78 ± 0.09 mm TL), at which time 90% of the larvae already had an inflated swim bladder. The differentiation of the digestive tract into buccopharynx, esophagus, and anterior and posterior intestine was completed by 3 dph (2.82 ± 0.07 mm TL), 1 day after the onset of exogenous feeding. The alimentary canal started coiling and formed its first loop at 2 dph, while the pancreas and liver were differentiated at 3 dph. Yolk sac reserves lasted until 7 dph (4.3 ± 0.1 mm TL), suggesting a brief period of endogenous and exogenous feeding. The first esophageal goblet cells appeared at 7 dph containing acid mucins and at 8 dph taste buds appeared on the buccopharyngeal epithelium. The stomach was morphologically differentiated at 9 dph (5.5 ± 0.1 mm TL) when gastric glands became abundant in the cardiac region, and the first pyloric caeca appeared at 14 dph (10.1 ± 0.9 mm TL). Supranuclear eosinophilic vacuoles were observed in the posterior intestine between 3 and 11 dph (6.3 ± 0.9 mm TL). Their number decreased as the stomach differentiated, suggesting a change in the protein digestion mechanism. The results of the study suggest a rapid development of shi drum and its digestive system and underline the possibility of weaning larvae to artificial feed even earlier than the 12 dph employed in the present study.     

Development of digestive system and swim bladder of larval nase (Chondrostoma nasus L.)

Digestive tract and swim bladder differentiation during nase (Chondrostoma nasus L.) larval development was evaluated using histological methods. The mouth, pharynx and oesophagus were impervious at the time of hatching. The straight tubular intestine consisted of undifferentiated cells and was situated over the yolk sac. The intestine, lined with a single layer of columnar epithelial cells, opened on 2 day posthatching (dph). First evidence of protein digestion and absorption by the enterocytes was observed on the sixth day, while lipid digestion and absorption began on the ninth day of larval development. Intestinal mucous cells developed and became active by the fourth day. The posterior chamber of the swim bladder inflated on the third day, while the anterior one filled 12 dph. Complete yolk sac resorption took place on the ninth day.     

斜带石斑鱼消化系统胚后发育的组织学研究

利用形态学和连续组织切片技术，对出膜后1-60d的斜带石斑鱼各期仔鱼、稚鱼和幼鱼的消化系统进行了光镜观察，描述了其消化器官发育过程和组织学特性。研究表明，实验水温为22.0-27.8℃时，孵化后第4天，上下颌形成，卵黄囊被吸收，消化系统明显分化成食道、胃、肠、直肠以及肝脏、胆囊和胰脏等，鱼体由内源性营养转向外源性摄食营养，表明其消化系统的形态变化与食性的变化相适应。此后随着鱼体的生长，其消化系统从功能和结构上逐步完善成熟。胰脏在出膜后第4天出现，是和肝脏相互分开的一个独立的器官，但是发育到第35-60天，位于肠道后部的胰脏组织内出现许多大空泡。     

Histomorphological changes in digestive tract of golden mahseer (<Emphasis Type="Italic">Tor putitora</Emphasis>) during different developmental stages

Histomorphological changes in digestive tract of golden mahseer (Tor putitora) were examined in larvae [starting from hatching to 45 days post-hatching (dph)], fry, fingerling, and adult. Digestive tract appeared during hatching, on the dorsal side of yolk sac, as a straight tube with a narrow lumen. Mouth opening and appearance of liver and pancreas were observed at 2 dph, and subsequently anal opening, appearance of goblet cells in esophagus, and posterior intestine were evident at 3 dph. The remodeling of oral cavity in terms of epithelial stratification, appearance of taste buds, and goblet cells were observed in a window of 4–5 dph. Intestinal folding was found to be initiated at 8 dph. From 12 to 45 dph, thickening of oral and esophageal mucosal/extramucosal layers, increase in intestinal folding, increases in the density of goblet cells in entire gut were observed. Within the same time window, other histological changes such as disappearance of vacuoles in liver, and abundance of zymogen granules in pancreas were also observed. Supranuclear vesicles in mid-to-posterior intestine were found to be prominent from first feeding to 45 dph; however, this phenomenon was no longer evident in fry and fingerling. Overall, the increase in intestinal folding and complexity of extramucosal layer were found to be continuous from the first appearance to adult, and this inferred the fact that the nutritional physiology, in terms of digestion and assimilation, progressively changes throughout the life stages of golden mahseer. Findings of this study will, therefore, help in preparing diets for different life stages of this fish, and in addition, the present information widens the understanding of digestive physiology of golden mahseer.     

Proteolytic Activity in California Halibut Larvae (Paralichthys californicus)

The digestive tract of many marine fish larvae undergoes numerous morphological and functional changes during ontogeny that can substantially influence larval survival under culture conditions. Increasing our knowledge of the digestive capacity and nutritional requirements of the larvae of new candidate species for aquaculture will aid in the development of optimal feeding protocols and greatly improve production under hatchery conditions. In this study, we assess the proteolytic capacity of California halibut (Paralichthys californicus) larvae using biochemical and histological analyses. Newly hatched larvae were reared in a semiclosed recirculating system and fed with highly unsaturated fatty acid (HUFA)–enriched rotifers from hatching until 19 d posthatch (dph) and HUFA‐enriched Artemia nauplii thereafter. Total and specific activity of trypsin and leucine‐aminopeptidase (LAP) and acid and alkaline protease activities were assessed throughout development using spectrophotometric techniques. Trypsin‐like activity and LAP and alkaline protease activities were detected shortly after hatching and before the opening of the mouth. Acid protease activity was not detected until 36–40 dph, concomitant with the development of the gastric glands. The specific activity of trypsin and LAP showed two distinct peaks at 8 and 20 dph. The second peak coincided with the shift from rotifers to Artemia. Hence, newly hatched California halibut larvae possess alkaline proteolytic activity before first feeding. Based on the digestive capacity evaluated in this study and the timing of the development of the functional stomach, we propose that California halibut can be adequately weaned to formulated microdiets around 36 dph.     

Ontogenetic development of digestive tract and digestive enzymatic activities in Squaliobarbus curriculus larvae

Squaliobarbus curriculus is an economically important freshwater fish. The ontogenetic development of the digestive system of S. curriculus larvae was studied histologically and enzymatically from hatching to 30 days posthatching (DPH). Amylase, lipase, alkaline phosphatase and pepsin activities were detected from the hatching stage, indicating that these enzymes were genetically preprogrammed. Marked increases in intestinal amylase, trypsin and alkaline phosphatase activities between 10 and 20 DPH corresponded to feed acquisition and transformation. Larval development in S. curriculus could be divided into three phases: phase I (endotrophic period): 1–3 DPH; phase II (endo‐exotrophic period): 4–5 DPH; and phase III (exclusively exotrophic period): from 6 DPH onward. At hatching, the digestive tract of the larvae was an undifferentiated straight tube. On 3 DPH, the digestive tract differentiated into the mouth cavity, oesophagus and intestine. On 6 DPH, feeding was totally exotrophic and the yolk sac was completely exhausted. During the growth of S. curriculus larvae, the intestinal mucosa formed and the number of goblet cells and microvilli increased, demonstrating maturation of the digestive system. The study about the digestive development of S. curriculus larvae will contribute to better larval‐rearing strategies.     

Precocious development of the digestive system in relation to early appearance of piscivory in striped bonito Sarda orientalis larvae

ABSTRACT: Striped bonito Sarda orientalis larvae and juveniles were reared in a laboratory from hatching to 14 days post hatching, and the development of their digestive system was investigated histologically. On day 2 (first feeding day), the larval-type digestive system was differentiated. On day 3, a blind sac with densely distributed gastric glands developed and pyloric ceca appeared, suggesting that the digestive system was attaining adult-type structure. The yolk was completely absorbed on day 4. Bonito larvae preyed on rotifers and/or Artemia nauplii on day 2, but almost the whole gut was occupied by fish larvae from day 4. In the striped bonito, the adult-type digestive system was established right after first feeding, and this precocious development was well synchronized with the appearance of piscivory.     

四指马鲅(Eleutheronema tetradactylum)消化系统胚后发育组织学观察

运用组织学切片技术研究四指马鲅(Eleutheronema tetradactylum)出膜后1-30 d(Day after hatching,DAH)仔鱼的消化系统形态和结构的变化.结果显示,在温度29-32℃、盐度25-28、溶氧4.80-5.5 mg/L、pH 8.0-8.3条件下,1 DAH仔鱼消化管尚未分化,由紧贴腹壁和卵黄囊的肠管组成.卵黄囊在3 DAH时完全吸收完毕,仔鱼开口摄食,消化管也与外界相通,此后逐渐分化出食道、胃部和肠道,一定程度上能自主消化吸收食物维持自身生长,逐渐从内源性营养期过渡为外源性营养期.15 DAH时出现幽门盲囊和胃腺,消化道结构和功能成分进一步发育完善,消化能力显著提高,30 DAH仔鱼消化道具备与成体相似的结构和功能.肝脏和胰腺分别在2 DAH和5 DAH时分化,随着仔鱼的生长而快速发育,至30 DAH时具备与成体相似的结构和功能.四指马鲅消化系统的发育表现为结构与功能密切联系且逐渐发育成熟的特点.本研究根据四指马鲅消化系统的发育特点讨论了育苗过程中的三大危险期并提出应对措施.     

Activities of digestive enzymes and histology of digestive system during larval development of devil stinger (Inimicus japonicus)

Devil stinger is a valuable demersal scorpaenid fish while the rearing of stinger larvae still relies on live prey. This study was conducted to illustrate the development of the main digestive enzymes and digestive system during larval development of this species to provide evidence for the application of artificial feeds. Enzymatic and histological assays were conducted from 1 day post hatching (dph) to 36 dph in larvae. The result showed that the selected digestive enzyme activities increased significantly after 15 dph. Specifically, the total trypsin activities increased significantly from 18 dph to 33 dph. The total pepsin and amylase activities increased significantly first and thereafter decreased significantly. The lipase activities followed the similar pattern with trypsin. With regard to the histological study, the stinger larvae open their mouth to first feeding at 3 dph and turned into totally exogenous nutritional stage at 6 dph. In addition, mucous membrane, rich in goblet cells, was widely distributed in oesophagus epithelium at 18 dph. The height and amounts of gastric gland in cardia and main body of the stomach increased gradually with the development of stinger larvae after 15 dph. The intestine length of stinger larvae was short, and goblet cell was abundant in anterior intestine after 12 dph, not the posterior intestine. The ontogeny of liver and pancreas started from newly hatched stage, and the differentiation of liver was prior to pancreas. The above findings would provide evidence for the use of artificial feeds from the larval stage of stinger larvae (at least from 21 dph).     

Histological study of the gastrointestinal tract in longfin yellowtail (<Emphasis Type="Italic">Seriola rivoliana</Emphasis>) larvae

This work contributes basic knowledge on larval development of Seriola rivoliana. A histological study describes the development of the digestive tract and accessory glands in S. rivoliana larvae reared under laboratory conditions at 24 °C from hatching to 30 days post-hatching (DPH). At hatching (2.6 ± 0.12 mm), larvae had an undifferentiated digestive tract with a closed straight tube and a large yolk sac with an oil globule. The liver and pancreas were observed at 1 and 2 days, and the mouth and anus opened at day 2. Enriched rotifers were visible in their digestive tract. At the beginning of the pre-flexion stage, a mixed nutritional period was observed. At day 3, exogenous feeding began; the digestive tract became differentiated into the buccopharynx, esophagus, an undifferentiated stomach, and the intestines. Zymogen granules were visible in the exocrine pancreas. At day 4, supranuclear vacuoles were present in the posterior intestine, indicating the beginning of intracellular digestion. At day 5, goblet cells were present in the esophagus and became functional at day 7 in the esophagus and intestine. The buccopharynx goblet cells developed at day 15. The presence of gastric glands and differentiation of the stomach in the fundic, cardiac, and pyloric regions during the post-flexion stage occurred at day 20. This was the onset of the juvenile period and the beginning of weaning; however, a long co-feeding phase is recommended. Pyloric caeca were observed at day 30 (13.6 ± 1.6 mm). These results provide valuable information on S. rivoliana larvae biology and digestive physiology, which should be useful to improve cultivation techniques and identify ecological features involved in ontogeny.     

Yolk utilization and growth during the early larval life of the Silver Perch,Bidyanus bidyanus (Mitchell, 1838)

The aim of this research was to investigate the yolk sac and oil globule utilization by silver perch (Bidyanus bidyanus) larvae produced from domesticated broodfish. The larvae were kept unfed in the holding tank, sampled, and investigated by image analysis software to determine various characteristics, such as the diameters of ova, water-hardened eggs, yolk-sac, oil globules, and the total length of larvae. The research illustrated that, with the exception of oil globule diameter, all other morphometric parameters were significantly lower (P < 0.05) when compared to the larvae from the wild broodfish. The yolk sac was completely absorbed at 96 h post-hatching (hph) and the oil globule was visible until 240 hph. The larvae exhibited predatory movements and tried to catch rotifer at 4 days post hatching (dph). However, the onset of feeding took place at 5 dph, while 100% of feeding occurred at 6 dph. During the first 96 h (h), larvae grew significantly faster than the next 144 h. Larvae encountered low mortalities (<10%) during the first 96 hph, before increasing significantly in the next 24 h and no unfed larvae survived post 240 h. The results also suggested that the exogenous feed should be available at 96 hph, which is well after the yolk sac is completely depleted. In addition, although most of eggs and larval performance from domesticated broodfish were inferior compared to the wild one, it has larger oil globule that could make longer of its mixed feeding period and therefore could have better in viability.     

Morphological development and allometric growth of yellowtail kingfish Seriola lalandi V. larvae under culture conditions

Morphological development and allometric growth patterns of Seriola lalandi larvae were assessed to characterize normal growth patterns under culture conditions. Early ontogenetic stages of yellowtail kingfish exhibited an exponential growth in terms of standard length as a function of age. Five development stages were characterized from hatching to the juvenile stage: larval stage I (0–2 days post hatch, dph) with endogenous feeding, characterized by a small yolk sac, unpigmented eyes, primordial finfold surrounding the body and a closed mouth; larval stage II (2–15 dph) characterized by mouth opening, complete pigmentation of eyes and the beginning of the exogenous feeding; subsequently, in the larval stage III (15–25 dph) the posterior tip of notochord of the larvae bended upward and the first rays appeared in fins, concomitant with a change in swimming behaviour; thereafter, larval stage IV (post‐flexion stage; 25–30 dph) began when larvae resembled in morphology to a juvenile organism; however, caudal and dorsal fins were not completely development. Lastly, the juvenile stage was reached 30 dph characterized by a morphology and fin structures similar to those of the adults. Growth and development of structures and organs associated with vital functions such as feeding, sensorial and breathing systems seemed to be more critical previous to 23 dph, which was reflected with a positive allometric growth of head and eyes during this period. The results from this study can be used as a tool‐guide to assess normal development in larval research with S. lalandi to improve existing rearing protocols in hatchery production.     

Effect of the lipid droplet adherence on growth and survival of the European hake (Merluccius merluccius) larvae

The effect of the adherence of the lipid droplet in the yolk‐sac on the growth and survival of the European hake (Merluccius merluccius) larvae was studied under starved and feeding conditions. Recently hatched hake larvae with adhered and non‐adhered lipid droplet (ALD and non‐ALD) were maintained during 8 days under starvation conditions. The adherence of the lipid droplet positively affected larval survival during starvation. At the end of the yolk consumption period (7–8 days after hatching), 90% of the surviving larvae had an ALD, while 10% of the survivors had a non‐ALD and showed a higher mortality rate. In a month feeding experiment, results show that the presence of non‐ALD in newly hatched hake larvae negatively affected their growth and survival. Dry weight and length (TL) of 20‐days‐old larvae with ALD were significantly higher than those with non‐ALD. Furthermore, lifespan of those larvae with non‐adhered lipid droplet was shorter, with most of them dying during the first 15–20 days of culture. As conclusion, the percentage of the lipid droplet adherence at hatching could be used as an indicator of ‘quality’ for European hake larvae: the greater the number of larvae with adhered lipid droplet, the better larval growth and survival.