| Yellow catfish (Pelteobagrus fulvidraco) is an important commercial freshwater species in China. It has a promising market potential in China, Japan, South Korea, East and South Asia. Due to its high market value, the culture of this species has increased rapidly in recent years. However, larvae rearing became a major bottleneck because of its high mortality which caused by uncorrected culture feeding strategies simply derived from the traditional carp culture. Little is known on the early life stages of P. fulvidraco, especially on their morphological and internal development relating to functional capabilities. To date, no study has been documented on ontogeny of the digestive system in P. fulvidraco, thus correct feeding strategy related to morphological development is unknown.In order to enhance the success of larvae rearing of P. fulvidraco, we need to know the ontogeny of its digestive system thoroughly and the feeding habits. The purpose of this study was to understand the morphological structure of digestive tract and the its feeding habits during the ontogeny of P. fulvidraco. We hope that this information would provide fundamental knowledge for larvae rearing management for this species. The main results are shown as follows:1 The development morphological characteristics of feeding and digestive organs of larvae of P. fulvidraco were studied. Eye diameter, lengths of head, maxillar barbells, mandibular outer barbells and gut increased with the development of fish larvae. Ratio of eye diameter to body length reached the maximum at 16 DAH, and then gradually reduced. Ratio of length of maxillar barbells, mandibular outer barbells to body length increased rapidly to 17 DAH, and then continue to increase with slowing down growth. Jaw teeth appeared at 4 DAH. The number increases with age, arranged 2 lines in upper maxillar and 1 line in mandibular. Gill raker appeared at 6 DAH, and the morphological structure similar to adult fish gill rakers at 20 DAH. It is concluded that the visual play an important role in preying at first then the tentacles were more important when larvae converted to benthic.2 Development of the digestive tract in P. fulvidraco followed the general pattern described for other fish species with some peculiar findings. At hatching, it consisted of an undifferentiated straight tube laying over the yolk sac. The digestive tract was differentiated into buccopharynx, esophagus, primary stomach and intestine by 2 DAH. The liver and pancreas were also appeared at this time. The intestine became differentiated into anterior and posterior regions separated by the intestine bend at 3 DAH. Gastric gland appeared in cardiac stomach at 3 DAH, the earliest appearance time among fishes studied to date. Oxynticopeptic cell contained pepsinogenic granules and abundant tubulovesicular systems at 3 DAH. As larvae grew, more pepsinogenic granules but less tubulovesicular systems were found in oxynticopeptic cell. The abundant visible tubulovesicular systems suggested that oxynticopeptic cell was still in rest phase with little hydrogen chloride (HCl) secreted at the first appearance time. The ultrastructure of oxynticopeptic cell indicated the asynchronous development of acid-secreting and pepsinogen-secreting function. The epithelial absorptive cell of the anterior and posterior intestinal segment showed electron-opaque lipid droplets and heavy pinocytosis, respectively at 3 DAH. Heavy pinocytosis could be observed in the posterior intestine until 25 DAH. Lipid vacuoles accumulation appeared in liver at 13 DAH, the same time as the storage of abundant glycogen. These results suggested the development of the digestive tract of P. fulvidraco larvae was functional rapidly, however it was still incomplete at 3 DAH. The functions of digestive tract and accessory glands were developed gradually until 25 DAH.3 The retinal structure and the density of three visual cells of P. fulvidraco larvae at 0-30 DAH were examined histological during different development stages. The retinal of the newly hatched larvae has only single cone as sensory cell. Over the whole range of development the density of cones and ganglion cells decreases while the density of the rods increases correspondingly. The forms of various layers of retina gradually develop to be perfect. The data concerned shows that the structure of the retina changes apparently in the age between 11 DAH and 13 DAH, which is a transitional period in which its visual characteristics changes obviously. It is revealed that the changes of visual structure of P. fulvidraco are adapted to the ecological shift from pelagic to benthic habitats.4 The circadian feeding rhythm for larvae culturing in pond, and juvenile at different feeding frequencies was investigated. Apparent day and night feeding rhythm is observed in larvae which culturing in pond. The feed intake rate at night was higher obviously than that at day through all the experiment stage. Similar results were found in juvenile. That is, there is typical nocturnal feeding activity with the highest levels of feeding activity at 20:00 pm and the lowest levels at 8:00 am in the condition of over-feeding 8 times per 24 hour. There are different significantly feeding rate between different feeding time (P<0.01). The similar circadian feeding rhythm occurred in the condition of over-feeding only 1 time in different time per 24h. However, there is no different feeding rate between different feeding time in latter feeding condition (P>0.05). Results suggest the circadian feeding rhythm of larvae and juvenile of P. fulvidraco belong to nocturnal.5 The food composition of P. fulvidraco larvae which culturing in pond were studied at different age. During initial bait phase, the main food were zooplankton, mainly rotifers, small Clandocerans and nauplii. The proportion of large Clandocerans gradually increased with age. Zooplankton were still main food according to number percentage of prey at 13 DAH. However the zoobenthos such as Chironomus were more than zooplankton in terms of weight percentage of prey. The main food changed from zooplankton to zoobenthos after 21 DAH.6 Effect of feeding frequency and different food items on growth and survival rate of larvae and juvenile of P. fulvidraco were studied. Trial 1 were conducted to investigate the effect of different feeding frequency (satiation feeding 1 (F1),2 (F2) or 3 (F3) times/day) of zooplankton on the growth and survival. Trial 2 were conducted to investigate the effect of different food items (feeding zooplanktons (groupâ… ), feeding zoobenthos (groupâ…¡) or feeding zooplanktons and zoobenthoes together (groupâ…¢)) on the growth and survival. The results showed that:â‘ During 3-29 days after hatching, with increasing feeding frequency, specific growth rate of total length and wet weight increased significantly in linear style (SGR1=1.0366D+3.2347 R2=0.9025; SGRw=3.0013D+9.4829 R2=0.8833).â‘¡During 17-25 DAH, the growth of groupâ…¡was slower than groupâ… and groupâ…¢. The growth of groupâ…¡was still slower than groupâ…¢, but there was no different between groupâ…¡and groupâ… up to 28 DAH. In 34 DAH, there were different among the growth of 3 groups. The growth of groupâ…¢was the fastest, followed by groupâ…¡and groupâ… .â‘¢There were no survival rate difference among group of feeding different food items during 3-34 DAH. It can be concluded that it is more useful that satiation feeding 3 times/day than 1 or 2 times/day, and feeding zoobenthoes is better than feeding zooplankton after 28 DAH.7 The "point of no return" (PNR) of P. fulvidraco larvae were investigated. The result indicated that feeding intensity is more suitable than feeding rate to calculate the PNR for P. fulvidraco larvae. A method of calculating the PNR via the feeding intensity index as a complement to traditional methods was presented. Under the experimental conditions, the PNR of P. fulvidraco larvae appeared at 11 DAH.Effect of prey density on growth of P. fulvidraco larvae was also studied.7 groups of different prey density (0,0.1,0.3,0.5,0.7,1.5,3.0 preys per millilitre) were arranged. There are different optimal prey density at different growth stage of larvae. The optimal prey density was 0.7 preys per millilitre before 8 DAH. While 0.5-1.5 preys per millilitre were the optimal prey density between 8 DAH and 17 DAH. The use of more than 1.5 preys per millilitre of live prey density had negative effects on larval growth.According to the development characters of P. fulvidraco larvae, a series of scientific methods to improve the surviving rate were suggested. |
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