Studies on the development and relative growth in the carp, Cyprinus carpio (Linne)

Abstract

The main object of the present investigations was to work out on the problems in relation to the culture and the propagation of the carp. From this point of view a detailed study of the spawning behaviour, embryology, larval growth, digestive system, and other organs of the carp reared in the experimental pond of Fisheries Laboratory, Kyushu University, was made on the basis of microscopic preparations. The following distinctive remarks would be noteworthy. 1. The carp spawns on vegetations or like substances at an optimum water temperature of 22-24℃ for nearly 6 hours at any time of the day, but usually at dawn from April to July at short and irregular intervals. During the spawning act the male nudges the female from behind and approximates the genital part, lying almost parallel to her side. Both lash the caudal region forcibly, discharging simultaneously the ova and milt, and then separate after swimming together for about 30-50 cm. The total number of eggs in a spawning female, 590 mm in total length and 4.9 kg in weight, is nearly 765,000, which is a considerably a high number for the production of fry. 2. In the embryonic and the larval stages, the blood vessels form a network on the yolk sac and fin folds, and are representations of well developed respiratory devices as an adaptation to less favourable conditions. Large secretory cells are marked around the head and the yolk sac of the embryo and the larvae, and last till the yolk absorption. These seem to aid the larvae in sticking to the substratum, which is a natural device of safety against driftng by the current of stagnant water. The first sign of the rudiment of the gut is marked as an endodermal thickening by about 40 hours after fertlization, which assumes a cylindrical form of double layered cells by 70 hours (18 hours before hatching). By 78 hours after hatching a lumen is formed by the retreat of cells rather than the evagination as in other vertebrates (Fig. 5P, R, S, T). The mouth is formed at about 8-10 hours before hatching (Fig. 5 U, V). The liver is formed on the latero-ventral sides of the gut (Fig. 5 T). Pharyngeal folds are marked at 41-45 hours after fertilization as two dorso-lateral endodermal outgrowths which proceed to meet an inwardly pushed layer of ectoderm to form gill slits by 56-57 hours after fertilzation (Fig. 5 Q). Pectoral fins begin their formation at about 55 hours after fertilization and the median fin fold is prominently visible by 70 hours. 3. The period of incubation varies with the temperature (Table 2) and at 13-17℃ the larvae hatch in 90 hours after incubation and measure 3.9-5.6 mm. Hatching is completed wthin 2.5-3.5 days. The yolk sac is completely absorbed in 8 mm. All the fins are marked in 10 mm. The posterior end of the notochord is begining to turn upward in 7.5-7.7 mm larvae. The heterocercal caudal fin starts its formation at 7.5-7.7 mm. Hypurals number 7 (Fig. 10). Scales first appear behind the opercle at 17-18 mm, and cover the entire body surface of the carp by 24-30 mm. Young carp of this length resembles the adult in appearance (Figs. 6, 13). Liver cells start arranging themselves into lobular forms in 4.2 mm larva within 10 hours after hatching. The pancreas is differentiated along the left dorsal side of the intestine as a deeply stained mass of cells among the liver cells by 5.3 mm larva. The gall bladder is marked at this stage (Fig. 7 F, H). The hepato-pancreas assumes the young form by 9.6-10.5 mm. The anlage of the air bladder is marked a few hours before hatching (Fig. 5 W) and becomes to be filled with air by 6 mm (Fig. 7 C) and pushes the first part of intestine. The anterior one is formed in about 10 mm larva. The newly hatched larva begins its active life after the absorption of yolk (i. e., 7-8 mm larva). Active preying habit on the zooplankton is quite prominent by this stage. Schooling behaviour, though, beginning at 9.5 mm, at this stage or even after is not so significant. They start feeding 24-36 hours after hatching. The juvenile and the young prefer to eat at short intervals, which seems to be advantageous for the absence of stomach. The power of discriminating eatable substances appears to be intense. Weberian ossicles are marked by 10.6 mm joining the ear bones anteriorly and the air bladder posteriorly. The longitudinal grooves and fenestrations are formed by 17-18 mm (Fig. 11). 4. The club cells appear at about 12mm larva on the head skin between the upper lip and the nasal cavity and occasionally multinucleate giant cells are marked more along the dorsal side of the head than the ventral one (Fig. 16 E). By 6 mm the dorsal skin of the head shows wrinkles which are the cytoplasmic extensions of the cells. They seem to disappear at 12-13 mm, which is the time of the appearance of club cells at that position. Taste buds first appear on the head skin (between the upper lip and the nasal cavity) at 18 mm. Occasionally, promucous cells like those of the buccal epithelium are observed from the head skin epithelium. A girdle of elongated cells intercepted by collagenous fibres hanging down into the dermis from the basement membrane is observed: this has not been reportd before (Fig. 16 F, G). Mandibular barbels first appear at 18.5 mm in total length and measure about 217 μ, while the maxlilary ones are noticed at 22 mm which is the stage of the appearance of the taste buds on the barbels. Mucous cells and club cells are evidenced abundantly in the proximal or post-proximal regions of the barbels. The epidermis enveloping the anterior side of the nasal flaps is thicker than that of the posterior one. Taste buds are marked from the distal anterior end. Club cells are absent from the middle and the distal region of the flaps (Fig. 19 A-D). The outer epidermis of the operculum appears to be thicker than the inner one and a few taste buds are noted from the outer side only. Conjunctiva of the eye and the caudal fin also possess taste organs in their epidermis (Fig. 20 AF). Their presence on all these organs are suggestive of their wide adaptability to environments. 5. Lips of the young and the adult are provided with low ridges and shallow furrows (Fig. 22) which appear to be helpful during the feeding activity from the bottom. Horny type of dentitions resting upon the membranous structure are marked on the inner posterior side of the lips of the larvae, juveniles and the young ones; these may help in holding the food materials (Fig. 27 C, D). The maxillary valve is marked with the opening of the mouth in the embryonic stage. It is a small, almost cresentic membranous structure hanging freely into the buccal cavity behind the upper lip. In 6.9-6.8 mm larvae the mucosa of the buccal cavity (roof) is marked to enter as a thin Iayer into the proximal of the maxillary valve. Mucous cells are greater and larger on the ventral sides of the valve than the dorsal one. One to 3 typical taste buds born on papillae are recorded only from the ventral surface of the valve, which come in direct touch with food substances (Fig. 27 C, E-G). A few prominent papillary folds marked in the mouth cavity as early as 9-10 mm become regularly to be arranged by 15 mm in total length and 3-4 longitudinal folds are marked from the centre of the buccal cavity (roof). The roof is concave and the floor is slightly convex so as to fit nicely during the food taking. Papillae on the pharyngeal roof are more abundant than those of the buccal one and they decrease posteriorly; on the Aoor, they reach tiI1 the fourth gill arch (Fig. 23 A-F). Mucous cells are almost uniformly distributed in the buccal cavity. Promucous cells and club cells are occasionally marked (Fig. 27 M). Three types of taste buds are recognized from the epithelium of the cavity: buds with truncated, protruding, and depressed apices. Besides lymphocytes, vacuoles or oval cells are occasionally found towards the middle or distal ends of the buds (Fig. 29 E-J). Five developmental stages of the mouth are marked (Fig. 21) in this study which are related to the change of feeding habits. Pharyngeal teeth are apparent in 6.8 mm in total length. In the larvae and the juveniles they are arranged asymmetrically in 3-4 rows totalling 7-8 on each arch. They are curved, serrated, and backwardly directed, which befits the carnivorous feeding habits. They hold the prey tightly and transfer them to the oesophageal cavity. The flatness on the crowns of teeth on the first and second row become apparent by 28 mm (Fig. 24 F), and furrows are established by 45-50 mm in total length. This change of teeth formation is in correlation to the change of food habits from early carnivorous to the omnivorous one. The relative length and the width of the pharyngel arch appears to be greater (i.e., 3-4.26) till about 30 mm in total length after which it nearly becomes on an average of 2.87 (2.2-3.46) by 45-50 mm. The higher value is a characteristic of the carnivores and the lower one of the herbivores (Fig. 25). The horny pad is biconvex and almost four cornered (Fig. 26 C). In the larval stage it is a soft structure but considerable hardness is felt by 28 mm, this becoming quite apparent by 45-50 mm in total length. This appears to be in relation to the development of furrows or grooves on the crowns of the pharyngeal teeth. Hardness of the pad is correlated with more herbivorous habits than carnivorous ones. Well developed taste buds are recognized on the palatal organs and in the tissues of the pharyngeal teeth but the finding of the same in the middle layer of the horny pad seems to be of special significance in the gustatory sensitiveness during the act of feeding (Fig. 26 B, E). Mucous cells are absent from the lips. These cells and the taste buds appear to be related to each other in their distribution in the pharyngeal and oesophageal cavities. The former increases posteriorly and the latter vice versa. The anterior part of the oesophagus is provided with more mucous