The so-called gizzard in mugilid fishes is a pyloric portion of the stomach, which is thickly muscularized, and adapted to the detritus feeding.To clarify the critical stage in the development from larval to juvenile, which is often found corresponding to the change of feeding habit as well as the development of digestive organs, the menada mullet,
Liza haematocheila (Temminck and Schlegel), was tested for the understanding of the problem.Samples, selected in series from the batch hatched and cultured artificially in laboratory, were studied anatomically and histologically, with special attention to the development of gizzard.
The differentiation of the organs in digestive system can be categorized into 10 (I-X) stages, each characterized by the type of intestinal convolution taking place in the larvae to juveniles, 3-30 mm in total length or during about 2 months after hatching.The development of the gizzard and other organs in the system-lver, gall-bladder, pancreas, stomach, pharyngeal pad, etc.are clearly distinguished by the 10 stages as to their development and differentiation. The blind sac shows its anticipative body at the junction between oesophagus and intestine when larvae grow to 6.2 mm in length and intestinal convolution falling on IV to V stages, and gastric gland appears beneath the epithelium of anticipative blind sac.Upon the establishment of the blind sac, the gizzard begins to develop at pyloric portion in the larvae 9.0 mm long with intestinal convolution of V stage, and the organ establishes in VI stage, and they change habit from pelagic to benthic swimming during the development.With the growth in size of the gizzard, the blind sac dwindles gradually until the former fully establishes its shape in the juveniles 20 mm long.The occurence of gastric gland in cardiac portion and blind sac of stomach (instead of pyloric portion) was confirmed in
Liza haematocheila and also in other mugilids,
Liza carinata (Valensiennes) and
Mugil cephalus Linnaeus.
It is well known that the
L.haematocheila brings out extremey high yield of seedling (fry) under artificial breeding as compared with other fishes such as
Seriola quinqueradiata Temminck and Schlegel, Pagrus major (Temminck and Schlegel), and
Plecoglossus altivelis Temminck and Schlegel, which are also bred under nearly the same process for the mullet. This may be explained by the reason that the larvae of the mullet in early stages are already provided with digestive function of high fidelity which is derived from early differentiation of the gastric gland, early growth of the strong pharyngeal organ, and faster development of the gizzard.
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