オイカワ属魚類の摂餌に関する形態学的研究

抄録

The present report aims the comparative morphology of two cyprinid fishes, Zacco platypus (Temminck et Schlegel) and Z.temminckii (Temminck et Schlegel), particularly with reference to the relative growth of two parts of the mouth which may be closely related to the capture of food, the floor of oral cavity, the gill raker, the pharyngeal bone, and the intestine.Through the detailed comparative study of these organs, the authors could make success in making clear the significance of the structural adaptations of these organs in the two species in relation to their feeding behaviors.
The growth of two parts of mouth (length of upper jaw and width of lower jaw) was examined against changes of total length.Upon the relative growth method of analysis, it is clear that the sizes of these parts for Z.temminckii are obviously larger than those for Z.platypus.Therefore, it might be considered that Z.temminckii is capable of seizing larger food organisms.
The prominent fleshy folds, called the oral folds, are present on the floor of oral cavity.Development of these folds varies in its extent according to the species.In Z.platypus, the folds develop gradually and increase in their number with growth of the fish.When the fish reaches about 90 mm in total length, all of the folds are highly elevated above the surface of the floor.In Z.temminckii, they are scarcely developed throughout the life history excepting the first two which are highly elevated in the fish measuring more than 60 mm in total length.
The outer surfaces of oral folds are provided with prominent papillae.There are also some specific variations in the degree of development of these papillae.In Z.platypus, the first indication of formation of the papillae can be observed in the fish measuring about 45 mm in total length.With growth of the fish, they develop gradually and increase in their sizes.In Z.temminckii, the papillae appear on the outer surfaces of the first two folds when the fish exceeds 55 mm in total length, and increase gradually in their sizes.However, the rest of folds are furnished with minute papillae which are only sparsely set on their surfaces.It appears that the development of oral folds and their papillae is positively related to the herbivorous behavior of the fish.
The modal number of gill rakers on the first arch is 11 in Z.platypus and 10 in Z.temminckii.In the former species, the gill rakers are rather slender, tapering gradually towards the pointed tips.While in the latter, the distal ends of gill rakers on the ceratohyal bone are truncated or the knob-like appearances when the fish reaches about 120 mm in total length.
The dental formula of pharyngeal teeth is 1, 4, 4-4, 4, 2 in Z.platypus and 2, 4, 5-4, 4, 2 in Z.temminckii.From the statistical comparison of the growth of three parts of pharyngeal bone (length of bone, width of the same and length of the longest tooth) against changes of total length between Z.platypus and Z.temminckii, it is clear that the relative sizes of three parts for the latter species become obviously larger than those for the former when the fish exceeds about 45 mm in total length.
Intestines of the two species bear a close resemblance to each other in their forms in the fish measuring less than 35 mm in total length.However, owing to the occurence of somewhat complicated coiling, the intestine of Z.platypus becomes longer than that of Z.temminckii when the fish exceeds 35 mm in total length.It is presumed that the elongation of the intestine of the former species is a structural adaptation for assimilating minute adherent algae.
Considering from the facts described above, it may be deducible that the structures of feeding and digesting organs change ontogenetically in relation to the behavior of minute adherent algae feeding inkn-abstract=