藻類食性コイ科魚類の形態学的, 生理学的研究- (II)

カワチブナ, ハクレンおよびニゴロブナの発育に伴う消化吸収率の変化

抄録

By using unialgal culture of the two species of green algae (Closterium moniliferum : 300μ in length and Selenastrum sp. 10μ) which were labeled with ¹⁴C, the assimilation efficiency in terms of carbon of three cyprinids, Kawachibuna (Carassius auratus cuvieri), Nigorobuna (C. auratus grandoculis) and Silver carp (Hypophthalmichthys molitrix) was studied at the various life stages. In addition, radioactivities of different fractions (oligo-and poly-saccharides, organic acids, lipides, protein and residue) of the both species of ¹⁴C-algae and of the fecal algae egested by Kawachibuna and Silver carp were measured.
In Kawachibuna and Nigorobuna, the assimilation efficiency for Closterium increased with their growth until they attained to 20 mm in total length (B. L. 14-15 mm), and reached the maximum level of more than 20 mm length (57-63%) at the successive stage. In Silver carp of 20 mm total length, the assimilation efficiency for Closterium was lower than for Kawachibuna and Nigorobuna of the same length. In Silver carp of more than 33 mm total length (B. L. 26-27 mm), 54-64% of carbon was assimilated.
Kawachibuna and Silver carp are already known to change their food items from zooplankton to phytoplankton when they grow up more than 15 and 24 mm in body length respectively. The body length at which the maximum values of assimilation efficiency were obtained coincides approximately with the body length at which the change in diet occurs. It is suggested that the increase in the assimilation efficiency for Closterium with the growth of the fishes is not directly correlated with the increase in the length of their intestines but with a functional differentiation of their digestive systems.
The three cyprinids assimilated 12-16% of carbon of Selenastrum and the rate was fairly stable throughout their lives, in contrast with those for Closterium. No difference was found in photosynthetic activity between fecal algae of Selenastrum egested by young Kawachibuna and fresh ones, while the activity of fecal algae of Closterium was approximately one-fifth of the fresh ones. The difference in assimilation efficiency may be related partly to the physiology of digestion of the cyprinids and partly to differences in chemical nature and cell structure of the two species of algae.