テラピアの鰓換水

Ventilation in the tilapia, Oreochromis niloticus

Creators Yamamoto Ken-ichi

Creators Handa Takeshi

Creators Yokota Motohiro

Creators Takahashi Kouji

In the tilapia, Oreochromis niloticus (body weight 1,100g), the ventilation properties under normoxic and hypoxic conditions were examined by continuous direct measurement of the ventilation volume, the hydraulic pressure in the buccal and opercular cavities, and the difference of the hydraulic pressure in between two cavities. The flow velocity and the mean flow passage areas at the mouth, the gill slit, and among the secondary lamellae were calculated using the measured values. And the reliability of the equation in search of the velocities was examined hy directly measuring the velocity at the gill slit, with the fiber-optic laser velocimeter. Under severe hypoxic condition, the ventilation volume increased at about 11 times that under normoxic from 148ml//min/kg to 1,615ml/min/kg. The mean flow velocities at the mouth and among the secondary lamellae increased at about 2 times those under normoxic condition from 13.1cm/sec to 26.5cm/sec and from 16.3cm/sec to 30.8cm/sec, respectively. However, the velocity at the gill slit was almost constant in 24.6-27.2cm/sec. The mean flow passage areas under severe hypoxic condition increased severalfold those under normoxic condition : 5 times at the mouth (from 0.35cm^2 to 1.69cm^2 ), 6 times among the secondary lamellae (from 0.16cm^2 to 0.88cm^2 ), 13times at the gill slit (from 0.24cm^2 to 3.13cm-^2). From the relational expression of the mean flow passage area and the mean flow velocity, the hydraulic pressures of opening were estimated to be-0.8mmH_2O for buccal valve and 3.1mmH_2O for opercular valve. And the hydraulic pressure in which the water begins to flow in respect of the secondary lamella was guessed to be at 0.8mmH_2O. The calculated velocity values were almost equal to those measured with the fiber-optic laser velocimeter, and the stroke ventilation volume calculated with the relational expression closely agreed with that measured directly. These results highly prove that the values calculated using the relational expression were considerably reliable.

[ISSN]0370-9361