8. Let $F(x, y)=(x-y)^{2}+\left(\frac{x}{3}+\frac{3}{y}\right)^{2}(y \neq 0)$. Then the minimum value of $F(x, y)$ is ( ).
(A) $\frac{12}{5}$
(B) $\frac{16}{5}$
(C) $\frac{18}{5}$
(D) 4
8. C.

Let the moving points be $P\left(x,-\frac{x}{3}\right)$ and $Q\left(y, \frac{3}{y}\right)$. Then $F(x, y)=|P Q|^{2}$.

Thus, the trajectory of point $P$ is the line $y=-\frac{x}{3}$, and the trajectory of point $Q$ is the hyperbola $y=\frac{3}{x}$. The distance from any point $\left(x_{0}, \frac{3}{x_{0}}\right)$ on the hyperbola to the line $x+3 y=0$ is
$$
d=\frac{\left|x_{0}+3 \times \frac{3}{x_{0}}\right|}{\sqrt{10}} \geqslant \frac{6}{\sqrt{10}} .
$$

When $x_{0}= \pm 3$, the equality holds.
Therefore, the minimum value of $F(x, y)$ is $\frac{18}{5}$.