  A marked strongly invertible knot is a triple $(K,h,\delta)$ of a knot $K$ in
$S^3$, a strong inversion $h$ of $K$, and a subarc $\delta \subset
\operatorname{Fix}(h)\cong S^1$ bounded by $\operatorname{Fix}(h)\cap K\cong
S^0$. An invariant Seifert surface for $(K,h,\delta)$ is an $h$-invariant
Seifert surface for $K$ that intersects $\operatorname{Fix}(h)$ in the arc
$\delta$. In this paper, we completely determine the equivariant genus (the
minimum of the genera of invariant Seifert surfaces for $(K,h,\delta)$) of
every marked strongly invertible knot $(K,h,\delta)$ with $K$ a $2$-bridge
knot.
