  Let $\star$ be a semistar operation on a domain $D$, $\star_f$ the
finite-type semistar operation associated to $\star$, and $D$ a Pr\"ufer
$\star$-multiplication domain (P$\star$MD). For the special case of a Pr\"ufer
domain (where $\star$ is equal to the identity semistar operation), we show
that a nonzero prime $P$ of $D$ is sharp, that is, that $D_P \nsupseteq \bigcap
D_M$, where the intersection is taken over the maximal ideals $M$ of $D$ that
do not contain $P$, if and only if two closely related spectral semistar
operations on $D$ differ. We then give an appropriate definition of
$\star_f$-sharpness for an arbitrary P$\star$MD $D$ and show that a nonzero
prime $P$ of $D$ is $\star_f$-sharp if and only if its extension to the
$\star$-Nagata ring of $D$ is sharp. Calling a P$\star$MD $\star_f$-sharp
($\star_f$-doublesharp) if each maximal (prime) $\star_f$-ideal of $D$ is
sharp, we also prove that such a $D$ is $\star_f$-doublesharp if and only if
each $(\star, t)$-linked overring of $D$ is $\star_f$-sharp.
