  The flow of the low energy eigenstates of a $U_q[sl(2|1)]$ superspin chain
with alternating fundamental ($3$) and dual ($\bar{3}$) representations is
studied as function of a twist angle determining the boundary conditions. The
finite size spectrum is characterized in terms of scaling dimensions and quasi
momenta representing the two families of commuting transfer matrices for the
model which are even and odd under the interchange $3\leftrightarrow \bar{3}$,
respectively. Based on the extrapolation of our finite size data we find that
under a variation of the boundary conditions from periodic to antiperiodic for
the fermionic degrees of freedom levels from the continuous part of the
spectrum flow into discrete levels and vice versa.
