  Isolated HI clouds with no optical counterparts are often taken as evidence
for galaxy-galaxy interactions, though an alternative hypothesis is that these
are primordial 'dark galaxies' which have not formed stars. Similarly, certain
kinematic features in HI streams are also controversial, sometimes taken as
evidence of dark galaxies but also perhaps explicable as the result of
harassment. We numerically model the passage of a galaxy through the
gravitational field of cluster. The galaxy consists of SPH particles for the
gas and n-bodies for the stars and dark matter, while the cluster includes the
gravitational effects of substructure using 400 subhalos (the effects of the
intracluster medium are ignored). We find that harassment can indeed produce
long HI streams and these streams can include kinematic features resembling
dark galaxy candidates such as VIRGOHI21. We also show that apparent clouds
with diameter < 20 kpc and velocity widths < 50 km/s are almost invariably
produced in these simulations, making tidal debris a highly probable
explanation. In contrast, we show that the frequency of isolated clouds of the
same size but velocity width > 100 km/s is negligible - making this a very
unlikely explanation for the observed clouds in the Virgo cluster with these
properties.
