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Isolated HI clouds with no optical counterparts are often taken as evidencefor galaxy-galaxy interactions, though an alternative hypothesis is that theseare primordial 'dark galaxies' which have not formed stars. Similarly, certainkinematic features in HI streams are also controversial, sometimes taken asevidence of dark galaxies but also perhaps explicable as the result ofharassment. We numerically model the passage of a galaxy through thegravitational field of cluster. The galaxy consists of SPH particles for thegas and n-bodies for the stars and dark matter, while the cluster includes thegravitational effects of substructure using 400 subhalos (the effects of theintracluster medium are ignored). We find that harassment can indeed producelong HI streams and these streams can include kinematic features resemblingdark galaxy candidates such as VIRGOHI21. We also show that apparent cloudswith diameter < 20 kpc and velocity widths < 50 km/s are almost invariablyproduced in these simulations, making tidal debris a highly probableexplanation. In contrast, we show that the frequency of isolated clouds of thesame size but velocity width > 100 km/s is negligible - making this a veryunlikely explanation for the observed clouds in the Virgo cluster with theseproperties.

Kinematic clues to the origins of starless HI clouds : dark galaxies or tidal debris ?