I agree with Kate that we should not assume species are NOT rare simply
because we have not sampled enough of the possible habitat to verify that
assumption.  However, for exactly the same reason, we cannot assume that any
deep water marine species IS truly rare.  The miniscule portion of the ocean
floor that has been sampled to date doesn't allow us to make any valid
generalizations at all about the relative scarcity of a species in nature.
We can compare the frequency of various species in the samples we have
obtained; but this may or may not have any relevance whatsoever to the actual
abundance in nature.  Kate stated that Bartram found what is apparently the
only grove of a certain tree remaining on earth.  That claim can be made with
a reasonable level of confidence ONLY because most of the possible habitat
for such trees has previously been explored.  If less than 1% of the
available habitat had been studied, then it would be foolish to speculate
that this was the only grove of trees.  Indeed, it would be highly probable
that additional groves existed.  In the same way, the specimens we have
obtained from the few tiny scratches we have made on the ocean floor simply
do not constitute a sufficient random sampling to allow us to generate any
meaningful statistics on population densities.  We can say which species are
rare in samples to date, and we can say which species are rare in
collections; but there is not a single deep water marine species which we can
say with certainty is rare in nature.  History has repeatedly shown that the
"rare" species of yesterday become the common species of today.  Granted, it
is possible that a particular species is currently on the verge of natural
extinction.  But, for any given species, that situation is relatively
unlikely.  The more likely explanation is always that we have not yet sampled
the area where the principle breeding populations live.
Paul M.