  We present submillimetre data for the L1689 cloud in the rho-Ophiuchi
molecular cloud complex. We detect a number of starless and prestellar cores
and protostellar envelopes. We also detect a number of filaments for the first
time in the submillimetre continuum that are parallel both to each other, and
to filaments observed in the neighbouring L1688 cloud. These filaments are also
seen in the 13CO observations of L1689. The filaments contain all of the
star-formation activity in the cloud. L1689 lies next to the well studied L1688
cloud that contains the rho Oph-A core. L1688 has a much more active
star-formation history than L1689 despite their apparent similarity in 13CO
data. Hence we label L1689 as the dog that didn't bark. We endeavour to explain
this apparent anomaly by comparing the total mass of each cloud that is
currently in the form of dense material such as prestellar cores. We note
firstly that L1688 is more massive than L1689, but we also find that when
normalised to the total mass of each cloud, the L1689 cloud has a much lower
percentage of mass in dense cores than L1688. We attribute this to the
hypothesis of Loren (1989) that the star formation in the rho-Ophiuchi complex
is being affected and probably dominated by the external influence of the
nearby Upper Scorpius OB association and predominantly by sigma-Sco. L1689 is
further from sigma-Sco and is therefore less active. The influence of sigma-Sco
appears nonetheless to have created the filaments that we observe in L1689.
Accepted by MNRAS.
