  We present Hubble Space Telescope imaging of 14 gas-rich, low surface
brightness galaxies in the field at distances of 25-36 Mpc, with mean effective
radii and $g$-band central surface brightnesses of 1.9 kpc and 24.2 mag
arcsec$^{-2}$. Nine meet the standard criteria to be considered ultra-diffuse
galaxies (UDGs). An inspection of point-like sources brighter than the turnover
magnitude of the globular cluster luminosity function and within twice the
half-light radii of each galaxy reveals that, unlike those in denser
environments, gas-rich, field UDGs host very few old globular clusters (GCs).
Most of the targets (nine) have zero candidate GCs, with the remainder having
one or two candidates each. These findings are broadly consistent with
expectations for normal dwarf galaxies of similar stellar mass. This rules out
gas-rich, field UDGs as potential progenitors of the GC-rich UDGs that are
typically found in galaxy clusters. However, some in galaxy groups may be
directly accreted from the field. In line with other recent results, this
strongly suggests that there must be at least two distinct formation pathways
for UDGs, and that this sub-population is simply an extreme low surface
brightness extension of the underlying dwarf galaxy population. The root cause
of their diffuse stellar distributions remains unclear, but the formation
mechanism appears to only impact the distribution of stars (and potentially
dark matter), without strongly impacting the distribution of neutral gas, the
overall stellar mass, or the number of GCs.
