A new species,
Tharyx vivipara is described, together with details of its viviparous early development. This is the first record for the genus of a simultaneous hermaphrodite with viviparous development. The species inhabits silty sand and soft silt in estuaries at densities over 400 per m
2.The gross morphology of this species superficially resembles that of
Chaetozone setosa Malmgren, 1867 but the diagnostic features of the latter species, namely acicular setae, are lacking.
T. vivipara differs from other members of the genus in the presence of a small, acutely pointed prostomium without eyes, short capillary setae of equal length in both notopodia and neuropodia, branchial scars present on every setiger, and its small body size (maximum length 15 mm). In addition, its reproductive biology is markedly different from that known for any other member of the genus. Oogenesis and spermatogenesis occur simultaneously and fertilized eggs undergo development within the parent’s coelom until the embryos have differentiated the tentacular palps and have aquired 3–4 setigers.
