Unknown, but see Range for known populations. At most reported locations in Australia, H. abdominalis appears to be rare or scarce. Mean peak densities in the Derwent Estuary, Tasmania were 0.12–1.11 individuals per 100 m² in 2000–2002, but subsequently declined significantly. In Tasmanian macroalgal (Ecklonia) habitats, densities of fewer than one individual per 500 m² are generally recorded (K. Martin-Smith, pers. comm.). Numbers from trawls (NZ) suggest that on soft bottoms, the species may be widespread if scarce, but further documentation is needed.

Exceptions to sparse populations are aggregations on some artificial structures and one documented report of large numbers aggregated on rafting seagrass (J. Manna, pers. comm.)

Unknown