Distribution & density
Hediste diversicolor is an euryhaline species and can withstand great variances in salinity. Smith (1956) reported that, in the Tamar estuary, England, individuals of this species living at the upstream limit regularly experience salinities less than 0.5 ppt. In marine dominated habitats, Hediste diversicolor behaves as a brackish water animal and is found in the least saline portion of the available ground (Smith, 1956). The distribution of Hediste diversicolor in high salinity areas is likely to be reduced as result of competition in the form of interspecific aggressions (Kristensen, 1988). In a study focussing on the distribution of nereid polychaetes in Danish coastal waters, Kristensen (1988) found that Hediste diversicolor could only maintain high population densities in marginal environments when the fitness of stronger competitors such as %Nereis virens% was reduced.

In estuaries the maximum density of the Hediste diversicolor population normally occurs in the middle regions, with density decreasing both towards the head and mouth of the estuary. Smith (1956), found that the maximum population density of Hediste diversicolor in the Tamar estuary corresponded to that portion of the estuary with the greatest salinity variation. The density of worms varies between locations and throughout the reproductive cycle. Numbers of juveniles may be over 100 000 per m² (Clay, 1967(c)). In the Ythan Estuary, Scotland, the density of adult Hediste diversicolor was reported to be 961 per m² (Chambers & Milne, 1975).