Cultivation of the common ragworm—Hediste diversicolor—has attracted a great deal of interest in recent years. Growth optimization is a key aspect for its intensive production. We have assessed the effect of body weight (Bw) and temperature (T) on growth-related parameters in common ragworm using correlation and multiple regression analyses. We used ragworms of 13 different weight classes in 15-day growing assays at 12 different temperature values. These polychaetes were stocked at a density of 1000 individuals m−2, and fed with commercial fish feed. Our results show that growth increases with T; when expressed as an absolute growth rate (AGR), growth increases as Bw increases; and when expressed as a specific growth rate (SGR), growth decreases as Bw increases. A change in the growth pattern was observed from an individual Bw of about 400 mg. Simulations performed with the equations that provided the best fit revealed that optimum T for growth changes with Bw, so that in individuals below 400 mg, optimum T is 24.9 °C, and above this temperature growth decreases. In individuals above 400 mg, growth increases slightly with temperature, but as weight increases, the effect of temperature on growth is less and less, and from a weight of 1050 mg, growth decreases as temperature increases. Mortality increases significantly at temperatures above 22 °C, especially in individuals with a Bw above 400 mg. Simulations of individual growth showed that up to 400 mg growth is quite fast at warmer temperatures, but from 400 mg to 1000 mg, the influence of T on growth rate is not significantly relevant in operational terms. This study demonstrated the huge usefulness of growth modelling for production planning.

The polychaete Hediste diversicolor is a suitable species for industrial aquaculture; however, cost-effective culture techniques need to be developed for its intensive production. The aim of this study is to evaluate the effects of worm density and substrate height and their interaction, as well as feeding frequency, on the rearing performance of H. diversicolor. Two trials were conducted. In trial 1, the effects of two substrate heights—6 and 12 cm—and two rearing densities—1000 and 4000 individuals m−2—were assessed in terms of worm growth and biomass production. Worm initial wet weight was 48 mg, and specimens were fed with commercial fish feed during a 70-day assay. The results show no interaction between rearing density and substrate height, and confirm density as a key factor in growth; however, a density of 4000 individuals m−2 results in a significant increase in production (final biomass three times higher for the highest rearing density) without affecting survival. In trial 2, the effect of three levels of feeding frequency—seven days a week; three times a week; and once a week—on growth in individuals of three weight classes—small (25–50 mg); medium (100–150 mg); large (250–350 mg)—was evaluated in a 15-day growing assay. Feeding frequency showed a major influence on the smallest size class, with the best growth indicators obtained at the highest feeding frequency. This study shows that Hediste diversicolor can be reared at a high stocking density to obtain a higher biomass production, and that feeding frequency must be considered as an important factor and adapted to the culture phase.