Effects of temperature, salinity and diet on growth, survival, and fatty acid composition of the ragworm Hediste diversicolor (OF Müller, 1776) (Annelida: Nereidae)

Abstract

[Introduction]: The common ragworm Hediste diversicolor is an omnivorous, burrowing polychaete with a broad tolerance to environmental conditions such as temperature, salinity and oxygen levels. This nutritious annelid has received a great deal of attention lately for its potential as an extractive species in IMTA systems to recapture high-value compounds from fed aquaculture particulate side streams (sludge) and its suitability for subsequent utilisation as a novel, omega-3 rich aquafeed ingredient (Wang et al., 2019). It has recently been shown that this species has the capacity for endogenous production of omega-3 long-chain polyunsaturated fatty acids (Kabeya, et al., 2020), however it is yet not understood how environmental cues affects this ability. We conducted two sets of experiments to assess the combined effects of diet, temperature and salinity on growth, survival and total body fatty acid composition in H. diversicolor juveniles. [Materials and Methods]: Polychaetes (H. diversicolor) were collected from Leangen Bay, Trondheim, Norway (63°26′24.5″N, 10°28′27.7″E). To investigate the effects of diet and temperature on growth, survival and fatty acid (FA) composition of H. diversicolor, worms were fed mixes of solid biogas digestate (SBD) and salmon aquaculture sludge (SS) along a 4-step feed gradient ranging from pure SBD to pure SS, and a 5-step temperature gradient ranging from 5.8 to 17.1 °C, for 15 days, using fish feed as a control (Experiment 1). A second experiment was conducted to investigate the effects of salinity and temperature on the same variables. Here, the worms were fed the diet which yielded the highest growth rates in the first experiment (33:66 % SS:SBD) along 5-step salinity and temperature gradients ranging from 5 to 40 ppt and 7.7 to 17.9 °C, respectively, for a duration of 28 days. In both experiments, worms were fed isonitrogenous diets equalling 30 % of the worms' total body nitrogen per day (Wang et al., 2019). Both experiments were conducted in a temperature gradient table (modified after Thomas et al. (1963)) using a 18h:6h light:dark cycle. Worms (n=7-8) were stocked in glass beakers (800 mL) containing an eight cm thick layer of sand and sand- and bagfiltered (1 µm) seawater from the Trondheim fjord collected at 60 m depth. The worms were allowed to evacuate their guts in clean seawater for minimum 4 hours before each sampling and weighing. Water exchange and feeding was conducted every second day. Data analyses were performed using the inbuild statistical package of SigmaPlot v. 14.5.

[Materials and Methods]: Polychaetes (H. diversicolor) were collected from Leangen Bay, Trondheim, Norway (63°26′24.5″N, 10°28′27.7″E). To investigate the effects of diet and temperature on growth, survival and fatty acid (FA) composition of H. diversicolor, worms were fed mixes of solid biogas digestate (SBD) and salmon aquaculture sludge (SS) along a 4- step feed gradient ranging from pure SBD to pure SS, and a 5-step temperature gradient ranging from 5.8 to 17.1 °C, for 15 days, using fish feed as a control (Experiment 1). A second experiment was conducted to investigate the effects of salinity and temperature on the same variables. Here, the worms were fed the diet which yielded the highest growth rates in the first experiment (33:66 % SS:SBD) along 5-step salinity and temperature gradients ranging from 5 to 40 ppt and 7.7 to 17.9 °C, respectively, for a duration of 28 days. In both experiments, worms were fed isonitrogenous diets equalling 30 % of the worms' total body nitrogen per day (Wang et al., 2019). Both experiments were conducted in a temperature gradient table (modified after Thomas et al. (1963)) using a 18h:6h light:dark cycle. Worms (n=7-8) were stocked in glass beakers (800 mL) containing an eight cm thick layer of sand and sand- and bagfiltered (1 µm) seawater from the Trondheim fjord collected at 60 m depth. The worms were allowed to evacuate their guts in clean seawater for minimum 4 hours before each sampling and weighing. Water exchange and feeding was conducted every second day. Data analyses were performed using the inbuild statistical package of SigmaPlot v. 14.5.