The decrease in wild fish stocks has led to a search for novel feed sources to supply the aquaculture industry. Polychaetes have recently been identified as suitable feed ingredients due to their favourable nutritional composition, especially their lipid profile which is high in unsaturated fatty acids. As ragworm farms have started to develop, there has been a need to understand and improve rearing techniques, in particular the dietary requirements and nutritional profile of the ragworm. In this thesis, research was focused on the king ragworm. Nereis virens. The protein and energy requirements were identified by increasing inclusion levels of nutrients in the diet as well as manipulating feed rations from starvation to satiation. Weight gain, survival, ingested feed as well as protein and energy retention efficiencies were evaluated. Results demonstrated that N. virens gained a proportional amount of nutrient in relation to the amount ingested up to a critical amount when gain either stagnated or decreased. A pattern of nutrient retention and maintenance requirements for different weight classes were calculated. This data was then used for bioenergetic modelling to calculate nutrient requirements using the equation: Requirement: a x BW (g) + c x growth The energy maintenance requirement was found to be 18 J g-1 worm day-1; for protein, the requirement was 0.19 mg g-1 worm day-1. The predicted weight gain (g) for a worm of any given size (g) was y = 0.015g1.106. The total nutrient requirement is the sum of maintenance and growth, including the constant c which is the cost of nutrient deposition. Alternative feed sources were also investigated to observe the extent at which N. virens can utilize novel sources and their abilities to preserve or convert highly unsaturated fatty acids such as EPA and DHA. Results showed a high adaptability to terrestrial animal or vegetable based feed sources but an inability to convert shorter chain or n-6 fatty acids when fed non-marine based feeds. There may however be potential for N. virens to utilize other feed sources from its natural environment to supplement n-3 HUFA content.