The deliberate cultivation of seafood through aquaculture is expanding with a focus on environmental sustainability, specifically decreasing global emissions and waste. In New Zealand this industry is growing at a rapid pace with intentions to scale up the farming of species such as salmon, pacific oysters and Greenshell™ Mussels. During processing by-products such as mussel shell are produced and often sent to land fill. Greenshell™ mussel shells consisted of approximately 90% calcium carbonate (Hamester, Balzer, & Becker, 2012) making it a valuable resource. This study focuses on the creation of polymer composites and foams which have been inspired by the marine environment and the materials in it such as calcium carbonate from mussel shells. It specifically aimed to incorporate mussel shell powder (0-30wt% for composites and 0-10wt % for foams) into polylactic acid (PLA) and low-density polyethylene (LDPE). These polymers were chosen for their bioderived nature and degree of recyclability respectively. This work creates, characterises, and tests these materials to determine potential applications. Composite materials were successfully extruded and then processed into 3D printing filaments to extend the applications of these materials. These samples were tensile tested, and granulated composites were used for thermal analysis. PLA composites showed far superior tensile strength and stiffness over LDPE samples, reaching 60.69 MPa yield stress and 3748 MPa Youngs’ modulus at 8wt% of mussel shell powder, with LDPE composites reaching 8.9 MPa yield stress and 197 MPa Youngs’ modulus. At 30wt% of mussel shell both PLA and LDPE samples showed less plastic deformation with PLA + 30wt% sample reaching failure almost instantly. Thermal stability of PLA and LDPE composites showed varying properties. The most notable difference between the two polymers was the change in thermal degradation temperature as the weight percentage of mussel shell powder was increased. PLA samples showed a reduction in thermal degradation temperature due to the introduction of moisture and hydrolysis as the weight percentage of mussel shell powder increased, decreasing from 340℃ (0wt%) to 287℃ (30wt%). LDPE samples showed an increase in thermal degradation temperature due to the high volumes of ceramic mussel shell powder and no hydrolysis. ii Thermal degradation of LDPE composites was seen to increase by 22℃ between the control sample to 30wt% samples. Extruded foams were moulded into small blocks for compression and microscopy. PLA foams showed higher expansion ratios than LDPE foams and much lower density. PLA 0.5wt% reached three times the expansion of un-foamed PLA and showed the highest expansion ratio of all samples tested. The expansion of PLA foams decreased between 0.5wt% and 10wt% as expected while the compressive strength showed a good improvement from 5 MPa to 20 MPa. Overall, LDPE foams had both lower compressive stress (13 MPa) and expansion (1.5) than the PLA samples. LDPE samples showed far greater recovery at 70%, which was 40% higher than PLA foams. In summary, PLA displayed best reinforcement at 8wt%, best thermal properties between 0-2wt% and best foam nucleation results at 0.5wt%. Finally, LDPE samples showed comparable results with best reinforcement at 8wt%, best thermal properties between 20-30wt% and best foam nucleation results at 0.5-1wt%. This thesis has thoroughly investigated composites and foams that could be introduced into a circular economy, resulting in a beneficial impact on the environment. Options for biodegradable (PLA) and recyclable (LDPE) polymers were shown while valuable by-product material (mussel shell) was introduced to composites and foams as a method of reducing stress on landfills and improving the properties of these materials. These materials could be made into complex containers, sandwich composites, and equipment in the medical, automotive, and aerospace industries.