Echinoids (sea urchins and sand dollars) undergo thyroid hormone (TH) regulated larval development and several species can endogenously synthesize these hormones. Although iodine is the essential component for TH synthesis, nothing is known about iodine uptake mechanisms in echinoids. This thesis primarily aimed to characterize integumental iodine uptake in larvae (echinoplutei) of the purple sea urchin Strongylocentrotus purpuratus. The two mechanisms considered were: iodine uptake via sodium dependent transport utilizing orthologs of vertebrate sodium iodide symporter (NIS) and apical iodide transporter (AIT) versus hydrogen peroxide dependent diffusion (PDD) of iodine. Pharmacology and radioiodine (125I) experiments characterized the effects of various compounds on echinoid iodine uptake. The results demonstrate that purple sea urchin echinoplutei acquire iodine from feeding on microalgae and through integumental transfer. Integumental transfer of iodine is inhibited by cyanide and is temperature dependent. Echinoplutei are not affected by perchlorate exposure demonstrating that NIS/AIT is not involved in S. purpuratus iodine uptake. NIS and AIT are both members of the Sodium Solute Carrier 5 (SSF5) transporter family and phylogenetic analysis of various vertebrate and invertebrate SLC5 members (obtained through BLAST searches) indicated that NIS-like transporters might be a vertebrate synapomorphy. Hydrogen peroxide exposure and oxidative stress induced an increase in iodine influx, whereas reducing agents and peroxidase inhibitors disrupted iodine uptake supporting a PDD-based iodine uptake model. In situ hybridization, immunohistochemistry and real time-PCR analysis demonstrate that the sea urchin dual oxidase 1 (Udx1) (an NADPH oxidase that produced hydrogen peroxide) is expressed throughout all stages of larval development in both S. purpuratus and Lytechinus variegatus specifically occurring in epithelial cells. These findings are the first data to demonstrate Udx1 presence and potential activity outside of fertilization and embryogenesis in echinoplutei. The expression patterns and pharmacological results make Udx1 an attractive candidate for involvement in integumental iodine transfer through PDD. These data provide the first evidence for PDD in an animal. The results also suggest that NIS/AIT may be a vertebrate synapomorphy and PDD of iodine across the integument may be widespread across organisms. Future characterization of iodine uptake mechanism in diverse taxa will address this issue. / Studies were funded by the Natural Sciences and Engineering REsearch Council (NSERC) to A.H. [grant number 400230], Equipment purchased with funding from the Caiadian Foundation for Innovation CFI and NSERC [grant number 400587] to A.H.