Fig. 1. (A) A hypothesis for iodine-based cross-kingdom communication in marine ecosystems. Marine organisms incorporate iodine into an organic matrix (organification) and synthesize thyroid hormone (TH)-related signaling molecules, which are then shuttled through the food chain (phytoplankton to zooplankton). This hypothesis is supported by previously published evidence and our own new data (see Fig. 2) of TH-like molecules being present in marine algae and various marine invertebrates and their larval forms. (B) One functional aspect of this cross-kingdom communication is the involvement of TH-related compounds in development to metamorphosis of various marine invertebrate larvae, represented schematically in B for molluscs (veliger larvae) and echinoderms (pluteus larvae). New evidence suggests that THs are used as developmental signals by larvae and that the primary source may be exogenous (Chino et al., 1994; Heyland and Hodin, 2004), although we were able to find evidence for endogenous synthesis as well (Heyland and Hodin, 2004). The role of THs in metamorphosis per se remains to be elucidated. The blue frame indicates larval development (L) and the red frame metamorphic development with metamorphic competence (C) and settlement (S). Please note that here we are using the term THs generically for thyroid-like hormones, since the specific chemical identification of THs in these lineages requires further confirmation using microanalytical methods (e.g. mass spectrometry and NMR). Images of echinoid larvae modified from Hyman (1995).