Still even on a much more narrow phylogenetic level, TH detection and signal transduction system may have diversified. Preliminary data suggest that important differences exist between
non-feeding and feeding echinoid larvae. As pointed out earlier,
phenotypic effects of high food concentration on larvae correlate
strongly with TH effects on echinoid larvae (Heyland and Hodin,
2004). Assuming that nutritional and hormone content in algae
are correlated, positive selection would drive the evolution of
detection systems for THs from exogenous sources in feeding larvae. For non-feeding larvae, however, the detection of exogenous
sources would be less relevant. Instead endogenous synthesis
mechanisms should be more important. In order to gain insight
into the evolution of TH signaling in echinoids it will be critical
to compare endogenous TH synthesis in feeding and non-feeding
larvae. Both, larval types can also receive THs maternally. The
adults of many echinoderm species feed on algae that contain high
amounts of iodine and may also synthesize THs or TH precursors
(Kupper et al., 2008; Mairh et al., 1989; Wong et al., 2002). Maternally derived THs may result in faster larval development and
therefore reduce the time offspring has to spend in the plankton.