Figure 8.

Proposed model of modulatory actions in metamorphic competence and settlement of sea urchin larvae. Round symbols indicate agonistic relationships while flat lines indicate antagonistic interactions. Data presented here suggests a modulatory role of histamine (HA) in metamorphic competence. Based on our data we propose that HA leads to the acquisition of competence in pre-competent larvae and maintains competence in competent larvae by inhibiting caspase mediated apoptosis. Previously published data also suggest a function of thyroid hormones and nitric oxide (NO) signaling in modulating metamorphosis. Note that published data [40] established a link between NO and HA signaling in the sea urchin S. purpuratus. Future research will focus on the interactions of these signaling pathways in metamorphic competence and settlement