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  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.