In Haliotis, NO means YES

Settlement and metamorphosis of marine invertebrate larvae is an ecological process of profound importance to the structure of benthic marine communities. Yet, metamorphosis is also a developmental process and our understanding of the mechanisms that regulate it remains relatively poor. Over the last 15 years, a nitric oxide (NO) based signaling system has emerged as the most phylogenetically widespread regulator of settlement and metamorphosis. Among a sea urchin, three sea squirts, three molluscs, a crustacean, and an annelid, NO (and often its downstream effector cGMP) regulates metamorphosis (Froggett and Leise, 1999; Bishop and Brandhorst, 2001; Bishop et al., 2001, 2006, 2008; Comes et al., 2007; Pechenik et al., 2007; Biggers et al., 2012; Zhang et al., 2012; Romero et al., 2013). Two important aspects to this pattern of NO function during metamorphosis are that (i) NO always acts as an inhibitory signal and that (ii) the functionality of NO signaling varies according to the settlement ecology of the larva. Ueda and Degnan’s recent work has challenged the first pattern and supported the second. First, why should NO signaling be inhibitory in these phylogenetically disparate, morphologically distinct (and, in some cases, CDB would argue), independently evolved larval forms? Based on a model in which the directionality of NO signaling during metamorphosis is stochastic over evolutionary time periods, there is a small probability, given current taxonomic sampling, that all larvae so far investigated would employ NO in an inhibitory manner. So either NO signaling during metamorphosis is conserved because larvae are homologous or because the use of NO in this manner during metamorphosis has been selected repeatedly. In the foundational study of NO during metamorphosis of the mud snail Illyanassa obsoleta Froggett and Leise (1999) hypothesized that the function of an inhibitory signaling system is to delay the onset of metamorphosis until chemical inducers or “cues” are detected, signaling habitats that are favorable for growth and development of the animals (Hadfield and Paul, 2001). Thus, Froggett and Leise provided a way to think about the directionality of a signal (i.e., stimulatory or inhibitory) in the context of a larval behavior that is presumably ecologically relevant and adaptive (Pechenik, 1990). It seemed as if widespread inhibitory NO function could be placed in the context of an equally widespread ecological reality. Nobuo Ueda and Sandie Degnan at the University of Queensland in Brisbane, Australia, have shown that NO positively regulates metamorphosis in the ascidian Herdmania momus (Ueda and Degnan, 2013) and, in the present study, positively modulates (see below for the difference between a regulator and a modulator) in the abalone Haliotis asinina. Therefore, the inhibitory function of NO in metamorphosis does not appear to be a fixed character. In this current work, NO is demonstrated to serve as a necessary inductive facilitator for metamorphosis in response to a natural inducer that is needed to transduce the metamorphic chemical cue from red coralline algae Amphiroa. Although in neither study did the authors test whether NO generators could antagonize NOS inhibitors (or other “epistasis”-type experiments) and did not use a non-enzymatic method of reducing NO activity, all of their pharmacological evidence does point to a novel directionality of NO signaling and so is highly notable in this regard. Interestingly, pharmacological manipulation of NO levels alone was not sufficient to induce larval settlement and metamorphosis; NO signaling could only modulate larval responses to algal-derived settlement cues. This takes us to the second emerging aspect of studies on marine invertebrate metamorphic signaling: the relationship between the function of identified signaling pathways and ecological specialization of larval settlement. One particular aspect of settlement ecology concerns differences in the behavior of competent larvae in the absence of settlement cues, depending upon whether larvae are settling into specialized niches or not (Elkin and Marshall, 2007; Toonen and Tyre, 2007). The “desperate larva” hypothesis, coined by Toonen and Pawlik (1994), but informed by earlier seminal experiments (Knight-Jones, 1953; Wilson, 1953) states that larvae should become less choosy about where they settle as a function of time spent in a settlementcompetent state. As a mechanism that