Brett Christopher Gonzalez:
Morphologically characterized by the presence of a dorsal covering of paired segmental scales (=elytra), scale worms are well represented throughout the scientific literature, and are a result of one of the most successful radiations of annelids. However, the phylogenetic relationships of elytrigerous annelids remain unresolved, in spite of the numerous systematic revisions the group has undergone. Since the first morphological cladistic analysis, molecular and combined investigations have shaken up the systematics, yielding surprising character transformations within Aphroditiformia, and paraphyly within various groups previously thought to be monophyletic, highlighting the necessity for further detailed systematic revisions. Herein, we attempt to resolve these controversies within scale worms, by inclusion of previously underrepresented taxa (i.e., caves and interstitium) in order to improve the overall resolution of the phylogenetic relationships within Aphroditiformia. To date, this is the largest and most diverse phylogenetic sampling of scale worms, being the first to include anchialine as well as several previously neglected interstitial representatives. Using combined and total evidence approaches, our phylogenetic analyses integrated morphological and molecular datasets, with subsequent sensitivity analyses to identify those groups with unstable positioning. Our inclusion of species from extreme environments showed several independent radiations among the deep sea, (anchialine) caves, and the interstitium, recovering six monophyletic clades within Aphroditiformia: Acoetidae, Aphroditidae, Eulepethidae, Iphionidae, Polynoidae, and Sigalionidae (inclusive of the former ‘Pisionidae’ and ‘Pholoidae’), respectively. Tracing of morphological character evolution showed a high degree of homoplasy throughout Aphroditiformia, with several secondary losses of scales among the groups. Ultimately, our investigations into cave and interstitial scale worms have given rise to interesting phylogeographical questions, as these groups have wide distributions despite their habitat specificity and seemingly low dispersal potential. While anchialine polynoids are highly habitat specific, we found strong geographic structure between Bahamian islands, with intra-island gene flow presumably through spelean corridors. Alternatively, our studies of interstitial taxa showed broad distributions associated to geographical regions, with high levels of homoplasy in group-specific morphological characters. Due to the high levels of homoplasy found by our cladistics analyses, we performed large-scale phylogenetic comparative analyses to unravel how ecology correlates to variation across e.g., feeding, motility, and colonization of habitats, especially the subterranean and interstitium. Focusing on these extreme habitat colonization events, we found evidence of shared morphological adaptations in response to aphotic conditions, as trait similarity appeared to be linked to functionality, thereby reflecting a high degree of adaptability and convergent evolution between relatively closely related scale worms. While some morphological and behavioral modifications in cave polynoids reflected troglomorphism, other modifications like eye loss were found to stem from a common ancestor inhabiting the deep sea, further corroborating the deep sea ancestry of scale worm cave fauna. In conclusion, while morphological characterization across Aphroditiformia appears deceptively easy due to the presence of elytra, convergent evolution during multiple early radiations across wide ranging habitats have confounded our ability to reconstruct the evolution of morphological characters. The high morphological variability include from loss of elytra and development of specialized male copulatory structures, to that of reduced body size and incorporation of elytral brooding. Our combined investigations into adaptation and evolution of morphological and molecular characters on species, genera, and across families has illustrated the degree to which ecology drives morphological plasticity, making it equally important to consider when addressing morphological and phylogenetic evolution.