UC San Diego

Two organismal systems were studied for this thesis. In the first, Mesorhizobium, a genus of bacteria best known for nitrogen-fixing symbiosis with terrestrial legumes, were identified covering the cuticle of Meganerilla bactericola, an annelid living in the anoxic Santa Barbara Basin. This was shown via phylogenetic analyses of several genes and fluorescence in situ hybridization (FISH). The presence of Mesorhizobium suggests a potential role for nitrogen-fixation in this symbiosis. The second, and main, component explored crinoid echinoderms and their myzostome annelid associates. Nearly all myzostomids live on or inside echinoderms, chiefly crinoids, and may steal incoming food from the host's food grooves or consume the host's tissue directly. A molecular phylogeny for Comatulidae - the most abundant and diverse family of crinoids on modern tropical reefs - was inferred using seven genes from 56 terminals (= species). The systematics of Comatulidae was revised accordingly and morphological characters were also coded and traced onto the molecular phylogeny. Most were homoplastic, and apomorphies were lacking for subfamilies, tribes, and most genera. However, combinations of features generally allowed for diagnoses for genera and species. A non-dichotomous key and photographic guide were developed to assist in field identifications, and DNA sequences on Genbank allow identification of specimens to the species level. Myzostomida phylogeny was inferred via molecular data from 75 taxa. Parasitic forms arguably arose two or three times, once in crinoids and once each for asteroids and ophiuroids. New and undescribed species almost outnumbered known myzostomid taxa in this dissertation, and a ̀turbo- taxonomic' approach was developed and applied to describe 21 new species of myzostomids. To compare relationships of myzostomids with their hosts, a phylogeny was inferred for 56 hosts and a tanglegram constructed with 90 associations. Host specificity was high and phylogenetic conservatism was also found, with clades of myzostomids interacting only with particular clades of echinoderms. This finding suggests that myzostomids (which have a free- living planktonic stage) are limited to one or a few closely related hosts, despite most crinoids co-occurring on the same reefs, many within physical contact of each other