While geographic distance is a variable often used to explain population genetic differentiation, dynamic processes leading to stochastic population structure are more likely driving factors. The following thesis presents the population structure of a common reef sponge, Stylissa carteri, and yields hypotheses on the influence of environmental heterogeneity as a predictor of the observed population structure. This project represents the largest population genetics study thus conducted in the Red Sea and also includes the first population genetics data gathered for sites off the coast of Sudan and Soccotra. The study herein presented includes both a large scale (36 reef sites covering over 1000km of coastline) and small-scale (16 transects of 50m each) analysis of gene flow in a benthic dwelling organism. The variable effect of geography and environmental conditions on S. carteri population structure is assessed using a seascape genetics approach. Environmental factors from a nine-year dataset accessed from the NASA Giovanni website including chlorophyll a, sea surface temperature, dissolved and particulate organic matter for both the annual and winter temporal scale were considered.

Between 33 and 91 percent of marine species are currently undescribed, with the majority occurring in tropical and offshore environments. Sponges act as important microhabitats and promote biodiversity by harboring a wide variety of macrofauna and microbiota, but little is known about the relationships between the sponges and their symbionts. This study uses DNA barcoding to examine the macrofaunal communities associated with sponges of the central Saudi Arabian Red Sea, a drastically understudied ecosystem with high biodiversity and endemism. In total, 185 epifaunal and infaunal operational taxonomic units (OTUs) were distinguished from the 1399 successfully-sequenced macrofauna individuals from 129 sponges representing seven sponge species, one of which (Stylissa carteri) was intensively studied. A significant difference was found in the macrofaunal community composition of Stylissa carteri along a cross-shelf gradient using relative OTU abundance (Bray-Curtis diversity index). The abundance of S. carteri also follows a cross-shelf gradient, increasing with proximity to shore. The difference in macrofaunal communities of several species of sponges at one location was found to be significant as well, using OTU presence (binary Jaccard diversity index). Four of the seven sponge species collected were dominated by a single annelid OTU, each unique to one sponge species. A fifth was dominated by four arthropod OTUs, all species-specific as well. Region-based diversity differences may be attributed to environmental factors such as reef morphology, water flow, and sedimentation, whereas species-based differences may be caused by sponge morphology, microbial abundances, and chemical defenses. As climate change and ocean acidification continue to modify coral reef ecosystems, understanding the ecology of sponges and their role as microhabitats may become more important. This thesis also includes a supplemental document in the form of a spreadsheet showing the number of macrofauna individuals of each OTU found within each sponge sample.