On tropical reefs, coral growth and morphology drive habitat structure, which impacts diversity and abundance of reef associated species. Coral symbionts can alter coral growth and morphology, indirectly affecting organisms that utilize coral for food or refuge. In Moorea, French Polynesia, I discovered epibiotic amphipods and worms that induce the growth of long, branch like coral “fingers” on colonies of Montipora. Montipora is one of the dominant genera of reef building corals in the lagoons of Moorea. It is typically encrusting or plating, but by building tubes that are overgrown by coral, the symbionts alter Montipora to resemble branching corals. The symbiont induced growth forms are ubiquitous across the lagoons, and add significant three dimensional structure to the reef. This radical change in shape affects both the coral and creatures using coral for food and habitat. For example, experiments where amphipods were removed from fingers show that corals with amphipods grow faster than those without. The vertical structure of the fingers also enhances coral survival, and may enhance Montipora’s ability to asexually reproduce through fragmentation. Montipora colonies with fingers are also protected from crown of thorns sea stars (Acanthaster planci). Surveys and experiments show that the added structure deters sea stars from attacking and hinders their ability to consume corals. Living tissue remaining following a sea star attack can survive and continue to grow, so the presence of fingers mitigates the impact of crown of thorns outbreaks and enhances recovery afterwards. Furthermore, the presence of fingers alters crown of thorns prey selection, indirectly affecting the susceptibility of other corals to predation. The added structure also impacts reef fish. Corals are important refuge for juvenile fish, and experiments reveal that fingers provide protective habitat for many fish species. Because fingered Montipora can grow in areas where other corals cannot, the fingers also act to increase the total amount of reef area that fish inhabit, increasing the abundance and diversity of fish in many parts of the reef. These experiments elucidate the impacts of the symbionts on the coral reef community, and demonstrate how species interactions create ripple effects that can alter the structure, health and diversity of ecosystems.