Marine Symbioses: Metazoans and Microbes 180° 120° W 60° W 0° 60° E 120° E 117 Sea surface 180° 200 m Continental shelf Volcanic Island Continental slope 60° N 30° N Oceanic ridge 4000−6000 m 0° Abyssal plain 30° S Oceanic trench 60° S Magma chamber 10 000 m (a) 180° 120° W 60° W 0° 60° E 120° E 180° (b) Figure Distribution of symbioses (a) Global distribution of photosynthetic symbioses (sun, between 301 N and 301 S) and confirmed locations of vent sites with chemosynthetic symbioses (star) (b) Example distribution of photosynthetic (sun), chemosynthetic (star), lightproducing (light-bulb), and protective (shield) associations in discrete regions of the ocean Map courtesy of S Soares (Oceanography Department, University of Hawai’i at Manoa) Relative position of symbionts Ectosymbiont Symbiont Host cells Nucleus Host vacuole Endosymbiont Intracellular Extracellular (a) Symbiont transmission In many cases, the mode of transmission may indicate the necessity of the interaction, especially for partnerships with vertical symbiont transmission However, necessity may not be a two-way street; one may be an obligate partner, requiring the association for survival, and the other may be facultative, with individuals able to live with or without the association One final layer of complexity to consider when describing a symbiotic association is the specificity between partners Does the association occur between two partners defined at the species, clade, or subclade level, or does it lie under some higher level of classification? Can one partner be found in association with several potential partners, either occurring in a range of hosts or as hosting a range of symbionts? Marine symbiotic associations occur in a variety of structural forms, with different combinations achieving the same symbiotic function Functions of Symbiosis Vertical transmission Donates symbiont Symbiotic parent Symbiotic offspring Horizontal transmission Symbiotic parent Aposymbiotic offspring (b) Figure Structure of symbiosis (a) Relatively position of symbionts (brown circles) to host cells (orange ovals with black nuclei) (b) Transmission of symbionts from host parent to offspring The diversity of marine symbioses can be categorized according to the function provided to the host by the symbiont For example, imagine hosting small plants that live inside your skin and feed carbohydrates directly to your body Or how about a small compartment just under your skin that glows in the dark because it houses light-producing bacteria? This article describes marine mutualisms grouped by three main functions: nutrient exchange, production of light, and production of protective compounds Nutrient-exchange symbioses bring together organisms with complementary metabolic capabilities Light-production symbioses typically occur in the deep, dark oceans of the world or in association with nighttime activities in which they provide hosts with a mechanism for capturing prey, avoiding predators, or communicating Symbioses that produce protective compounds function to deter grazers, competitors, predators, or pathogenic microbes Humans also host numerous microbial symbionts known to function in nutrient exchange and the production of