The nudibranch, a shell-less snail of tropical waters, maintains an unusual symbiotic relationship with the unicelled plants called zooxanthellae. Juvenile nudibranchs, foraging on corals and similar organisms, ingest algae-bearing polyps. Algae thus 57 the nudibranch. Almost immediately, rapid development of the algae ensues in the nudibranch’s intestinal tract. As this occurs, the formerly colorless nudibranch gradually develops colored patterns underneath its translucent skin. Its reliance on nourishment from external sources declines and eventually ceases as it increasingly derives nourishment from metabolic products of the algae. Thereafter, nudibranchs exhibit a type of self-contained agriculture: algae living in their intestinal tract produce, via photosynthesis, sugars and other nutrients fully sufficient to sustain them. No other 58 is required.