even a fungus and an algae (such as in lichens). Many sea slugs have developed such relationships. Small crustacea called copepods are often found living on dorid nudibranchs. They have not been studied well enough to know whether each nudibranch has its own species of copepod or whether the copepods live exclusively on the nudibranchs. The copepods are easily recognised because their two large egg sacs look like a pair of large "tails". Another more spectacular example is the shrimp Periclimenes imperator which is always found living on large dorid nudibranchs such as the Spanish Dancer, Hexabranchus , or on chromodorids such as Chromodoris tinctoria . Another quite different group of relationships have been discovered with sea slugs and involves plants and plant organelles. As I have described for Elysia pilosa many sacoglossans keep chloroplasts and other plant plastids alive to obtain the sugars they produce from photosynthesis. Some nudibranchs show various degrees of ability to harness solar power by keeping whole single-celled plants (zooxanthellae) alive in their tissues for the same purpose. This is described on the page on " solar-powered nudibranchs "