Lichen (group Lichenes) is an example of bisystem and alternative system design in nature. The "initial system" is an alga which can synthesize organic matter (plus), but cannot obtain water in dry places (minus). A fungus we can consider as the alternative system which can get water (plus) but lacks chlorophyll for photosynthesis (minus). Ideal Final Result - lichen, a complex plant made up of an alga and a fungus.
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The New Orleans Aquarium
"Upside-down jelly fish" incorporate algae in their bodies. The algae use sunlight to produce nutrients, which feed the jelly fish. So the jelly fish either swim upside down (tentacles up, caps down) or rest on the bottom, positioned so that the algae get the maximum amount of sunlight. Neither behavior is used by jelly fish that have to hunt for their food.
Mormyrids are electric eels that live in particularly muddy water. Other electric eels use a constant electric field, and sense disruptions in the field to detect their prey, but the mud interferes with the sensing. The mormyrids used pulsed fields and sense the echoes of the pulses. This lets them use more energy for the pulses, less energy (and less complexity) for the sensing organs, and lets them inhabit areas that other eels can't use, so they have no competitors.
The turtle is an example of the "Nested Doll" principle. (or is that too easy?)