Marriage has been taken to moving new heights and depths by parasitic
flatworms of the genus
Diplozoon.
Now, we're talking about parasites here: not the most attractive organisms, admittedly. But everyone has to make a living.
The phylum Platyhelminthes is a big one, and includes such things as free-living flatworms as well as tapeworms, liver and blood flukes, and other disgusting beings. Disgusting to us, largely because we're more likely to be the victims than the perpetrators of parasitic behavior.
The flatworms in question here, Diplozoon, live parasitically on the gills of fish. Each has both male and female sex organs; but the plan is not self-fertilization. When immature individuals meet and fall in love (say, who am I to say that they don't?) they thereupon become firmly and permanently physically attached to each other, their tissues fuse, and their reproductive systems grow together so that each fertilizes the other. They live the rest of their lives together in a state of perpetual copulation, through sickness and health, and so forth. They may be the only completely monogamous species.
Love is so important to these tiny beings (they're only 2 or 3 centimeters long) that the hapless young worm who does not find a mate fails to mature, and soon dies.
(PHYLUM, Platyhelminthes; CLASS, Monogenea; SUBCLASS, Polyopisthocotylea; GENUS, Diplozoon)
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Buchsbaum, Animals Without Backbones, University of Chicago Press, Third Edition 1987
New York Times