"Many species of polychaetes undergo epitoky whereby sexually immature worms transform into pelagic morphs capable of sexual reproduction. After fertilization, they release their gametes through rapid disintegration." worms are out here having insane sex we can't even comprehend
Weird worm sex by phylum and class:
Phylum Annelida, class Errantia
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The segmented worm palolo (Eunice viridis), close relative of the infamous Bobbit worm, is normally a seafloor-dweller, but upon reaching maturity it develops paddle-like limbs and starts swimming in mid-water, where it’s easier to find partners. Its sexual organs develop, while the digestive system atrophies. The worm itself does not mate, and will eventually return to the seafloor -- but its tail does, as the palolo’s posterior half (the epitoke) breaks off and swims alone to the surface, complete with its own limbs and its own eyes. This occurs precisely four times a year, in the early morning of two consecutive days in October and again in November, always in the final quarter of the waning Moon. The swimming tails, packed full with eggs and sperm, disintegrate, mixing their gametes in the surface water. Inhabitants of Melanesia and Polynesia gather in great number to fish the bursting tails from the water, which there are a delicacy.
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In its close relative Autolytus pachycerus, of Australian waters, you can even see the little growing individuals develop their own palps and eyes before they detach from each other.
Phylum Annelida, class Echiura
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Spoonworms have stocky, slug-like bodies that stay hidden under rocks while a long, concave, bifurcate proboscis roams around to find and collect food. In the green spoonworm Bonellia viridis can sport a Y-shaped proboscis two full meters long, even as the trunk measures less than 15 cm. That’s only true for the female, however, for the male measures only a few millimeters in length, and spends his whole existence in a pocket of the female’s uterus. Contact with a female prevents the male from ever growing out of the larval stage, except for his sexual apparatus.
Phylum Platyhelminthes, class Monogenea
Monogeneans are a group of parasitic flatworms that hang from the skin or gills of fish with their hooked faces. Diplozoon paradoxum, though, is more romantic than most. It’s a rare case of a monogamous worm -- in fact, the most monogamous animal in nature. A lonely Diplozoon larva withers and dies, but when two larvae meet on the gills of a carp, they fuse together into a single body, with two evident halves but no partition. Then, and only then, they mature into adults, one half into a male, the other into a female, permanently joined by the genital area, through which gametes flow seamlessly to produce new larvae.
Another Monogenean, the salmon fluke Gyrodactylus salaris, is viviparous, which means its offspring grows inside the mother’s body as in mammals. Gyrodactylus however goes a bit further than mammals: the maturing embryo (they are all hermaphrodites) starts developing already pregnant with a daughter of her own, so that three generations are nested inside each other like a matrioshka. When this first daughter is born, the embryo she carries forms a new daughter as well, while the newly relieved mother developes male sexual organs as well, and may conceive again by mating with other flukes or with herself.
Phylum Plathyhelminthes, class Turbellaria
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Unlike other flatworms, most Turbellarian live free in water, where they move by fluttering magnificently with the edges of their paper-thin body. They are hermaphrodites, each individual able to produce both eggs and sperm, so that each can fertilize the others, like slugs and earthworms do. However, producing eggs costs a lot more energy and nutrients than producing sperm; consequently, in many flatworm species each individuals would rather take the part of the male. How to decide? Penis fencing. Upon meeting, two flatworms of these species will each extrude a pair of sharp stylets, with which they will fight as with daggers, until one (or both) manages to break through the other’s skin and inject sperm into its cavities.
Worm sex is truly beyond our ken.
