Sex is different in the dark. The really, really dark of the abyss. For instance, it's difficult to find another of your own kind. Some species get around the challenge by never letting go of their partner.
"Odd gelatinous anglerfishes approach, the females casting fishing tackle attached to their foreheads, complete with rods, lines, luminescent lures, and sometimes hooks. Their male partners accompany them in the form of tiny parasitical appendages attached by their jaws to the females’ skins, where they subsist on embryoniclike connections to her blood supply. Thus conjoined, the males atrophy to nothing more than their gonads, awaiting the moment to spawn."
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Chapter 26The Existence of a World Previous to Ours