Deep in the ocean, below the reach of wind-driven currents but well above the seafloor, lies an in-between world—the eerily still midwaters, dominated by alien-looking, gelatinous life forms. Here ctenophores swim about using glittering comb-legs, colonies of siphonophore clones stretch a hundred feet long, and giant larvaceans secrete elaborate mucus structures. These strange inhabitants of the…