It doesn’t! (at least, not at all like we understand it). Male shipworms release their sperm into the water column, and female shipworms filter it out of the water using their gills. Eggs are then fertilized and “brooded” internally for a while, before the young shipworms are released to float off and find a new place to colonize!
Starfish are either or male or female and on moonlit nights (very romantic!!) in the Spring, they release eggs or sperm into the sea. So one starfish can have thousands of “babies” with lots of other starfish of the opposite sex.
Abyssal grenadiers mate through external fertilization. The female releases the eggs into the water and the male is nearby and releases his sperm. We think this happens in mating groups, where many females and males are spawning together. Fertilized eggs have tiny oil droplets in them, which makes them buoyant and float up to the thermocline (~1000 m below sea level). In the thermocline, water is warmer and the pressure is less intense – this lets the fish develop under less environmentally stressful conditions. As only a few ripe females and no spent individuals have been collected, abyssal grenadiers are thought to only mate once and then die (this reproductive strategy is called semelparous). Basically, the females save up all their reproductive resources for 30 years and then throw a big orgy party before dying!
If they were humans, people would say they do it in a very romantic way. When a male and female decide they like each other and would like to mate, they dance together for sometimes hours or even days (on and off, they do rest and feed in between). Their dance is like a slow waltz, side by side, swimming up and down in the water column. Then when finally the timing is right, the male tilts at a slight angle and slides up alongside the female so that the female transfers a “sheet” of eggs on the abdomen of the male. The eggs have a glue(ish) substance that they stick to the male, so as the male moves up, the sheet of eggs attached down his belly. Once they mate, the male can’t mate with anyone else until the embryos develop and hatch, so they part ways.