|Image from the "FunSea" video below|
Theyposition themselves into the water column in order to take advantage of better water currents for their gametes to disperse. Or at least, that's what seems to be the case. This reproductive posture is actually not all that well understood.
|This image borrowed from A snails' Odyssey blog here! Great info!|
Since these guys are likely trying to take advantage of the same water current to carry their gametes to their destination, several individuals (from different species) are likely to find locations in the same general area.
Their postures are fairly arched and position the body well into the water column.
Interesting that both of these are on sandy bottoms. Coincidence? Difference in species? Hard to say.