Ocean Lily Symbiosis

Ocean Lily
A saltwater water lily. The stalks can grow to be 60 feet (allowing them to live in water as deep as 60 feet) and the pads can get up to three feet in diameter. The Lilies are pollinated by bees, which, in combination with their depth limit of 60 feet, limit them to coastal areas, or self-pollinated by Underwater Horn Ants. The seeds of the Ocean Lily are golf ball-sized nuts with a shell that is semi-permeable to seawater. The nut floats, slowly taking in water until, eventually, it sinks under the increased weight to the bottom of the ocean, where the Underwater Horn Ants strip the husk and bury the seed so it can germinate. The nuts are farmed by the local intelligent life (humans, etc.), and the farmers must be careful to ensure their crop doesn't slowly migrate away with the current.

Underwater Horn Ants
These underwater ants live in a symbiotic relationship with the Ocean Lily, and live in hollow "horns" in the base and pad of the lily. The ants have limited swimming capability, but do have a kind of swim bladder, allowing them, if dislodged from the lily, to sink to the bottom and crawl back. The ants keep the Ocean Lilies safe from herbovores, while the Lilies provide the ants with a home and food (the husks of the lily nuts). Additionally, the ants will eat kelp, anything they kill while protecting the lilies, anything dead they find, and pretty much anything else.

Ocean Lily/Underwater Horn Ant symbiotic relationship and evolution
Initially, the Ocean Lily and Underwater Horn Ant evolved (force evolved?) separately, the Ocean Lily first, followed by the Underwater Horn Ant, which quickly moved in to the spacious horns of the Ocean Lily upon discovering them. Ocean Lilies previously dropped seeds that sank, until an entrepreneurial farmer decided that nuts encased in floating husks would be far easier to harvest. This was ultimately a dominate trait, so the non-husk Ocean Lilies started dropping seeds for husk Ocean Lilies. Further, the husk protected the seeds from being eaten by smaller fish, so husk Ocean Lilies were better able to survive. This caused a problem, however, whereby the husks prevented the seeds from germinating on their own. This problem was solved by the Underwater Horn Ant, which ate the husks and trampled the seeds into the ocean floor. As a result, all husk Ocean Lilies without Underwater Horn Ant symbiotes died off.