A rhizocephalan doesn’t really have a common name. Perhaps that’s because it doesn’t look like much. But this spineless invertebrate spins a tale of high romance, of commitment and sacrifice, and an intimacy of the closest kind.

The scientific name of this Hugh Grant of the animal kingdom is Sacculina, a slightly unappealing title but nonetheless appropriate. Another name is for the creature is parasitic barnacle, a rather more descriptive term that nonetheless requires some élucidation.

A barnacle is a type of crustacean, the family of animals that includes crabs and prawns, but also crayfish, krill, woodlice, sea lice and lobsters (but not Pthiris pubis, known socially as ‘crabs’). They are one of the most diverse groups of animals, ranging in size from the microscopic to the enormous Tasmanian giant crab. With their cement-like shells, barnacles are those creatures that can make a coastal walk very unpleasant. They attach themselves to rocks and pylons, and some have taken to riding on the backs of whales. The actual animal is like a shrimp, lying on its back and kicking its legs in the air as it scoops microscopic organisms out of the water. When the tide is low, a trapdoor on top closes to stop the barnacle drying out.

A parasitic barnacle looks nothing like a barnacle. Only in its youngest stage may the connection be noticeable—the larvae are quite similar. They drift in ocean currents for several days, moulting and surviving on the yolk in their guts. At this stage they are somewhat shrimp-like—or at least somewhat like a shrimp larvae. Even though they are only four days old, the sexes already have radically different lifestyles. The female is the homebuilder. She drifts until she finds what she’s looking for—a hair on the leg of a crab of a particular species.

This is when things take a turn of the Ridley Scott kind. The female moults one last time, and gets rid of her legs and head and all sensory organs and muscles, in fact almost everything that makes her an animal. She becomes a sort of blob of barnacle cells. These cells grow a dart that pierces the shell of the crab, and the blob of cells disappears into the crab’s body.

The lady barnacle drifts around in the crab’s bloodstream until it gets near to the crab’s gut. The lady barnacle thinks “what a lovely, warm place to set up home” and settles down. There she starts growing roots that wrap around the crab’s gut and starts stealing its nutrients while it grows, and actually develops some organs like ovaries. This is where the name rhizocephala comes from, from the Greek words for ‘root’ and ‘head’.

Here I should probably provide a disclaimer: shit gets very, very weird. If the crab the barnacle lands on happens to be a lady too, all well and good. If, however, the crab turns out to be a gentleman, the barnacle TAKES OVER THECRAB’S MIND AND CASTRATES IT. It does this by effectively genetically reengineering the crab: producing hormones that tell the crab to stop being a male and damn well be a lady. It’s just like Mrs Doubtfire.

The barnacle isn’t done yet though. In a further act of mind control, it forces the crab to moult. This allows the barnacle to find a crack in the surface and start growing into a large, tumescent… growth. This grows and grows, becoming more obvious until it is visible to the human eye. At which stage it looks nothing like a barnacle or any other type of animal.

Here enters the male barnacle—literally. The man-barnacle lands on the surface of the blob, which is in fact a growing mass of eggs, and fertilises them. Here he stays, living inside his mate for the remainder of his life as a sperm donor, the most committed and intimate partner in the animal kingdom. The crab, female or now-female, lovingly cares for the eggs, thinking that they are its own. Periodically the egg mass releases larvae into the water, where they swim off to find another crab and live happily ever after.

Such romances occur nearby in Western Port Bay—in fact likely anywhere there are crabs. Because of their ability to sterilise crabs, there is some talk of using these barnacles as biological weapons, or controls rather, to reduce populations of pest crabs, much like cane toads were used to control beetles. It couldn’t possibly go wrong. Perhaps another Alien prequel is in the making.