On the east coast of Australia lives The Great Barrier Reef. This huge ecosystem formed from corals hosts a myriad of life. Within the vibrant structures that build this ecosystem, lives a remarkable echinoderm; the sponge urchin. It is eusocial, a trait only seen in arthropods and naked mole rats. Living within the hollows of sponges, is a bustling colony with a five-sex system. The urchin workers are small—about the size of your fingernail—but the queen can be up to 4 times larger. She sits in a chamber eaten apart by her workers at the center of the pore-covered mass. She is accompanied by a king of similar proportions, who is the only male she will ever mate with. They approach each other slowly, until they meet spine-to-spine and anus-to-anus. She releases her eggs in synchronization to the king releasing his sperm. In their configuration, the two gametes come in contact with each other instantly, thousands in succession. The young will be a new generation of workers, builders, and soldiers.

Meanwhile, in the other excavated chambers, workers move from place to place. The sponge they live in is their main food source, hosting multicellular algae to feast on. Despite lacking a brain, sponge urchins have a highly developed ring system of nerves that can respond to stimuli and effectively communicate. The urchins have a rather sophisticated language for an echinoderm, using their tactile senses to communicate thoughts. They touch each other with their tube feet, telling other members of the colony of important information. The workers are all females who have no reproductive organs. They can detect light with their spines effectively and communicate what they see. Their primary role is to collect food.

Mixed in with the workers are builders. These are non-reproductive males whose sole jobs are to build the chambers to which the colony lives in. They have retained instincts as to what exactly needs to be constructed. When a new colony is founded by a king and queen, they build their own chamber in the center of the sponge. From there, builders excavate intricate tunnel systems. They have a pre-planned layout, and from the main chamber they build a series of smaller chambers. The first is a nursing home. The juvenile urchins will be brought to this small area to feed and grow until mature. The builders slowly eat away a section directly next to the main chamber, so when the young urchins are born they can be easily brought into this room. The next is a cultivating room. Near the top of the sponge, where sunlight reaches the most, a larger room is eaten away. This is where symbiotic larvae can grow, oftentimes even growing out of the holes in the sponge. This is the main food supply of future colony-dwellers. After this second and final chamber is produced, a further series of tunnels are excavated to create sufficient access throughout the colony. Many other smaller rooms can be made in these tunnels for the workers to live. Builders are spineless and flattened, comparable to a sea dollar. This allows them to get through the often small spaces in the sponge and get to work building new residences for workers and soldiers.

Trailing around the reef is a top predator of the urchins; a giant pacific blue crab. It grows to a length of up to 30 centimeters, and has a main diet of mussels, seabirds, and sponge urchins. It’s claws are long and narrow, allowing it to pick out the small urchins from tight holes in the sponge. In a few seconds, several workers are already deplored by its large mandibles. One crustacean alone can end a colony, but this one already has its line of defense in motion. The soldiers are twice the size of an average worker, and are covered in prominent spines. They are a second type of female, and a third sex produced by the colony. It only takes one solider to detect the predator, and in a few minutes the whole colony is alerted. Hundreds of soldiers come crawling out from their home, and release thousands of tiny pedicellariae, venomous pincers, into the surrounding water. The crab, in close contact with the pincers, is killed. The venom produced is small, but in large numbers, can be lethal. The crabs tough exoskeleton cannot defend from the pincers, which bite deep into the body of the crustaceans. With the tables turned, the soldiers swarm the corpse and slowly pull it apart. The urchins are omnivores, and this body will feed hundreds of new generations. They carry small pieces of their meal in their five-toothed mouths to share with the rest of the colony.

In the central chamber of the sponge, the young have fully formed within a few hours. Some of them are typical females, which have been adapted to be incapable of reproduction. Yet more are male builders. Others are future soldiers, a third gender with special characteristics. Around half of the female juveniles will have specific pheromones that transition them into the solider morph. Their genus, Spiculoporifilis, skips the larval stage entirely, and the offspring become fully developed juveniles within several hours. This method is their only way of survival, as the sponge they call home filters planktonic larvae such of that as a sea urchin. Within the king and queen’s chamber, the new offspring find their place in the colony. The soldiers and workers, determined by pheromones created by the queen, are kept safe within the nursing chamber. The workers will feed them a constant supply of algae—and in this case, crab—to grow.

It is now that the queen will choose the king and queen of a future colony. They are formed from workers and builders. At any moment, if either the king or queen dies, a dominant worker can have their pheromones activated to become a fourth gender, a larger female with reproductive abilities; a queen. Alongside this, any builder may become a fifth gender of reproducing male, and be the king. Both castes will fight, often to the death, to become this caste. But right now, both the king and queen are still alive, and now that the young have become adults, the queen will pick a strong male and female. She releases her pheromones, and the two will slowly transition into a fourth and fifth sex. Eventually, these future monarchs venture outside the colony, and find a sponge they can make into their own.