Every fifty years, the merfolk hold a Choosing, in which is selected a new Between, the mermaid who will care for the Seadragons. Without proper care, Seadragons will be unable to maintain the Balance, which preserves all life undersea. But Seadragons are now dying out. There are only two left, and they haven't laid an egg in 500 years. Elan, a young woman from a race of merfolk called Merras, wants to be the next Between, but no Merra has been chosen in centuries.
Mermaid's Song contains examples of:
- Balance Between Good and Evil: Most species find it easier to be evil than to be good. The Seadragons are the only species that's more good than evil, so they are crucially important to maintaining the Balance.
- Bottomless Pits: The last trial in the Choosing requires maids to swim into a pit that leads to a series of caverns that are so far down that the pressure makes it hard to breathe and threatens to rupture their ears. Maids with Merra ancestry can hear voices in the pit, telling them to swim down to their deaths. A few Merramogs (people with Merra and Ghrismog ancestry) succumb to the voices. Elan has organized a group of Merras to sing at the exit hole, and follows their voices to safety.
- By the Hair: As Elan is swimming out of the pit, her ruthless rival Kili grabs her by the hair in an attempt to drag her down. After Elan fights her way to safety, Kili lies that Elan almost died and Kili had to pull her to safety by the hair.
- Child by Rape: Kagor was conceived when the cruel Ghrismog Groff raped the Merra Tia as punishment for not obeying him. When he learned she was pregnant, he had her imprisoned so she couldn't go to a healer for an abortion. Shortly before Kagor's birth, Tia escaped and fled into the open ocean to raise him in peace. Groff tracked her down almost three years later and kidnapped Kagor to raise as his son, but it soon became apparent that Kagor had no feeling for anyone, and he grew into a remorseless hunter who takes great delight in watching the sharks he controls rip his victims to shreds. Groff wonders if the violence of his conception and Tia's hatred for Groff caused him to turn out that way.
- Death by Childbirth: Elan's adopted mother, the mogmaid Maadu, is a few days away from giving birth, so Elan searches for effron, a rare plant that can help a maid survive childbirth. But when she arrives at the caverns with the effron, Maadu's biological daughter Tsui thinks it's some Merra poison, rips it up, and scatters it on the currents. Elan can't find more before Maadu goes into labor. She and her child both die.
- Eye Scream: While fighting with Kagor and his sharks, Elan puts one shark's eyes out with her thumbs.
- Fantastic Racism: There are two main races of merfolk - Ghrismogs and Merras. Ghrismogs are more fish-like in appearance, and are more practical-minded than the artistic Merras. Centuries ago the two races lived in harmony, until another race called the Olaki arrived, plotting to take over the Caverns because of the abundance of food in the region. The Ghrismogs saw the danger, but the Merras had no conception of greed, and continued to trust the Olaki until it was too late. The Olaki massacred most of the Merras and enslaved the Ghrismogs, subjecting the males to Crippling Castration so that for a generation, all Ghrismog children were half-Olaki. Eventually the Ghrismogs rebelled and overthrew the Olaki, but ever since then, the Ghrismogs have hated the Merras and blamed them for the Olaki invasion. Merras are so mistreated that although once all Betweens were Merras, social pressures have prevented them from participating in a Choosing ever since.
- Giant Squid: While searching for effron, Elan is attacked by a giant squid that drags her to his lair. She sings to him, causing him to relax and release her. The squid, amazed to hear a Merra singing that song for the first time in centuries, teaches Elan to cross-speak. He becomes her friend and teacher, and helps her train for the Choosing.
- Humans by Any Other Name: "Split-tails."
- Left for Dead: After Groff stole Kagor from Tia, he had his guards put her eyes out with urchin spines and leave her to wander the open ocean until some predator ate her. However, she managed to make it back to the Caverns, where she lived as a hermit in the abandoned caves.
- Lighting Bug: An aquatic version. Ghrismogs use caged bioluminescent fish to light their caves.
- Living Lie Detector: Some people have the ability to taste others' emotions on the water, including whether a person is lying.
- Mermaid Problem: Mermaids mate via a hidden vertical slit below their navels.
- My Significance Sense Is Tingling: After the Seadragon goes berserk and kills scores of merfolk, several characters feel the resulting upset in the Balance from miles away.
- Nuclear Mutant: After the Seadragon reaches his Despair Event Horizon, Elan finds him curled up around a wrecked submarine full of radioactive materials, inhabited by creatures including crabs with too many legs, fish with eyes in the wrong places, and an eel with two heads.
- Post-Victory Collapse: In the second trial of the Choosing, two killer whales are forced to attack each other, with maids between them. The goal is for the maids to use their magic to protect themselves from the whales. Elan swims directly between the whales and sings to them until they both swim away. Then, exhausted, she faints and sinks to the seafloor.
- The Power of Hate: When the Seadragon has completely lost the will to live, Elan's hatred towards Kagor is so strong that the Seadragon uses the last of his strength to bite Kagor in half.
- Psychic Link: When the Seadragon goes on a rampage, Elan completely opens her mind to his in order to calm him down. From then on, the two can feel each other's emotions. The Seadragon's sorrow and despair is so strong that Elan almost loses the will to live, while Tia tells her that the link with her mind is the only way the Seadragon can feel any peace.
- Seeing Through Another's Eyes: When Kagor and his sharks attack Dria, Elan, who is some distance away, looks through Dria's eyes in order to give her better cross-spoken instructions.
- Strange-Syntax Speaker: The dolphins all use subject-object-verb order.
- Synchronized Swarming: The first trial in the Choosing requires the mermaids to control schools of lightfish and get them to swim in certain ways. Kili has her fish form a mantle around her. Elan, however, wins the competition with her fish's complex, constantly changing patterns, and is so good at it that as less skilled maids lose control of their fish, they join Elan's school.
- Tears of Joy: Elan searches for the Seadragon, listening for his song, even though he's fallen silent. When she finally hears him again, she cries.
- Telepathy: Merras can use cross-speak, a form of telepathy that allows them to converse with both each other and many kinds of sea creatures from miles away.
- Unishment: When a mogmaid is attacked by an eel, Elan is able to calm the eel and get it to release her. The other Ghrismogs accuse Elan of causing the attack in the first place and have her sent to a Dispenser, a judge who deals with minor crimes. The Dispenser knows Elan did nothing wrong, but because she sang to the eel, which Merra are forbidden from doing, he sentences her to swim alone for a day in the open ocean beyond the protection of the reefs, knowing the punishment won't bother her.Dispenser: You are afraid of the open sea?
Elan: No, Sabat.
Dispenser: Say yes.
Elan: Yes. - Unscaled Merfolk: Merras are part human and part dolphin.
