Rhodoreef: In the story, this is the coral atoll in which the mer-city of Calliathron is located. This secluded tropical location is based on the Great Chagos Bank of the Indian Ocean—the largest coral atoll in the world. The protagonist's surname comes from this reef. The word rhodo means "rose-colored", and the reef is named so due to the abundance of pink, partly from pink coralline algae from the algal group known as Rhodophyta.

Sea cow: The Steller's sea cow was hunted to extinction in the 18th century. Its living relatives are the dugongs and manatees. The story features a new species of sea cow.

Sirenian: An order of fully aquatic, herbivorous mammals. Examples are dugongs and manatees.

Seagrass: These are the only flowering plants in the ocean. Marine grain can be harvested from these plants—similar to land grasses like rice and wheat. Merpeople have domesticated it, and they farm many varieties in paddy fields outside the city. Wild seagrass meadows exist as nature reserves.

Calliathron: The name of the mer-city. It's derived from Calliarthron, a genus of pink coralline algae.

DSV: Abbreviation of deep-submergence vehicle. These are used by research teams for deep sea exploration, while in Calliathron, they are used by authorized personnel when venturing into the deep.

Otter: Sea otters are not native to this area, but they are popular companion animals.

Earpiece: The merpeople of Calliathron possess a sophisticated digital device with sonar tech, which complement their natural echolocation capabilities.

Bioplastic: These are made from algae like kelp. Edible water containers, sauce sachets and more can be made from algal plastic. Ari Jónsson, a design student at the Iceland Academy of Arts, had created a biodegradable water bottle by combining red algae powder (agar) with water. New York designer Charlotte McCurdy has fashioned a water-resistant jacket from algae plastic, which captures existing CO2 from the atmosphere.

Peduncle: The tail stock of a merperson, from the waist to the two flukes. This is similar to a dolphin.

Ogi: The main digital device used by merpeople. It's similar to a phone, but takes the shape of a foldable hand fan. It has a paper-like screen and incorporates a headpiece. Ogi is a Japanese hand fan.

Air kiosk: These somewhat resemble phone kiosks. Since it's not practical for merpeople to keep surfacing for air, Calliathron has a city-wide oxygen supply system.

Coral atoll: A ring-shaped coral reef surrounding a body of water called a lagoon. The mer-city lies within this atoll.

Seagrass meadows: These are important ocean eco-systems, providing habitats for a plethora of marine life, including fish, squid, turtles and dugong. They are also massive carbon sinks.

Echolocate: This involves emitting a sound wave and listening to the echo when it bounces off objects. Animals like bats and cetaceans use echolocation to get a picture of their environments.

Cumulonimbus cloud: Dense clouds that are also referred to as thunderheads. They can cause severe weather.

Cay: A low island on the surface of a coral reef.

C-Net: This is the merpeople's Internet. It makes use of sound waves to transmit data instead of electromagnetic waves (radio waves). The "C" is a pun on sea.

C-Wave: This is similar to Wi-Fi, but it uses sound waves.

Porpoising: A speedy surface run that alternates between swimming and long jumps. This behavior is observed in small cetaceans like dolphins and porpoises, as well as penguins and seals.

Ultrasound: Sound with high-frequency waves. These are used by bats and cetaceans to echolocate. They are also used for medical imaging.

CGI: Short for computer-generated imagery. Merpeople also use this in movies, video games and simulations.

Spyhopping: Cetacean behavior that involves poking the head out of the water and holding that position by kicking tail flukes. It's similar to humans treading water.

Logging: This means floating on the surface like a log. This is also seen in resting cetaceans.

Youth Council: The youth organization of one of Calliathron's two political parties.

Regent: Merpeople have city-states with constitutional monarchies similar to ancient Greece. A regent is currently the head of state in Calliathron.

City-state: An independent sovereign city which serves as the center of political, economic, and cultural life. They have existed throughout the world in history, such as the city-states of ancient Greece (Athens, Sparta etc.). Modern day examples would be Singapore and Vatican City.

Grey, grubbing groupers: This is a nod at Captain Haddock's famous "blue, blistering barnacles in a thundering typhoon" in The Adventures of Tintin.

Oxy-hose: A long tube connected to a wall outlet. Merpeople can use it to suck in oxygen.

Amoeba: A single-celled, aquatic protozoan that moves by extending one part of itself to gradually pull the entire cell along.

Grapes: This refers to sea grapes, which are a traditional food in East Asia. Sea grapes are a variety of seaweed. They're also called "green caviar".

Lettuce: Sea lettuce is a type of green algae that's similar to lettuce. This is also eaten in the real world.

Merlingo: The most popular language-learning service in Calliathron.

Sinhalese: Sinhalese, also called Sinhala, is the most-spoken language in Sri Lanka. In the story world, it's the most-spoken language in the country known as Serendiva.

Serendiva: An island country in the story world. The name comes from Serendib, one of the many names Sri Lanka was known by in the past. Diva means "island" in Sinhalese.

Salmalwatte: A suburb in the capital of Serendiva. The watte, meaning "yard", is a common suffix in Sri Lankan place names. Salmal is used to refer to cannonball tree flowers (mal means "flowers").

Seren City: The capital city of Serendiva.

Asparagus: Sea asparagus, also called salicornia, grows in salt marshes, beaches and mangroves. They're coastal plants known as halophytes (sea-tolerant). These are eaten in various parts of the world.

Purslane: Sea purslane is a common coastal herb that resembles land purslane. They are a food source with high nutritional content.

Grain: This refers to marine grain. Merpeople have domesticated several varieties of seagrass similar to how people have done so to land grasses like wheat and rice. Marine grain has massive potential—wild eelgrass has even been used in a Michelin-starred restaurant in Spain. It has 50% more protein than rice.

Beetroot: The sea beet is the wild ancestor of common vegetables such as beetroot and Swiss chard. Due to the pleasant taste of its leaves, it's also known as wild spinach. Merpeople also source other saltwater-friendly land plants and farm them—like tomatoes, cabbage and broccoli.

Curry: Coconut milk is what's used for curries in Sri Lanka. The merpeople of Calliathron obtain their coconuts from the trees that grow on the cays, which are islands above the coral reef. They also have desserts like coconut milk ice-cream and puddings made of coconut milk and agar jelly.

Seitan: Seitan is a food made from gluten, the main protein of wheat (known as miàn jīn in Chinese and milgogi in Korean). It's often eaten as a meat alternative due to its meat-like properties. Merpeople make their seitan with marine grain protein.

Burble: Burble is the word used by merpeople to refer to a post made on Burbler, a social media application similar to Twitter.

Burbler: A widely-used social media platform in Calliathron. It's similar to Twitter.

Trawling: This is the practice of pulling a large, cone-shaped net through deep waters in order to catch fish. David Attenborough from Blue Planet 2:
"As far down as 6km, there are more species of coral in the deep than on shallow tropical reefs...They, like their shallow water relatives, provide homes for all kinds of other creatures...But today, their timeless world is being reduced to rubble. As over-fishing empties the surface waters of the seas, trawlers have started to ransack the deep. Now countless numbers of reefs that have flourished here for millennia lie in ruins."

Deep-sea reefs: These corals are also known as cold-water corals. They live in deep, darker regions of the ocean and create oases on the seafloor for many organisms.

Coral bleaching: Rising global temperatures cause corals to expel the algae living in their tissues. This makes them turn completely white. Bleached corals can survive, but they are more vulnerable to disease and starvation.

Carbon emissions: Human activity such as burning fossil fuels and farming animals release massive amounts of carbon dioxide and methane into the atmosphere. These gases trap heat, causing global temperatures to rise. Trees act as carbon sinks, but forests are being destroyed, which is aggravating the climate change crisis.

FYP: This is short for Final Year Project. It's a thesis that students are required to submit to graduate from high school.

Transporter Boards: These are smart boards that resemble surfboards. They are common holiday gear used by merpeople on the cays above the coral reef.

Humanology: The study of humans.

MerTech: This is a top research university in Calliathron. It's known for technology and scientific breakthroughs.

Hyperiid: In the story world, this is a web browser that allows anonymous communication. It's similar to the Tor browser. Hyperiid is the name of a type of marine amphipod that has adapted to be practically invisible.

Dark C-Net: The merpeople's equivalent of the Dark Web—online content not indexed by common search engines.

Casa Bava: A trendy restaurant in Calliathron. The name comes from kasbava (කැස්බෑවා), which means "turtle" in Sinhalese.

Grain milk: This is made with marine grain. It's similar to rice milk.

Spirulina: Blue-green algae that is available in the form of tablets or powder. It's considered a superfood.

Acerola cherry: A tropical fruit that merpeople grow in their vertical farms.

SBI: Short for State Bureau of Investigation. It's the state security and intelligence service of Calliathron, as well as its highest law enforcement agency.

Empire Ocean: A military real-time strategy game that is popular in Calliathron. The name comes from the game, Empire Earth.

Nacre: An iridescent substance lining the inside of the shells of some marine animals. It's also called mother-of-pearl, since pearls are composed of this material.

Sirye: An upscale suburb in Calliathron, where the rich and influential live. It's closer to the city center. Sirye comes from the word "cirri", which is used to refer to the tentacles of animals such as the nautilus.

Second Ocean War: A recent war waged between a number of mer-cities.

Merwiki: The merpeople's equivalent of Wikipedia.

Nautilus Chamber: The secret underground vault of Talmus Mora, astate official.

Golden spiral: A logarithmic spiral whose growth factor is φ, the golden ratio. A nautilus shell is one of the finest examples of a natural logarithmic spiral. In mathematics, the golden ratio is an irrational number, approximately equal to 1.618. It has been used in art and architecture for millennia. It's also called the divine proportion due to its occurrence in nature.

The Exorcism of Anemone Pearl: A popular horror movie in Calliathron. It's somewhat like The Exorcism of Emily Rose.

Stingrays: An elite commando and intel unit of the Calliathron army.

Flechette: Underwater guns make use of flechettes as ammunition. Flechettes are pointed metal projectiles.

CSU: Short for Calliathron State University.

Fluber Ultra: Merpeople have a ride-hailing app called Fluber, which is similar to Uber. The most luxurious vehicle class offered by this service is Fluber Ultra.

Tuk: A motorized trishaw that is common in Asian countries. Calliathron has a streamlined, aquatic version of it.

Chork: An eating utensil that combines chopsticks with a fork.

Algae lamp: This an existing lamp, which draws energy from batteries charged by the algae's photosynthesis. There is no electricity involved. Also, a microalgae lamp is apparently 150 times more efficient in absorbing CO2 than a tree.

Karmant: The industrial district of Calliathron, which resides at its northern border.

Little Angler: A compact two-seater deep-submergence vehicle (DSV) with an extendable robotic arm. It resembles an anglerfish, a deep-sea fish with a luminous lure above its mouth.

Mudskipper: A rude term to refer to pro-human merpeople. It comes from the amphibious fish that can live on land.

Insular shelf: The shelf surrounding an island (in this case, the atoll) is known as an insular shelf. This area is relatively shallow.

Brutalist architecture: This type of architecture incorporates rigid geometry and modular elements.

Sector Eleven: An industrial sector that contains Star Valley, where a mer-factory complex is located.

Seastar Factory: A factory that manufactures products made of volcanic glass.

Star Valley: A valley between guyots that is rich in minerals and volcanic glass. It's named after the many species of starfish that reside there.

Volcanic glass: Volcanic glass is any glassy rock formed from lava or magma. An example is obsidian.

Spanked with a stingray tail: An idiom that exists in Sri Lanka.

Motorpod: The underwater equivalent of a motorbike.

Guyot: An underwater volcanic mountain with a flat top, more than 200 m below the surface of the sea.

High seas: The open ocean away from territorial waters.

Twilight zone: Formally called the mesopelagic zone, this is the oceanic zone that lies at depths between 200 to 1000 meters. It's below the topmost area that receives sunlight. As the name suggests, the twilight zone appears dark and inhospitable, though over 90% of the ocean's fish reside there.

Marine snow: Organic debris that showers down to the deep ocean. It's an important food source for lifeforms that live there.

Midnight zone: This is what's called the bathypelagic or bathyl zone. It lies beneath the twilight zone. It extends from 1000 to 4000 meters. The wreck of the Titanic is here.

Seamount: An undersea mountain formed by volcanic activity.

Hydrothermal vent: These are geysers on the seafloor. Due to the buildup of minerals, they often form massive chimneys as high as 18 stories. Brimming with organisms packed close together, these vents can harbor as much life as rainforests.

Black smokers: This is one type of hydrothermal vent—the other being white smokers. These vents are typically found in the midnight zone. They emit black fluid high in sulfur content.

Siphonophore: Even though it looks like one creature, siphonophores are colony organisms that can grow to gargantuan proportions—as long as a blue whale. They are thin, transparent floaters with brilliant displays of bioluminescence.

Giant phantom jelly: This gigantic deep-sea jellyfish has rarely been seen, but it's believed to be widespread throughout the world. Its long, velvety oral arms can be as long as 10 meters.

Abyssal realm: This refers to the abyssal or abyssopelagic zone. This is the bottom-most part of the ocean at depths of 4000 to 6000 meters. This zone makes up the largest habitat in the world, covering 60% of Earth's surface and 83% of the total area of the ocean.

Scudline: This is an invented word. "Scud" is a common word for an amphipod (crustacean with no carapace). Amphipods are known as the "bees of the sea", because they do the bees' job by pollinating seagrass.

Callian: A citizen of Calliathron.

Mermish: The common tongue used by all merpeople. Although there are several languages spoken in Calliathron, Mermish is the most-used language.

Abyssal plains: Underwater plains in the deep ocean floor. They account for over 50% of the Earth's surface, but also happen to be the least explored places.

Witch's Cauldron: A brine pool near the Sea Witch's think room. It gets its name from the misty layer that coats it.

Brine lake: This is brine that has pooled in a seafloor depression. The result is the startling sight of a lake at the bottom of the ocean—complete with a shoreline.

Hagfish: Primitive, eel-like fish with a scary maw. They are known to produce copious amounts of slime.

Ridi: A common currency used across the ocean.

Cypod Terra: A land mobility device developed by Anuk and his team. It takes the form of a wheeled armchair with a brain-machine interface.

Neural chip: A brain implant that allows the direct transfer of information from brain to machine. In the story, this is used to mentally control land gear. It's inspired by Elon Musk's Neuralink.

Hulang: One of the oldest and largest nomadic mer-tribes. The name comes from the Sinhalese word for "wind" (හුළං).

Brain-machine interface: A direct communication pathway between the brain's electrical activity and an external device such as a computer or robotic limb.

Finburgini: A manufacturer of luxury vehicles in Calliathron. It's similar to Lamborghini.

Cypod X3: The newest Cypod model. It's collapsible.

Scalp block: Regional anesthesia applied to the nerves on the scalp. It's used for awake brain surgeries.

Cyborg: A being that is partly biological and partly machine. The word is a portmanteau of cybernetic and organism.

Kelp beer: Sugar kelp, eaten in Asia, is used to make seaweed beer.

Abyssopelagic: Relating to the abyssal zone of the ocean (depth of 4000 – 6000m). This is the deepest region, except for the trenches.

Dumbo octopus: An adorable deep-sea octopus. Due to its ear-like projections, it's named after Disney's elephant character.

Glass octopus: A transparent, alien-looking cephalopod inhabiting tropical and subtropical oceans worldwide.

Hydromedusa: A member of the largest group of cnidarians (jellyfish belong here). They look like jellyfish, but are smaller.

Vampire squid: A cephalopod found throughout temperate and tropical oceans in extreme deep sea conditions. It has a dark red body and blue eyes. Although it doesn't actually have ink sacs, it does produce a cloud of bioluminescent mucus that is similar.

Mangrove forest: Mangroves grow in saline or brackish water along tropical coasts. Mangrove forests are also called mangals. They are biodiversity hotspots and important ocean biomes. They also protect the coasts from erosion.

Littoral zone: An area of the ocean that is close to the shore.

Irrawaddy dolphin: Irrawaddy dolphins are a critically endangered species that inhabits coastal waters in Asia. They have no beak. Their faces are expressive, and they can squirt water out of their mouths. They are almost hunted to extinction by fishermen who use car batteries to electrocute them. They also die due to electrofishing, water pollution and fishing nets.