How to Classify Fish, Mammals, and Reptiles

Fish and Other Aquatic Life

Fish are aquatic animals that have fins and gills and are cold-blooded, their body temperatures affected by environment. This includes eels, actual fish, lampreys, rays, and sharks, but not some animals that have the word “fish” in their name, like jellyfish and starfish. Technically it excludes dolphin and whales, which are mammals. Some can breathe air just like us and can survive several days without suffocating. Fish may not hear well and instead depend on sensing motion, but they have excellent color vision, taste, and smell. A number of small fish have developed the ability to glide through the air for over a hundred feet, typically to evade predators.

Some fish form schools or shoals, which are slightly different. In a school, fish move at the same speed and direction, being tightly synchronized as if of one mind. By contrast, shoals are more loosely organized, the fish independent but staying close. Like birds, they are sometimes assigned religious symbolism, which we can leverage. For example, if there was once a drought on land and fish had allowed people to survive, they might be revered.

As world builders, our primary use for fish is as food during dining scenes unless we want one to threaten humanoid species that enter the water or sail upon it. People can be stung, paralyzed, poisoned, and outright killed by sea life, whether immediately or in time. We can have someone meet their end by drowning, by being swallowed whole, or most dramatically, by being bitten to death. Piranha, sharks, and other animals with significant teeth are good models for threatening sea life.

Only the largest marine life is likely to threaten or destroy a ship-of-the-line, but giant squid and octopus have been done. You’ll want to invent something unique, either a single large creature or a coordinated group of smaller ones. If we invent some unusually smart sea life, maybe they’ll have another agenda or just be attracted to pretty things, like those armored knights seen upon deck. This comes close to inventing a monster (see chapter five).


Mammals are the largest and smartest animals, generally, though this can be different on our world. Most have four legs, though some have adaptations extreme enough that we may not realize they’re a mammal, such as whales and dolphins.

Other sea mammals include otters, polar bears, and seals, and while some aquatic mammals can survive outside of water, others will die. If inventing one, this is something we must decide on. All of them depend on the sea for food and can submerge far longer than humans. Many must come on land to breed. Either blubber, large size, or waterproof fur can be used to retain heat. Large animals use their weight to stay down where their food is (on the bottom) while lighter animals have food that is more likely to be nearer the surface. Habitat is either open sea or coastal, with the latter including kelp beds, beaches, reefs, and even rocky cliffs. Sea mammals are hunted not only for food and fur but a substance like spermaceti, which is used to make wax. These give us product ideas.

Other mammals have developed aerial locomotion. Cats have a limited ability to essentially parachute themselves to slow their fall.  Tree-dwelling animals can glide between tall trees that are spaced far apart. Bats can outright fly. These traits can be used in our work.

Living in trees poses challenges that cause adaptations, which include far better balance and ability to grip a vertical surface to prevent pitching backwards or slippage. Gaps between branches must be overcome by reaching between, jumping, or gliding. Longer limbs, claws, and a prehensile tail (i.e., one that can grab things) aid these.

Walking is a distinguishing trait that comes in three types. Primates (including humans) and apes are among those with plantigrade locomotion, meaning the toes and metatarsal bones (those between toe and arch) are on the ground, along with the heel. The disadvantage is speed, caused partly by shorter, thicker legs. The advantage is being more weight bearing. Digitigrade animals like cats and dogs walk on their toes and are faster and quieter as a result. Then there’s ungulate locomotion, meaning walking on the tips of the toes, which sounds painful to us, but these animals have a hoof that is perpetually growing and wearing down like our nails; these animals are usually herbivores, are faster, and often have antlers (on males).

Most mammals give live birth and nurse with milk, but a few lay eggs. Communal raising of young is the norm with pack animals in particular, unlike with non-mammals. Mammals are warm-blooded, meaning the body regulates temperature instead of relying upon ambient air or water to do so; the ability has limits, which is why mammals can die from heat stroke or hypothermia. Being warm-blooded causes higher metabolism and therefore greater need for food. Mammals can replace a tooth once or never, but we could always decide that our mammal can replace teeth every time one is lost, like sharks.

Lastly, mammals are used for food, leather, wool, experiments, pets, transportation, and entertainment, discussed in a subsequent section of this chapter.


Reptiles include turtles, crocodiles, snakes, and lizards. They either have four legs or none. Cold-blooded, they cannot control body heat without environmental help; while some have adapted to extreme temperatures, most stay in water or seek sun or shade as needed. A slow metabolism means less food is needed than for a mammal of the same size (as much as ninety percent less), and some can go a half year without food, though this means they aren’t moving much; movement burns energy that must be replaced with a meal. Reptiles can dominate areas with little food, because there isn’t enough to sustain birds and mammals. All of this also means reptiles don’t do long chases and have a sit-and-wait strategy as predators, but this doesn’t have to be true of ones we invent. Some small reptiles can glide through the air.

All Earth reptiles have lungs, but some have permeable skin, too, suggesting we can create a reptile without lungs if desired. Reptiles have watertight, horny skin/scales so they can live on land, unlike amphibians, but it isn’t thick like mammals and can’t be used for leather except in decorative fashion (as opposed to for protection or clothing). Most are carnivores or eat insects, but herbivores exist. Some reptiles consume rocks to help with digestion; such a stone is called a gastrolith. Reptiles are less intelligent than mammals and birds due to small brain size, but we can invent more intelligent and therefore more frightening ones. Most are diurnal (i.e., active during the day) but some that operate at night have a kind of thermal sight that we can make more extreme and useful, especially if we invent a humanoid species that’s reptilian.

Reptiles usually produce sexually but some are asexual (where’s the fun in that?). Genitals are stored within the body. Some do live birth while others lay hard or leathery eggs that almost immediately hatch.

Smaller reptiles rely on avoidance to not become a meal of birds or other reptiles. As such, they hide within underbrush and can often camouflage themselves, whether basic skin color does this or they can change it; the ability to lie still for long periods aids this. If unable to flee, they may hiss or make noise, like the rattlesnake; others make themselves appear bigger, like the cobra. Some are brightly colored to indicate they are venomous. Some actually play dead. Others can detach their tail and run away, the tail still wiggling for up to twenty minutes to distract their predator from their fleeing; some of these tails are brightly colored to encourage an attack there, but regardless, the tails grow back but not usually to the same length, and may be discolored compared to the original.