Every fact in the three fish lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the facts are simplified or less certain: the number of fish species is an estimate: the textbook we use gives about 31,000 [1], while an older NOAA page quotes 20,000 with perhaps as many again still to be counted [5]. Not every fish has a swim bladder, and fish can change the amount of gas in it, so the 'gas halves at 10 m' calculation in lesson 3 is a simplified model that treats the gas as an ideal gas at constant temperature and ignores the stretchy walls of the bladder [10]. The pressure rule 'about one extra atmosphere per 10 m' is for sea water and is rounded [11]. The divers and syringe models in the experiments show the physics, not a real fish.
All sources
[1] Biology 2e, 29.2 Fishes
OpenStax (Rice University) · Reliability: High
Type: Peer-reviewed open university textbook
Modern fishes include an estimated 31,000 species, by far the most of all clades within the vertebrates. Living jawless fishes (hagfishes and lampreys) lack jaws and paired fins. Chondrichthyes (sharks, rays and skates) are jawed fishes with paired fins and a skeleton of cartilage; electroreceptors called ampullae of Lorenzini let sharks detect electromagnetic fields, and the lateral line detects movement and vibration in the water. Osteichthyes, the bony fishes, have a bony skeleton; their skin is often covered by overlapping scales. Ray-finned fishes have fins supported by a fan of slender bones; lobe-finned fishes have fleshy, lobed fins. The swim bladder helps to control the buoyancy of the fish. All bony fishes use gills to breathe; water is drawn over gills in chambers covered by a protective, muscular flap (the operculum).
South Carolina Department of Natural Resources · Reliability: High
Type: State government natural resources agency (education page)
Fish are vertebrates: they have a backbone. They are cold-blooded animals that lay eggs and are well suited to living in water. The caudal (tail) fin propels the fish through the water. Scales protect the fish from injury. The lateral line senses vibrations or movements in the water. The swim bladder can inflate and deflate with air. Gills are the feathery tissue that lets fish breathe in water; the operculum is the bony flap that protects the gills. Fish can detect colours and see a short distance.
Arizona Game and Fish Department · Reliability: High
Type: State government wildlife agency (education sheet)
Fish are aquatic animals: they live in water. All fish have a backbone. Fish are ectotherms (cold-blooded): they cannot control their own body temperature. Instead of lungs, fish breathe through gills, small slits behind the head. Fins help fish swim; with their fins fish can move forward, backward, up, down and sideways. Scales cover the body and protect it from injury and disease.
American Museum of Natural History · Reliability: High
Type: Natural history museum (education page)
Fish need oxygen dissolved in water. Water holds much less oxygen than air, so fish must force large volumes of water over their gills. Gills contain many filaments, each with thousands of tiny folds called lamellae; dissolved oxygen passes through thin membranes in the lamellae into the fish's blood. Whales and other ocean mammals have no gills: they come to the surface to breathe air with their lungs.
The whale shark is the largest fish; it grows to more than 50 feet (about 15 m) long. A series of fine rings appears in a scale as it grows; scientists estimate a fish's age by counting the growth rings on scales or ear bones. Fish do not chew, because chewing would block the flow of water over the gills. The most often quoted estimate of fish species is 20,000, with perhaps as many again.
U.S. Geological Survey, Water Science School · Reliability: High
Type: National science agency (education page)
Cold water can hold more dissolved oxygen than warm water; oxygen levels are high in winter and early spring and usually at a seasonal low in summer. Surface water in summer typically has more than 8 mg/L of dissolved oxygen; hypoxia is when oxygen is below 2 mg/L. Dissolved oxygen is breathed by fish and zooplankton and they need it to survive.
Unlike fish, which breathe through gills, dolphins breathe air using lungs, through a blowhole on top of the head. Dolphins are warm-blooded mammals that give birth to live young and feed them milk.
Encyclopaedia Britannica · Reliability: Medium-high
Type: Edited encyclopedia
An ectotherm is any so-called cold-blooded animal whose regulation of body temperature depends on external sources, such as sunlight or a heated rock. Ectotherms include the fishes, amphibians, reptiles and invertebrates.
Type: National space and aeronautics agency (education page)
The buoyant force on a submerged object equals the weight of the fluid displaced by the object. An object's apparent weight in water is its weight minus the buoyant force. The density of water is 1.0 g/cm³.
At sea level the air presses on us at 14.7 pounds per square inch (1 bar). For every 33 feet (10.06 m) you go down in the sea, the pressure increases by one atmosphere.