The Fish: Part 1

• Genesis 1:26 “have dominion over the fish of the sea”
• Main food source
• Good form of fertilizer
• Used to make glue
• Entertainment (aquariums)
• Three classes of fish
• Class Agnatha
• Class Chondrichthyes
• Class Osteichthyes 

Class Osteichthyes

• Characteristics
• Most fish we think of are in this class.
• Some unusual members such as sea horses
• Typical flat, spindle-shaped body
• Others flat on the bottom, round like a pencil, can look like boxes, pyramids, or balls
• Usually less than 1 meter in length 
• Support and Movement
• Bony skeletons (Bone is cartilage hardened by mineral deposits)
• Vertebral column and skull of bone
• Ribs, pectoral, and pelvic girdles are often cartilage (but can be bone)
• Move through the water by the whipping motion of its body. 
• The power comes from wavy, muscular bands found in the fish’s trunk and tail.
• Fish are denser than water, so why do they float
• Swim bladder: thin-walled sac in the body cavity that enables fish to control its depth and maintain it without swimming.
• Gases diffuse in and out of the SB through the bloodstream. 
• Fins
• Used for guidance or slow swimming movement. 
• Unusual ones: 
• Sea robin uses its pectoral fins to “walk”
• Walking catfish can hold water in its gills, and walk across land for long distances
• Flying fish use them for gliding through the air
• Can go 55 km/h and up to 46 m

• Body Coverings
• Overlapping scales. The number does not change, just the size in proportion to their bodies.
• Some fish, like some catfish are scaleless. 
• Some catfish have bony plates. 
• Sturgeon have rows of large enamel-like plates.
• Special glands beneath the scales secrete mucus.
• Nearly waterproof coating
• Lubricates the fish for smoother movement through the water (reduces friction by 66%)
• Chromatophores: branched cells responsible for producing some of the pigment in fish scales.
• Some fish can change their color or pattern
• Change color in response to temp., diet, states of excitement, and physical condition.
• Digestion
• Feed on plankton, worms, insects, other fish, and even some mammals.
• Ingested through mouth (which often contains teeth to bite or hold prey)
• Fish mouths vary depending on their prey
• The direction of the mouth points to where it eats
• Food passes through a flexible throat, or pharynx and a short esophagus into a saclike stomach for storage.
• Digestion occurs in a short intestine.
• Short tubes called pyloric ceca (at junction of stomach and intestines) secrete digestive enzymes.
• Liver, near stomach, secretes bile to aid in digestion of fats.
• Most fish have a gallbladder (stores and exerts bile) and a pancreas (secretes other digestive enzymes).
• Indigestible food eliminated via the anus.

• Respiration
• Almost all fish have an operculum: a plate behind the eye on each side of the head.
• Under this is a series of gills 
• Gills consist of two rows of thin gill filaments on a band of cartilage called the gill arch.
• Each gill filament consists of a sac of thin epithelium and richly supplied with blood vessels.
• Gill rakers are a number of cartilage projections on the inner part of the gill arch. They keep food and debris from entering and clogging the gills.
• Circulation
• Circulation of blood enables oxygen and nutrients to reach every cell of the fish.
• 2 chambered heart
• Atrium receives the blood
• Ventricle pumps the blood to the arteries
• Single loop system in which the heart pumps blood only to the gills.

• Nervous System
• Major organs: brain & spinal cord.
• 10 Pairs of cranial nerves branch from the brain
• Many spinal nerves
• Large optic lobe: They could see well if it weren’t for the fact that they live in water.
• Great sense of smell. People taste water, fish smell it.
• Tongue for taste and touch
• No external ears, but can detect sound vibratos 
• Sensor hairs: changes in current (detect pressure and movement changes)
• Reproduction
• Some are ovoviviparous and even viviparous.
• But most are oviparous.
• Eggs are laid and then fertilized externally.
• Females in some species produce hundreds of thousands of eggs.
• When environmental conditions are right, the female spawns (lays eggs)
• The male then covers the eggs with milt (a milky fluid containing sperm)
• The embryo usually forms atop of food material called yolk.
• As the embryo grows, the yolk shrinks.
• Eventually a tiny larval fish escapes the egg and fends for itself.
• This process can take hours to 4 months.