Class: Chondrichthyes: Cartilaginous fishes

Characteristics:

1. Ventral mouth

2. Skin with placoid scales in most. (Describe placoid scales)

3. Skeleton composed of cartilage.

4. No swim bladder

General comments: Sharks may reach 15 yards in length. Shark fertilization is internal, some being live-bearers, with gestation periods up to 2 years which the longest of any vertebrate.

Dangerous to humans? Sharks are basically timid but they do attack humans. Most dangerous is great white shark which may be 6 yards long or even larger. Hammerhead shark and tiger shark also attack humans. During WW II there were reports on mass shark attack on victims of ship sinkings in tropical waters.

How locate prey? Shark eyes have no lids. Well equipped as predators although vision is not acute. Made up for by keen sense of smell which guide them to food and a lateral line system for locating objects & moving animals. Canal system extends along side of the body and over head; canal opens to surface at intervals; extremely sensitive to vibrations and currents in the water.

Jaws and teeth: Both upper and lower jaws have many sharp, triangular teeth, each backed up by series of developing teeth that replace worn or lost throughout life..

Reproduction of sharks: All dioecious. Internal fertilization with 3 variations of maternal support as follows:

1. Oviparous - egg layers; often deposit eggs in horny capsule called mermaid's purse which has tendrils which wrap around objects. Embryo nourished from the yolk for 6 to 9 months in some and as much as 2 years in one species before hatching into a miniature shark.

2. Ovoviviparous - retain eggs in the oviduct while they are nourished by the contents of the yolk sac until born. (No placenta)

3. Viviparous - a form of placenta develops through which embryo receive nourishment from maternal bloodstream.

Care for young: Regardless of type once eggs are laid or young born, all maternal care ceases.

The rays

Group includes skates, electric rays, stingrays and manta rays.

General traits: Most are bottom dwelling with greatly enlarged pectoral fins that are fused to head and used like wings in swimming.

Stingrays have slender whiplike tail armed with one or more saw-edged spines with venom glands at the base. Wounds terribly painful and heal slowly and complications.

Electric rays are sluggish fish with large electric organs on each side of the head. Produce high amp current which flows out into surrounding water; voltage rather low, but may be several kilowatts which is enough to stun prey or discourage predators. Ancient Egyptians used them for electrotherapy to treat diseases such as arthritis and gout.

Class Osteichthyes: Bony fishes Largest and most diverse group of vertebrates.

Characteristics of Osteichthyes

1. Bony skeleton.

2. Skin with embedded scales and mucous glands.

3. Fins both median and paired.

4. Respiration by gills covered with an operculum

5. Presence of swim bladder.

6. Two-chambered heart.

7. Dioecious (sex reversal in some)

Locomotion in water:

Appear to move fast to us; our judgment tempered by our own experience with water as viscous medium; most fish move 10 body length per second which is impressive compared to humans; actual speed is little over 6 miles per hour (trout). The larger the fish the faster it can swim. Trunk and tail muscles are what enable a fish to swim, not the fins which stabilize.

Swimming most economical form animal transport. Almost perfectly support by medium and do not have to expend energy to overcome gravity. (Think how heavy you feel when you come out of the swimming pool).

Role of the swim bladder. Sharks don't have swim bladder and so sink to bottom if they quit swimming. But in most bony fish the swim bladder serves as a flotation device. Those without swim bladder include tunas and bottom dwellers such as the flounder.

Archimedes' Principle is key to understanding (discuss it). Fish adjust volume of gas in its swim bladder to achieve neutral buoyancy at any given depth without muscular effort. If fish descends bladder gas is compressed so fish becomes heavier and tends to sink so that it must add gas. If fish swims up, gas in bladder expands, making fish lighter so it is in danger of shooting out of the water. Must take away gas to prevent this. (Discuss Boyles' Law).

Fish adjust gas volume in swim bladder in two ways:

1. Pneumatic duct - connects swim bladder to esophagus. These must come to surface and gulp air so are restricted to relatively shallow depths. Trout is an example of this.

2. Gas exchanged with the blood - Most fish secrete into and withdraw from the swim bladder from the blood. Depends on two specialized areas.

A. Gas gland - secretes gas into bladder from network of blood capillaries called the rete mirabile (Marvelous net). Gas gland secretes lactic acid which forces hemoglobin to release its load of oxygen so that it accumulates in the rete and builds to such a high pressure that oxygen diffuses into swim bladder.

B. Resorptive area called ovale - removes gas from the bladder.

3. Composition of the gas - varying amounts of nitrogen, carbon dioxide, argon, and even some carbon monoxide.

Respiration by bony fish

Gills richly supplied with blood vessels covered with a thin epidermis; located inside pharyngeal cavity and covered with movable flap, the operculum.

Functions of the operculum: protection to delicate gill filaments; streamlines the body; provides pumping system for moving water through mouth and across gills and out operculum.

Flow of water: Opposite to the flow of blood (countercurrent flow) which is best for getting the greatest possible amount of oxygen from the water; some can remove 85% of it.

Ram ventilation: Very active fish such as herring and mackerel have to continuously swim forward to force water across the gills; will suffocate if placed in an aquarium that restricts free swimming movements even if water is saturated with oxygen.

Fish which breathe air: A surprising number can live out of water for varying lengths of time by breathing air. Examples: African lungfish and Indian climbing perch.

Example #1: African Lungfish

size/weight/height: 6.5" when fully grown

adaptations/coloration: Heart: 4 chambers

Lungs: necessary to breathe air when water dries up

Life style: Burrowing: in soft mud �� curls up in chamber lined with mucus �� opening allows breathing through mouth �� metabolic rate slows �� energy comes from breakdown muscle tissue �� may survive maximum 4 yr. but usually only few months necessary

behavior: aggressive predators

reproduction/lifespan: Eggs: laid in nest; Newborn: larvae have external gills �� reabsorbed during metamorphosis; Parenting: males guard newly hatched young up to 2 mos.

diet: Wild: frogs & small fish

habitat/range: swamps �� small rivers �� West & South Africa

Example #2: Indian Climbing Perch: One of the best air breathers which spends most of its time on land near the water's edge, breathing air through special air chambers above the small gills. Also called walking fish, it is a small Asian freshwater fish noted for its ability to live and walk about out of water. Rather oblong, brownish or green, it grows to about 25 cm (10 inches). It lives in ponds and ditches and sometimes emerges for short periods, "walking" with a jerky motion, aided by its tail and by spines on the lower edges of its gill covers.

Feeding Behavior of bony fish

General comments: Despite what fishermen might think, fish devote more time and energy to eating or searching for food to eat than to anything else. There are five main feeding patterns:

1. Carnivores - most fish prey on animal foods from plankton to insect larvae to large vertebrates; some sea fish can eat victims nearly twice their own size; most cannot chew food since it would block current of water across the gills; usually swallow prey whole.

2. Herbivores - eat flower plants, algae, and grasses; few in number but essential in food chain, especially in freshwater rivers, lakes, and ponds that contain very little plankton. Example: carp.

3. Filter-feeders - eat abundant microorganisms of the sea; ranges from fish larvae to basking sharks; herringlike fish are best examples; strain food from water with sievelike gill rakers; most abundant of all fishes so important for carnivores; many freshwater fish feed this way.

4. Omnivores - feed on both plant and animal food. Example: bream.

5. Scavengers - feed on dead organic matter. Example catfish.

Digestion: Like other vertebrates except for a few fish that lack stomachs altogether; carnivores have very short intestine, but it is long and coiled in herbivorous forms; carp for example has intestine 9 times its body length.

Migration in bony fish - Example Pacific salmon

General comments: species include king, sockeye, silver, humpback, and chum. Each makes a single spawning run after which they die.

Life cycle: Migrate downstream from where hatched; ranges hundreds of miles over Pacific ocean for nearly 4 years; grows tremendously; return to spawn in headwaters of parent stream; guided upstream by characteristic odor; reach spawning beds of their parents where they were hatched; spawn and die; following spring newly hatched fry move downstream; are imprinted with distinctive odor of the steam from its vegetation and soil; imprint odors of other stream they pass while migrating downriver and use these in reverse order as a map when return as adults.

How do they find mouth of their stream from the vast ocean? Believed that do not require precise navigational ability but use ocean currents, temperature gradients, and food availability to reach the general coastal areas where "their" river is located. From this point, they navigate by their imprinted odor map.

Reproduction in bony fish

General comments: Most are oviparous, but a few (guppies and mollies are well-known example) are ovoviviparous.

Deep salt water fish produce large numbers of eggs and often come together in great schools without mating and release vast numbers of sperm and eggs into the water to survive or not. Example: a large female cod may release 4 to 6 million eggs at a single spawning and less than one in a million survive the numerous perils to reach maturity.

Marine near-shore fish typically make yolky eggs that don't float and are adhesive; some bury their eggs; many attach them to vegetation; some deposit them in nests; and some even incubate them in their mouths (example male banded jawfish which retrieves the female's spawn and incubates them until they hatch, leaving them for brief periods in the burrow only while he feeds).

Fresh-water fish almost invariably produce nonfloating eggs which they scatter among weeds or along the bottom. Perch is an example. No care given to the eggs.

Fish growth

General comments: Growth rate depends on temperature so some grow in summer and almost quit growing during winter; seasonal growth is shown in annual rings in the scales which enables on to determine a fish's age.

Grow whole lives: Unlike most animals fish get bigger the older they get; this is possible because the water offsets the pull of gravity.