The Fishes

Phylum Chordata

Subphylum Vertebrata

Cartilaginous fishes: Class Chondricthyes

Sharks, skates, and rays.

Characteristics:

1. Fins:

a. Caudal fin - tail or posterior fin.
b. Paired pectoral fins - ventral and posterior to external gill openings.
c. Paired pelvic fins - ventral and posterior to pectoral fins.
d. 2 dorsal median fins

2. Mouth: ventral with jaws; olfactory sacs do not open into the oral cavity.

3. Placoid scales on skin - scales that are composed of dentin and are covered with enamel-like substance (denticles sp.); modified placoid scales serve as teeth.

4. Endoskeleton entirely cartilaginous; notochord persists in adult; vertebral column is complete and separate from the notochord.

5. Digestive system of a "J"-shaped stomach, intestines, liver, gallbladder, and pancreas.

6. Circulatory system of several pairs of aortic arches; dorsal and ventral aorta, capillary and venous systems, hepatic (liver) portal and renal (kidneys) portal systems; 2-chambered heart; high []'s of urea and trimethylamine oxide in blood (nitrogenous waste products typical of carnivorous animals which take in more nitrogen than needed): protein (polymer of amino acids) -----> Single amino acids.

7. Gaseous respiration by means of 5 to 7 pairs of gills with separate, external gill slits.

8. NO SWIM BLADDER!

9. Brain of 2 olfactory lobes, 2 cerebral hemispheres, 2 optic lobes, a cerebellum, and a medulla oblongata; 10 pairs of cranial (skull) nerves. (page 644)
Cerebellum - controls balance and coordination - small region that lies just above the medulla.
medulla - most posterior region of the brain that is really a conical continuation of the spinal cord; together with the midbrain, makes up the "brain stem" or medulla oblongata. Regulates subconscious activities such as heartbeat, respiration, etc.

10. Sexes are separate; paired gonads (sex organs: ovaries in females and testes in males); reproductive ducts open into the cloaca (excretory structure); sex organs and excretory organs.

Sharks

Reproduction:

• Sperm from the testes of the male is carried by the wolffian ducts to the cloaca and is excreted during intercourse;
• The male uses specialized modifications in the pelvic fins (claspers) to deposit the sperm into the female oviduct;
• The oviduct retains unfertilized eggs from the ovary until they are fertilized;
• After fertilization, the zygotes develop within a modified portion of the oviduct which becomes the uterus;
• Within the uterus, a primitive placenta may form and attach developing embryos to the uterus wall and to the mophenus (coelom wall) until the shark is born; Following birth, the uterus and newborn shark are both released from the mother;
• The uterus may remain attached to the young shark for a short time;
• This type of reproduction where the embryo is attached to the mother's uterus wall is called viviparous reproduction;
• In some species of sharks, the mother retains the developing embryos within the uterus without attachment to the uterus wall ovoviviparous reproduction;
• Some other shark spp. will lay large, yolky eggs immediately after fertilization. this is called oviparous reproduction.;
• Finally, some sharks and rays deposit fertilized eggs in a capsule called a "mermaid's purse" which is often attached to seaweed. The young sharks or rays will later emerge from this structure.

Rays

• includes skates, electric rays, sawfishes, stingrays, eagle rays, and manta rays.
• All are bottom dwellers (benthic)
• have greatly-enforced pectoral fins that are fused to the head and used like wings for swimming;
• external gill openings are on the ventral side of the body; water (for breathing) is taken through the spiracle on the dorsal side;
• feed on mollusks, crustaceans, and occasionally, fish.
• Sting rays have a slender whip-like tail that is armed with one or more saw-edged spines with venom glands. The wounds are very painful and heal very slowly.
• Electric rays produce a high-amperage current by the simultaneous discharge of electricity. The voltage output is very low, but the power output may reach several kilowatts. This is enough to stun prey or ward off predators.

Chimaeras

• small subclass of cartilaginous fishes such as ratfish, rabbitfish, spookfish, and ghostfish;
• 25 spp. today; anatomically appear as an odd mixture of shark-like and bony fish features;
• instead of a toothed mouth, their jaws bear large, flat plates;
• The upper jaw is completely fused to the cranium, which is very unusual in fish; eat seaweed, mollusks, echinoderms, crustaceans, and other small fish; not a commercial sp. and are seldom caught; despite their ugly appearance, they are rather colorful.

Bony fishes: Class Osteichthyes

General Characteristics:

1. Bony skeleton (mostly); most primitive or simple skeleton in the vertebrate animals; numerous vertebra; notochord may persist; post anal tail present in adults.

2. 3 types of dermal scales:

a. Ganoid scales - diamond-shaped; simple scales with layers of silvery enamel on upper surface and bone on the lower surface.
b. Cycloid scales - thin, flexible scales of dermal material arranged in roughly, concentric circles with smooth margins.
c. Ctenoid scales thin, overlapping dermal scales of advanced bony fishes; exposed posterior margins are rough with fine, toothlike spines.
d. some without scales.

No placoid scales on any bony fish.

3. Fins both singly and a median; also paired lateral fins. Fins have rays of cartilage or bone.

4. Mouth with many teeth and movable jaws (some toothless fish do exist.). Olfactory sacs paired and may or may not open into the oral cavity.

5. Respiration by gills supported by bony gill arches and covered by a common operculum - opening for water passage to outside from internal gill cavity.

6. Swim bladder usually present with or without a duct connected to the pharynx.

7. Circulation system of a 2-chambered heart, arterial and venous vessels, and usually four pairs of aortic arches; blood contains nucleated red cells.

8. Nervous system of a brain with small olfactory lobes, cerebrum. Large optic lobes and cerebellum; 10 pairs of cranial nerves.

9. Sexes separate (sex reversal in some spp.); gonads are paired; fertilization usually external; larval forms may differ greatly from adults.

Classification:

Three distinct groups:

1. Lobe-finned fishes:

• have fins and bony rays and lobes of bone-less areas lacking rays.
• Simple, primitive type of fishes:
• The coelacanth is a simple bony, lobe-fin fish which was once thought to be extinct, but was found living off the coast of south Africa in 1938.
  

2. Lungfishes:

• Simple bony fishes represented today by only 3 genera;
• resemble lobe-fins (have lungs and lobed fins).
• Although they have gills and lungs, they cannot survive outside of water;
• can come to the surface and gulp air. Another sp. actually can breathe air for long periods of time;
• can burrow into mud and form a cocoon to survive the summer in tropical regions where some regions dry p. The medial fins are fused to make one large, continuous fin.
  

3. Ray-finned fishes:

• Large (21,000 sp.) group of bony fishes.
• Most common and familiar type of bony fish.
• Fins entirely composed and supported by rays; no lobes present.

Structural and Functional adaptations:

1) Locomotion in water - driven by powerful trunk and tail muscle that flex and extend in a "zigzag" pattern.

2) Buoyancy and the swim bladder:

Chondricthyes

• All fishes are slightly "heavier" (more dense) than water because their skeletons and other tissues contain heavy elements that are present only in trace amounts in natural waters.
• To keep from sinking, sharks must always keep swimming and moving forward in the water.
• The asymmetrical (heterocercal) tail (caudal fin) of sharks provides the necessary tail lift as it sweeps back and forth in the water.
• The broad shark head and flat pectoral fins act as angled planes to provide head lift.
• Sharks also have great buoyancy by having very large, fatty, livers composed of low density squalene(Squalus sp.) = fatty (oily) material that helps provide buoyancy.

Osteichthyes

• In bony fishes, buoyancy is provided by an efficient swim bladder - a gas-filled organ that regulates the buoyancy of the fish body.
• The amount of gas (oxygen diverted from gills or lungs (if present)) regulates the degree of buoyancy and the depth at which the fish remains.
• Can change depth (water level by adjusting the amount of gas in swim bladder.
• Some fish have to "gulp" air from above the water surface to fill the swim bladder. e.g. trout.
• The gulped air is forced into the swim bladder through a pneumatic duct which connects the swim bladder to the esophagus. The more advanced fishes have a gas gland which secretes gas into the swim bladder.
  

3)Fish Respiration:

A)Gills - respiratory organs which are composed of their filaments covered with a thin epidermal membrane that is folded repeatedly into platelike gill lamellae - tissue richly supplied with blood vessels for gas exchange. The gills are located inside the pharyngeal cavity and are covered by the operculum. Flap of tissue which covers the gill - protects the gills.

B)Lungs - allow some fish to breathe air, but are not developed enough to provide 100% of the fish's respiratory needs. Most of the oxygen that fish breathe is provided by the dissolved oxygen in the water and therefore must be respired by gills.

4)Osmotic regulation: Fresh water is very dilute with respect to [salt] (salinity). The fresh water salinity is much lower than that of the blood of freshwater fishes.
Water: tends to enter the cells osmotically and salt is lost due to diffusion. Most of the water intake and salt loss occurs across the gills.

Classification Osteichthyes

Subclass Actinopterygii, ray-finned fishes

Superorder Chondrostei, primitive ray-finned fishes

Superorder Neopterigii (Teleostei), modern bony fishes

Subclass Sarcopterygii, fleshy-finned (lobe-finned) fishes, e.g. coelacanth