Lecture 18

Fish or fishes

Fish for same species. Fishes for different species

What is a fish

Has aquatic vertebrate, gills, appendages in the forms of fins and skin with scales. Not a monophyletic group.

Type of scales

Placoid scales, ganoid scales and cycloid and ctenoid scales

Placoid scales

Small, conical, toothlike structures. Typical of chondrichtyes. Modified to teeth in sharks.

Ganois scales

Diamon shaped. Early bony fishes and living gars.

Cycloid and ctenoid scales

Arranged in overlapping rows. Typical of teleost fish.

Locomotion in water

Propulsive mechanism: trunk and tail musculature. Movement achieved through undulation of the posterior end. A less flexible body plan is conducive to speed.

Buoyancy

Swim-bladder is a gas filled organ. Present in most pelagic (open sea) bony fishes. Absent in tuna, abyssal (very deep) fish and most bottom dwellers. Chondricthyes (sharks) have no swim bladder.

Respiration

Most use gills, they breather dissolved oxygen. Gills in the pharyngeal cavity. Gills covered with an operculum in bony fishes. Some fish have lungs and are able to breathe air. Gills composed of thin filaments covered with an epidermal membrane. Membrane folded repeatedly into plate-like lamellae, they contain main blood capillaries. Water is pumped continuously in the mouth, over the gills and out through the gill slits. Gas exchange occurs across thin walls of blood capillaries.

Osmotic regulation

Maintenance of balance of fluids. Freshwater fish are hyperosmotic regulators. Marine fish are hypoosmotic regulators

Hyperosmotic regulators

Greater salt concentration in fish than in surrounding water. Scale and mucous protect the fish, but water can enter across membranes (gills). Water pumped out by kidneys. Salt-absorbing cells in the gill move salt from water to blood.

Hypoosmotic regulators

Contain a smaller concentration of salt than surrounding water. Salt-secretory cells in the gill move salt out of body. Salt is voided with feces or excreted by the kidney.

3 major groups of fishes

Agnatha: hagfishes, lampreys.

Chondrichthyes: sharks, rays and chimaeras.

Osteichthyes: Bony fishes. Ray-finned fishes, teleosts, lobe-finned fishes, coelacanths, lungfishes

Agnatha - Hagfishes

No vertebral column or jaw, but they do have a head and skull(cranium). Marine group. Scavengers and predators, not parasitic. Poorly developed eyes, keenly developed sense of smell and touch. Produce slime as a defence mechanisms. They rasps bits of flesh from its prey. Keratinized plates on tongue. Can itself into a knot, passes the knot foward along its body until it is pressed securely against the side of its prey (for leverage).

Agnatha - Lamprey

Class petromyzontida. 20 species in North America. Half are parasitic. They use tooth-like plates of keratin for rasping a hole, through which fluids and tissues are sucked. Sea lamprey are invasive in the Great Lakes.

Chondrichthyes distinct features

Cartilaginous skeleton, bone entirely absent, derived feature as they descend from acestors with well-developed bone. Placoid scales, made of material similar to teeth. No swim bladder. Heterocercal tail: assymetrical, provides lift. Large livers with squalene, a particularly buoyant lipid.

Chondrichthyes - Rays

Skates, stingray, electric rays and manta rays. Dorsoventrally flattened bodies.

Chondrichthyes - Chimaeras

Also called rat fishes. Closest living relatives are sharks. Instead of teeth their jaws bear large flat plates.

Chondrichthyes - Sharks

Apex predator, most feed at the top of the marine food chain. Exceptions like whale shark and spined pigmy shark.

Sharks tail

Assymmetrical heterocercal tail. Vertebral colums turns upward and extends into the dorsal lobe of the tail. Provides lift as the shark swims. Must constantly move to avoid sinking even while sleeping.

Osteichthyes

Bony fishes. 2 groups: ray-finned fishes and lobe-finned fishes. Gas filled pouches called lungs if used for gas exchange or swim bladder is used for buoyancy.

Gills vs lungs

Gills evolved first. Ancestors of ray-finned and lobe-finned fishes had gills and lungs. In lobe: lungs were preserved as respiratory structures and some also retained gill. In ray: gills were preserved and lungs adapted into swim bladder.

Ray-finned fishes

Symmetrical homocercal tail allows for greater speed. Permitted by the swim bladder and improved control of buoyancy. Gills and swim bladder.

Lobe-finned fishes

Small group: 6 species of lungfishes and 2 species of coelacanths. Ancestor of tetrapods is found within an extinct lineage of this group. Lobe fins with a single bone that articulates with the rest of the body. Diphycercal tails. Lungs and gills.

Lungfishes

Breathe with gills and lungs. Can live out of water for extended periods of time. E.g. African lingfishes live in streambeds during the dry season. They remain dormant until rain returns.