Chapter 12 - Marine FIshes

Evolution and Diversity of Fish

Fish are an ancient group of animals, with origins dating back more than 500 million years.
There are more than 25,000 species of fish found worldwide, exhibiting a remarkable variety of shapes, sizes, and colors.

Protochordates, the 'first chordates', exhibit structures found in vertebrate embryos such as a hollow dorsal nerve cord and a notochord.
Sea squirts, or tunicates, are covered by a clear, tough membrane and possess incurrent and excurrent siphons for water and waste exchange. They also have both male and female reproductive organs.
Lancelets are tiny, transparent fish-like animals that filter plankton from the water and retain dorsal nerve cords, notochords, and gill slits.
Acorn worms, superficially resembling invertebrates, have a dorsal nerve cord and gill slits, but no notochord. They burrow in the sand and represent an evolutionary link between invertebrates and vertebrates.

Jawless fishes, such as lampreys and hagfish, dominated the ocean 100 million years ago and are the most primitive of vertebrates.
The sea lamprey, a jawless fish, inhabits estuaries and feeds by attaching to living fish and sucking out their blood and tissues. It has no scales.
Cartilaginous fishes, such as sharks, skates, and rays, have a skeleton composed mostly of cartilage and exhibit characteristics like placoid scales and visible gill slits for breathing.

Cartilaginous fishes have fewer than 700 species and exhibit unique characteristics such as using spiracles to breathe, having more rigid fins, and relying on lift provided by their pectoral fins to prevent sinking.
Rays and skates are bottom-dwellers, with stingrays being well-concealed in the sand and possessing a sharp spine near the base of their tail.
Sharks, with over 350 known species, vary greatly in size and exhibit structures and behaviors that have allowed them to survive for over 300 million years.

Bony fish, belonging to class Osteichthyes, have adapted well to life in the sea with protective scales, a single band on the scales representing about one year's growth, and gills located in the head region covered by an operculum.
They breathe by water containing dissolved oxygen passing over their gills, and their fins enable them to move in all directions while stabilizing their bodies.
Bony fish maintain buoyancy using an internal gas-filled organ called a swim bladder and exhibit various feeding and digestive adaptations.

Most bony fish have separate sexes and external fertilization, while egg-laying fish produce a large number of eggs to ensure survival of some offspring.
Fish with internal fertilization either lay fertilized eggs in casings or live-bear young fish.

Fish exhibit a well-developed nervous system, good sense of hearing, and sensitivity to changes in the environment through their lateral line organ and swim bladder.
They also have an excellent sense of smell and vision, allowing them to carry out responses and detect prey or predators.