Biology: Fishes

Fishes

• Fishes are aquatic vertebrates with gills, appendages in the form of fins, and skin with scales of dermal origin.
• The modern concept of a fish is used for convenience, not as a taxonomic unit.
• Fishes do not form a monophyletic group, as the ancestor of land vertebrates (tetrapods) is found within one group of fishes (sarcopterygians).

Major Groups of Living Fishes

• Five major groups of living fishes:
  • Hagfishes
  • Lampreys
  • Cartilaginous fishes (sharks, rays, and chimaeras)
  • Ray-finned fishes
  • Lobe-finned fishes

Evolution of Fishes

• Fishes first appeared in the early Paleozoic era, about 550 million years ago.
• Early vertebrates were an assembly of jawless agnathan fishes.
• One group of ostracoderms gave rise to the jawed gnathostomes.

Hagfishes and Lampreys

• Living jawless fishes include hagfishes and lampreys, with approximately 119 species.
• Both groups lack jaws, internal ossification, scales, and paired fins, and share porelike gill openings and an eel-like body form.
• Hagfishes and lampreys diverged from each other at least 450 million years ago.

Cartilaginous Fishes

• Cartilaginous fishes (Chondrichthyes) include sharks, rays, and chimaeras.
• They lost the heavy dermal armor of early jawed fishes and adopted cartilage for the endoskeleton.
• Most are active predators.

Ray-Finned and Lobe-Finned Fishes

• Ray-finned fishes (Actinopterygii) include nearly all familiar bony fishes.
• Lobe-finned fishes (Sarcopterygii) include lungfishes and coelacanths.
• Lobe-finned fishes are remnants of important lineages that flourished in the Devonian period.### Fish Evolution and Characteristics
• Widened areas in the lines of descent indicate periods of adaptive diversification and relative number of species in each group.
• The lobe-finned fishes (sarcopterygians) flourished in the Devonian period, but declined and are today represented by only four surviving genera (lungfishes and coelacanths).

Hagfishes (Myxini)

• Hagfishes have a distinctive body shape, with a long larval stage as an ammocoete, a disk-like buccal funnel, and slime glands.
• They have a low-pressure circulatory system served by three accessory hearts in addition to the main heart.
• Teeth are formed from keratin and are used to rasp flesh from prey.
• Hagfishes have a unique ability to generate enormous quantities of slime, which makes them difficult to grasp.
• They have a distinctive way of feeding, using their tongues to grasp and tear flesh from prey.

Lampreys (Petromyzontida)

• Lampreys are characterized by their ability to grasp a stone with their mouth to hold their position in current.
• They have a sucker-like mouth with sharp keratinized teeth, which they use to attach to fish and rasp through their flesh.
• Parasitic lampreys migrate to the sea or remain in freshwater, where they attach to fish and consume their body fluids or flesh.
• Brook lampreys are a type of lamprey that never feeds after metamorphosis and dies soon after spawning.
• Most lampreys ascend freshwater streams to breed, and males begin building a nest and are joined later by females.

Characteristics of Fish Groups

• Cyclostomata (hagfishes and lampreys) have a slender, eel-like body with no paired appendages, and their skin is naked or has keratinized plates.
• Chondrichthyes (sharks and rays) have a fusiform or dorsoventrally compressed body, with a caudal fin that is heterocercal or diphycercal.
• Actinopterygii (ray-finned fishes) have a caudal fin that is heterocercal or homocercal, and paired pectoral and pelvic fins supported by bony rays.
• Sarcopterygii (lobe-finned fishes) have a caudal fin that is heterocercal or diphycercal, and paired pectoral and pelvic fins supported by stout bones and bony rays.

Fish Characteristics

• Hagfishes have a brain that is small but distinct, with 10 pairs of cranial nerves, and eyes that are poorly developed.

• Lampreys have a brain that is moderately developed, with 10 pairs of cranial nerves, and eyes that are moderately developed.

• Fish have a range of senses, including vision, hearing, smell, and vibration reception.

• Fertilization in fish is usually external, but can be internal in some species.

• Fish have a range of reproductive strategies, including oviparity, viviparity, and ovoviviparity.

• The excretory system in fish varies, with some groups having pronephric and mesonephric kidneys, while others have opisthonephric kidneys.

• Fish have a range of gill structures, with some groups having five to seven pairs of gills, while others have a single pair of gills.### Heart Structure

• Heart with a sinus venosus, atrium, and ventricle; single circulation

• Heart with a sinus venosus, atrium, and ventricle; single circulation; nucleated red blood cells

• Heart with a sinus venosus, atrium, and ventricle; single circulation; nucleated red blood cells; pulmonary and systemic circuits incompletely separated

Lamprey

• Life cycle of the "landlocked" form
• Parasitic stage in lakes; migration toward lakes
• Filter-feeding ammocoete larvae; metamorphosis
• Attachment to fish with keratinized teeth and suction; feeding on body fluids
• Anticoagulant into the wound; nonparasitic lampreys do not feed after metamorphosis

Chondrichthyes

• Cartilaginous skeleton; calcium salts; phosphatized mineral tissues in teeth, scales, and spines
• Almost all chondrichthyans are marine; 28 species live primarily in freshwater
• Composed of two subgroups: Elasmobranchii (sharks, skates, and rays) and Holocephali (chimaeras)

Elasmobranchii

• 12 living orders; 1150 species
• Body of a dogfish shark is fusiform (spindle-shaped); asymmetrical heterocercal tail
• Paired pectoral and pelvic fins; median dorsal fin; median caudal fin
• Median anal fin present in most sharks; clasper in males; paired nostrils associated with olfaction
• Lateral eyes are lidless; spiracle behind each eye
• Five (rarely, six or seven) gill slits anterior to each pectoral fin
• Tough, leathery skin covered with toothlike, dermal placoid scales

Senses

• Highly sensitive senses; detect prey from a kilometer or more away with olfactory organs
• Detect prey at moderate distances by sensing low-frequency vibrations with mechanoreceptors
• Switch to vision as primary method of tracking prey at closer range
• Excellent vision; electroreception to find prey buried in the sand
• Ampullae of Lorenzini on the head; detect weak electric fields

Feeding

• Track prey using highly sensitive senses in an orderly sequence
• Detect prey from a kilometer or more away with olfactory organs
• Detect prey at moderate distances by sensing low-frequency vibrations with mechanoreceptors
• Switch to vision as primary method of tracking prey at closer range
• Final stage of attack guided by bioelectric fields surrounding all animals

Internal Anatomy

• Mouth cavity opens into a large pharynx; separate gill slits and spiracles
• Short, wide esophagus runs to the J-shaped stomach
• Liver and pancreas discharge their contents into a short, straight intestine
• Spiral valve slows passage of food and increases the absorptive surface
• Rectal gland secretes a fluid containing a high concentration of sodium chloride
• Paired kidneys regulate salt concentration of the blood

Reproduction

• Internal fertilization; maternal support of embryos highly variable
• Some species oviparous; lay large, yolky eggs soon after fertilization
• Some species viviparous; retain developing young in the uterus
• Ovoviviparous (lecithotrophic viviparous) species; retain developing young in the uterus while nourished by the yolk sac
• Viviparous species; embryos receive nourishment from the maternal bloodstream through a placenta or from nutritive secretions