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Vertebrate Zoology Chapter 7: Origin of Osteichthyes and Radiation of Actinopterygians

Origin of Osteichthyes and Radiation of Actinopterygians

Bony Fishes: Osteichthyes

  • Superclass Osteichthyes
    • Greek Roots: osteon = bone; ichthys = fish
    • Definition: Includes fishes whose skeletons comprise endochondral bone, which replaces cartilage during embryonic development.
  • Other Bone Types:
    • Dermal bone
    • Perichondral bone
  • Notes: Dermal and perichondral bones are also present in basal lineages like placoderms.
  • Classes of Osteichthyes:
    1. Actinopterygii - the ray-finned fishes
    2. Sarcopterygii - the lobe-finned fishes

Factoids from the Introduction

  • Actinopterygians have more species than all tetrapod groups combined.
  • Since the year 2000, ichthyologists have described approximately 7,500 new species of teleosts, which represent more than 20\% of teleost species, with 60\% of these new fish coming from Asia, Africa, and South America.

Radiation of Fish and Tetrapods

  • Devonian Period (Paleozoic Era): Known as the Age of Fishes.
  • All major lineages of fishes, both extant and extinct, are represented.

General Characteristics of Bony Fish

  • Endochondral Bone: Cartilaginous skeleton ossified; includes both dermal and perichondral bone in basal gnathostomes.
  • Lung: Ventral to and derived from gut.
  • Operculum: Covers gills and is derived from dermal bone; respiration primarily via gills.
  • Teeth: Attaches to dermal bones such as premaxilla, maxilla, and dentary; can also be located on other bones making up the palate of the mouth.
  • Palate: Roof of the mouth covered in dermal bone.
  • Fins:
    • Pair and median fins supported by fin rays known as lepidotrichia, which are dermal bones evolved from scales.
    • The presence of fin rays defines the distinction between fins and limbs.
  • Lateral Line Canals: Present for sensory perception.
  • Nervous, Circulatory, and Excretory Systems: Complex systems are present in bony fish.

Bones Surrounding the Mouth

  • Example: Dermatocranium of a bowfin, Amia calva, displays bones shared by actinopterygians and sarcopterygians.

Evolution of Enamel

  • Origin: Sharks and rays diverged before the evolution of enamel; tetrapods and modern fishes diverged afterward.
  • Ganoine: A homologue precursor to enamel found in actinopterygians.
  • Cosmine: A layer of dentine covered in enamel, characteristic of sarcopterygians.

Differences between Sarcopterygian and Actinopterygian Fins

  • Sarcopterygians:
    • Described as “flesh fins”.
    • Rays of paired fins extend from a central shaft of bone in a feather-like manner, referred to as monobasic fin.
  • Actinopterygians:
    • Known as “ray fins”.
    • Rays spread outward in a fan-like manner from the bones at the base of the fins; they have lost the three basal bones of the fin.
    • Notable bones: Scapula, Humerus, Radius, Ulna.

Pectoral Fins of Living and Devonian Lobe-Fins

  1. Basal condition: Central endochondral axis (e.g., Latimeria, Neoceratodus, Glyptolepis).
  2. Derived condition: More branching (e.g., Sauruipteris, Eustenopteron).
  • Basic Limb Bones: Humerus, radius, ulna.

Circulation in Osteichthyans

  • Basal osteichthyans: Had lungs and a pulmonary circulation returning blood to a single atrium.
  • Teleosts: Most lost the respiratory function of the lung; modified to function as a gas bladder for buoyancy control, which subsequently led to the loss of pulmonary circulation.
  • Sarcopterygians: Evolved a divided atrium returning oxygenated blood from the lungs to the left atrium; a partially divided ventricle aids in maintaining separate flows of oxygenated and deoxygenated blood.

Actinopterygii: Basal Groups

  • Actinopterygii:
    • More numerous of the two classes, with over 27,000 species.
    • Includes prevalent species like salmon, cod, herring, tuna, marlin, pike, sardine, clownfish, and goldfish.
  • Divisions of Actinopterygii:
    • Encompasses 44 orders; 4 represent more primitive fish that existed early in the Paleozoic era, while the remaining constitute the more derived clade Teleosti.
    • Two Subclasses:
    1. Chondrostei: Consists of several relic species.
    2. Neopterygii: Considered the most derived and recent group of ray-finned fishes.
      • Infraclass Teleostei: Contains more than 20,000 species.
      • Infraclass Holostei: Encompasses a smaller number of more primitive species, including gars and bowfins.

Characteristics of Primitive Actinopterygians: Order Polypteriformes

  • Polypteriformes:
    • Comprising bichirs and reed fish, located in freshwater Africa.
    • Features:
    • Considered the most primitive surviving group of Actinopterygii.
    • Eel-like body shape, elongated but not flexible, reaching up to 1.2\text{m} in length.
    • Possesses a spiral valve intestine and paired lungs with pulmonary circulation.
    • Distinct dorsal fin divided into numerous finlets with independent rays.
    • Heavily armored with dermal bone and thick ganoine scales.
Order Polypteriformes: Bichirs
  • Body Design:
    • Paired fleshy pectoral fins and paired ventral lungs, enabling survival in low oxygen environments and situations where they may be buried in silt during dry seasons.
    • Larvae display external gills similar to amphibians, reflecting primitive conditions.
  • Predatory Behavior:
    • Hunt at twilight, lying in wait for prey, feeding on appropriately sized aquatic creatures.
  • Aquarium Popularity:
    • Increasingly popular among aquarium enthusiasts.

Primitive Actinopterygians: Order Acipenseriformes

  • Physical Characteristics:
    • Generally have a single lung or gas bladder dorsal to the gut.
    • Often lack endochondral bone, with poorly developed vertebrae, resulting in mostly cartilaginous skeletons (a secondary derived trait).
    • Feature a prominent heterocercal tail, enlarged rostrum, and are primarily scaleless.
  • Two Families:
    1. Acipenseridae (Sturgeons):
    • Elongated fish with downward-facing mouths, and bony plates along their torso.
    1. Polyodontidae (Paddlefish):
    • Notable for greatly elongated paddle-shaped rostrum that likely enhances electroreception.
Sturgeon Characteristics
  • Evolutionary Longevity:
    • Sturgeons have remained relatively unchanged for about 200 million years.
    • Comprises 24 species distributed across subtropical, temperate, and sub-Arctic waters in Eurasia and North America.
  • Distinct Features:
    • They can grow very large, up to 6\text{m} in length and 2000\text{kg} in weight.
    • Body covered in heavy bony scutes instead of standard scales and possess a few isolated rows of large plate-like bony ganoine scales.
    • Characterized by a flattened rostrum and barbels for sensory detection.
  • Feeding Habits:
    • Average behavior involves bottom-feeding, utilizing a protrusible jaw for suction feeding, evident by their snout stirring up substrate to expose prey.
    • Sturgeons can swallow large prey whole, since they lack teeth.
  • Commercial Significance:
    • Known for their meat and particularly their roe (caviar); hence, conservation statuses for all species are threatened due to overrunning spawning habitats, slow reproduction rates, and long life spans (some over 100 \text{ years}).
Polyodontidae: Paddlefish
  • Species:
    • Two species are identified: one found in North America (Mississippi watershed) and another native to China, the latter presumed extinct in 2020.
  • Physical Attributes:
    • Paddlefish can grow approximately 2\text{m} in length, featuring a distinctive elongated flattened rostrum believed to detect electric fields.
  • Feeding Mechanism:
    • Filter feeders that sieve zooplankton using gill rakers.
  • Conservation Issues:
    • Populations have decreased due to dam constructions impeding spawning routes and poaching for their roe.

Class Actinopterygii Summary

  • Subclass Chondrostei:
    • Order Acipenseriformes (paddlefish and sturgeons)
    • Order Polypteriformes (bichirs)
  • Subclass Neopterygii:
    • Infraclass Holostei (gars, bowfin) - 8 species
    • Infraclass Teleostei - over 20,000 species

Teleostei: Major Characteristics

  • Defines most contemporary fishes (>95\%) adaptively; replaced heavier armor of ancestry with lighter and more flexible scales that overlap.
  • Highly diversified into about 24,000 species, representing approximately half of all vertebrate species.
  • Habitat Range: From depths of -11,000\text{m} to heights of +4,500\text{m} and withstands a temperature range from polar conditions to hot springs at 43^\circ\text{C}.
  • Molecular Phylogenetics: Recently modified groupings based on DNA sequences to establish current phylogenetic relationships.
    Entry through Superorder Elopomorpha and Superorder Osteoglossomorpha, exemplifying their vast diversity.

Superorder Clupeomorpha

  • Comprises approximately 500 species of marine schooling fishes with notable commercial importance (e.g., herring, shad, anchovies).
  • Observations reveal enormous schools provide sustenance for more prominent marine predators.
  • Feeding mechanisms involve gill-straining apparatus; Dentition tends to be minimal or absent.

Superorder Ostariophysi

  • Comprising about 30\% of all living fish and predominantly freshwater species (approximately 12,000 species).
  • Typical members: Catfish, Cyprinids, piranhas, with diverse traits but generally manifest protrusible jaws and functions of pharyngeal teeth.
  • Unique Features:
    • Alarm substances released in response to danger trigger frights in conspecific fish.
    • Weberian Apparatus: A structure connecting the swim bladder to the inner ear, enhancing auditory capabilities.

Cyprinivore Description

  • Largest family within Ostariophysi (\sim 2,400 species).
  • Key examples include carp and goldfish, both characterized by toothless jaws that rely on pharyngeal teeth for grinding food.
  • Notably, they can efficiently consume hard-shelled prey, such as snails.

Catfish Overview

  • Diversity: Approximately \sim 1,800 species, featuring barbels resembling a cat's whiskers for foraging.
  • Categories: Commonly bottom-feeders with negative buoyancy due to reduced swim-bladders.
  • Aquaculture Industry: Developed endeavors focusing on raising catfish, notably in warm conditions.
  • Noteworthy Size Variation: Smallest (Asian Stone Catfish) vs. largest (Mekong Catfish).

Endangered Fish Species: Bestiary

  • Vampire Catfish (Candiru): Notable for its unique feeding behavior, capable of parasitizing larger fishes, primarily drawn by urine discharge.
  • Piranha: Smaller deep-bodied schooling fish known for their formidable dentition and reputation as fearsome carnivores.

Teleost Reproductive Modes

  • Distinctive Egg Diversity:
    • Oviparous: Variability in locations and laying methods.
    • Viviparity: Found in specific families.
    • Parental Care: Varies from none to mouth-brooding practices.

Fish Conservation Concerns

  • Pollution: Direct toxins affect freshwater species negatively.
  • Fishing Threats: Overfishing of marine species with impacts on population recovery.

Review and Key Concepts for Chapter 7

  • Synapomorphies of Osteichthyes such as operculum and lung or swim bladder.
  • Differentiation between Sarcopterygians and Actinopterygians based on fin structures.
  • Teleosts' homocercal tail structure's benefits.
  • Comparative analysis of jaw musculature configurations impacting feeding.

Chapter Objectives

  • 7.1: Osteichthyes and synapomorphy summarization.
  • 7.2: Overview of extant non-teleostean Actinopterygians.
  • 7.3: Description of adaptations in Teleostei affecting performance.
  • 7.4: Identification of synapomorphy in Elopomorpha and Ostariophysi.
  • 7.5: Anatomical characteristics of Acanthopterygii.
  • **7.