Chapter 6 - The Fishes

Fishes are a group of animals adapted to life in water.

Agnathans

  • “Without Jaws”; jawless vertebrates

  • includes Cyclostomes and Ostracoderms

  • Cyclostomes = “round mouth”

    • lampreys and hagfishes - jawless parasitic fish

    • Lampreys: known for their ability to attach to other fish and feed on their blood

    • Hagfishes: scavengers that feed on dead and dying fish, possessing a unique slime defense mechanism.

  • Ostracoderms

    • “shell skin”

    • earliest fossil “fishes”

    • Jawless, bony plates

    • Small, bottom, filter feeders

  • Gnathostomes

    • “Jawed Mouth” - first jawed vertebrates

    • Descendents of Ostracoderm-like ancestors

  • Placoderms

    • “plated skin”

    • more active and predictious

    • more streamlined shape, reduced plating, jaws

    • typically no teeth

    • they had more beak like mouths and they were very large

  • Chondrichthyes

    • cartilaginous fishes

    • cartilage instead of bone

    • sharks, skates, rays

    • evolutionary radiation into various forms

  • Osteichthyes

    • bony fishes

    • 95% of living fish species

    • more evolutionary change than Agnatha or Chondrichthyes

  • Bony fish subclasses

    • Sarcopterygii (lobe-finned)

      • lungfish, coelocanth

      • their fins are very thick and lobe shaped

    • Actinopterygii - (ray finned)

      • diverse group of bony fishes infraclass Chondrostei

        • Sturgeons and paddle fish

        • primitive (heterocercal) tail (the back bone goes up into the top part of the tail fin)

    • Holostei: (entirely bony)

      • gar, bowfin

      • more advanced tail (part heterocercal, vertebrae goes part of the way up their tail)

    • Telostei: final bony

      • most common fish

      • most advanced tail (homocercal) the back bone stops at the base of the tail (trout, bluegill, …)

General Characteristics of Modern Fishes

  • Vertebrates

    • animals with backbones; largest group with more than 30,000 species

  • Ectotherms

    • obtain hear from external sources

  • Bony scales

    • formed from bone, not epidermis (reptiles, mammals, birds)

  • Mucous glands

    • protects from ecto parasites and water gain/loss

  • Streamlined (fusiform) Shape

    • depends on habitat and niche

    • Long thin shapes are more common in moving water (trout) then short and flat are more for moving through weeds in lakes and ponds (bluegill)

  • Skeleton

    • weaker than most vertebrates (water support), they do not need strong bones for support

  • Swim/Air bladder

    • sad within the body cavity

    • provides equal density to surrounding water flotation.

  • Gills

    • specialized tissues on the sides of the head

    • respiration

    • rid body wastes

    • maintain water balance (osmoregulation)

    • Shove water through their gills using muscles to create a current, facilitating the efficient exchange of oxygen and carbon dioxide.

Osmoregulation

  • Gills and Kidneys

  • Osmosis: movement of water across a membrane

    • equalize concentration gradients on both sides of the membrane

    • When salutes are elevated on one side and only water can move across the membrane, water will move to the otherside so the level of water to solutes will be equal

  • Saltwater vs. Freshwater

    • In fresh water the ion gradient is higher on the outside of the fish, causing more water to be absorbed through their skin more

      • their urine is a lot more dilute

    • In salt water it is the opposite, where the ion concentration is greater outside the fish than inside, leading to water being drawn out from the fish's body to balance the osmotic pressure.

      • They will drink a lot more water because so much water is leaving through their skin

      • Their urine is more concentrated to get rid of some of those ions from the salt water

  • Most fish: fresh water, salt water, not both

    • some: brackish water (estuaries)

    • Catadromous fishes: mostly freshwater but spaun in saltwater

      • American Eels:

    • Anadromous fishes: mostly saltwater but spawn in freshwater

      • Salmon: Known for their incredible migration journey from ocean to freshwater streams to spawn.

Senses

  • Sight:

    • no eyelids - always open

    • eyesight fair to good - depends on species;

  • Smell

    • Varies with species

    • migratory fish, some sharks - highly developed

  • Taste

    • Well developed in some species

    • catfish - tastebuds on barbels (and their skin!!)

  • Hearing

    • typically minimal

  • Lateral line system (most important sense)

    • allows them to feel vibrations through water, aiding in navigation and detecting prey or predators in their environment.

  • Electric fields

    • communication in murky water

    • prey, predator, area, individual (each individual will have a different electric field, but not in every species) , sex ID (different electrical fields)

    • Electric eel - strongly electric - stuns prey (600v)

Behavior

  • Communication

    • mostly visual: colors, shape, and behavior

    • very few emit sounds or use electric fields

  • Foraging

    • most swallow prey whole

    • wide variety of prey types

    • bottom, middle, surface feeders

    • generalists vs specialists

    • mouth morphology adaptations

  • Reproduction

    • most oviparous (egg laying)

    • some viviparous (live bearing)

    • some ovoviviparous (eggs hatch in female)

    • most fertilization is external, some are internal

    • aquatic larval stage

  • Breeding and Parental Care

    • most exhibit no parental care

    • males often care for the embryos when there is parental care - why?

      • The males want to ensure that their genes have a better chance of spreading

      • Females will drop their eggs and leave. The male fertilizes them and stays with them to protect them from predators and ensure a higher survival rate, ultimately increasing the chances of his genes being passed on to the next generation.

    • mouth brooding

  • Schooling (grouping)

    • selfish herd

    • antipredatory

    • prey detection & capture

Conservation of Fishes

  • Fish are affected by numerous conservation issues

    • Overfishing is a problem - we are overfishing the oceans and the number of fish left is decreasing. Trophic levels are changing the fish are staying smaller

    • Dams are an issue - they prevent the migration of fishes that are migratory

    • Chemical issues - affect fish and humans -

    • Dead zone - a region in the ocean where the oxygen levels are so low that marine life cannot survive, often caused by nutrient runoff from agriculture.

      • Mississippi spills out all of the ag chemicals into the ocean

    • Silver Carp - released to clean up algae blooms, but they also pose a threat to native fish populations by outcompeting them for food and habitat.

Learning our fishes

  • Understand the morphological structure