Fish Notes - Marine Biology

Characteristics of Chordates

• 3 categories of chordates
  • Tunicates
  • Lancelets
  • Vertebrates

• Phylum Chordata is characterized by having a notochord, gill (pharyngeal) slits, and a dorsal nerve cord at some point in their life cycle

  • Notochord: firm tissue mass along the dorsal side (becomes vertebral)

  • Dorsal nerve cord: tube of nervous tissue just above the notochord

  • Estimated 49,000 species

Fish

• Largest type of vertebrates with over 30,000 species grouped into 3 different classes
  • All other vertebrates have 1 class
• Classes are jawless, cartilaginous, and bony fish
  • cartilaginous: made of cartilage/having a skeleton of cartilage; cartilage is the main type of connective tissue seen throughout the body
• They are ectotherms, meaning that their body temperature changes with the environment

Adaptations of Fish

• Gills: fleshy filaments with tiny blood vessels (it is why they're red) 
  • Water enters the mouth, passes over vessels in gills adding oxygen and removing carbon dioxide
• Fins: fan-like structures for moving, balance, and steering
• Scales: protects the body, most are overlapping plates of bones
• Acclimation: they can adapt to different water characteristics (temperature, salinity, etc.)
• Archimedes Principle = buoyancy
  • Fish have a swim bladder/gas bladder -- it is an organ inside their body that they fill up with gas to control their buoyancy

Coloration

• Chromatophores
• Structural colors
  • Iridohores
• What does this indicate:
  • Mood
  • Warning coloration
  • Cryptic coloration
  • Disruptive coloration
  • Countershading

Feeding

• Sharks are carnivorous 
  • Will eat anything regardless of size
    • This is due to the powerful jaws and them shaking their head to saw through the food
  • Whale sharks, basking sharks, and megamouth sharks are filter feeders
    • Utilize gill rakers to filter
    • Number of teeth and gill slits determine what they can eat
• Manta rays feed on plankton and small fish by filtering
• Most bony fish are carnivorous
  • Hunt and chase
  • Sit and wait
  • Food can be found in sentiments, water columns, rock surfaces, or on other organisms
  • They use well developed teeth to  capture and hold prey until it is swallowed whole
• Fish do not chew their food
• Their food preference varies
  • Depends on adaptations and preferences
  • Grazers eat primarily seaweed
    • Parrotfish
    • Plankton feeders are the most abundant

Digestion

• Pyloric Caeca: Slender tubes that secrete digestive enzymes
• Liver and pancreas also release enzymes such as those found in the bile
• Intestines are straight in carnivores and coiled in others
  • Spiral valves found in cartilaginous fish 

Gas Exchange

• Countercurrent system of flow is an adaptation where blood flows in the OPPOSITE directions of water flow
  • Oxygen transfers are based on diffusion
• Hemoglobin in red blood cells absorb the oxygen
• Red muscles are more heavily used muscle than white muscles
  • Redness due to extra oxygen stored by protein similar to the hemoglobin (myoglobin)

Irrigation of Gills

• Feathery gills so that they have a lot of surface area for gas exchange to occur
• The water moves into the mouth and out through the gills
• Gill arches: Cartilaginous or bony structures that support the gills
  • 2 rows of gill filament on inner surface of each arch
  • Gill rakers are on the inner surface of each arch
    • They use these to filter out food, as the food is going through their gills
• Lamellae - The row of thin plates or disks found on each filament

Locomotion

• Most fish swim with a rhythmic side-to-side motion of body or tail
  • Creates S-shaped waves that push against the body from head to tail
• Bony fish
  • Due to the swim bladder, bony fish use pectoral fish for maneuverability
    • Able to make right turns
• Sharks lack of swim bladder
  • Compensate with large, stiff pectoral fins that provide lift similar to planes

Regulation of Internal Environment

• Unlike other marine organisms, the blood of fish is less salty than seawater
  • Loss of water through osmosis
  • To prevent dehydration, they swallow seawater
    • Kidneys (in combination with CI cells of gills) excrete excess salts
      • Water conservation is then possible through small amounts of concentrated urine 

The Cartilaginous Fish Method

• In most organisms, the job of the kidneys is to remove wastes from the blood (urea) and eliminates it through the urine
  • Urea is toxic in most animals
• These fish retain the urea in their blood to elevate the amount of solutes in their blood to match that of seawater
  • Gills block the loss of urea by absorbing water
  • Any excess salts are eliminated by the kidneys, intestine, and rectal gland

The Method used by Freshwater Fish

• They have the opposite problem in that they are hyperosmotic
  • Hyperosmotic - solution with the greater concentration of solute
  • As a result, they are constantly absorbing water through their skin and especially gills
• Highly efficient kidneys produce extremely diluted urine
  • This causes a loss in salts and other solutes
  • To rectify this, they use an active transport system to exchange Na+ from the water for NH4+ in their bodies and CI- for CO3^2- to maintain their pH levels
• Lateral line
  • System of canals that run along the body
    • Inside the skin and in the bone (or cartilage) of the head
  • Lined with sensory cells that pick up vibrations
    • Open to the surface through visible pores
    • Vibration/pressure in the water comes from swimming animals and sound waves from water displacement
  • Allows for obstacle avoidance, predator/prey detection, orientation to currents and position in school 

Behaviors

• Territoriality
  • Especially common in highly competitive areas such as coral reefs
  • Fighting is rarely used
    • Bluffing is most common form of defense because it avoids injury
      • Raised fins, darting about, and open mouth
      • Sounds can be made by rubbing teeth or fin spines
      • Drum (you can actually hear it underwater) 
• Schooling
  • Some spend entire lives, some only part of life in a school
    • When part-time, it occurs as juveniles or during feeding
    • Hammerhead sharks are rare cartilaginous fish that school (they don't school all the time, but they tend to school during mating/feeding time)
  • It is a well-coordinated unit with no visual leader
    • Vision is a key part in maintaining orientation within group
      • Some examples of blind fish schools
    • Group breaks apart during feeding or attacks by predators

Special Attributes of Sharks

• Sharks and rays are successful predators:
  • Subclass: Elasmobranchii have cartilaginous skeletons
    • This characteristic saves energy. Saving energy is one of the things that have made them successful predators
  • Sharks have a sense of smell that detects incredibly diluted substances
  • Sharks have a "conveyor belt" of multiple rows of teeth
    • They swing into place as old teeth wear out and fall away 
  • Unique to elasmobranchs is electroreception - the ability to sense minute and electricity created by muscle nerves
  • Sharks and rays have organs called Ampullae of Lorenzini which you can see as visible pits near their snouts used to detect the electrical current 
• The largest fish in the ocean
  • Shark size ranges from hand-sized to the whale shark (the largest fish in the ocean)
    • Whale sharks can reach 14 meters (46 feet) 
    • Basking sharks can reach 10 meters (33 feet)
    • Megamouth sharks can reach 6 meters (20 feet)
  • All three are filter feeders that consume plankton 

Special Attributes of Rays

• Superorder Batidoidimorpha of subclass Elasmobranchii consists of the rays, which includes skates and guitarfish
  • Ray anatomy is well suited to life on sandy bottoms or midwater
  • Specially adapted to life in midwater are the eagle ray and the manta ray
  • Pectoral fins have become "wings" that stretch forward over the gills and are fused to the sides of the head

Characteristics of Bony Fish

• Class Osteichthyes are jawed fish with bone skeletons
  • Most have a swim bladder and scales
  • Most control buoyancy by adding or releasing gas to/from their swim bladder
    • They control the swim bladders with oxygen gas exchanged to and from blood circulation
    • Many have a special organ called the gas gland and the rete mirabile that take up gases from the bloodstream for the swimbladder
    • This allows many species to hover nearly motionless in midwater
• Most bony fish reproduce externally
  • The female lays her eggs, the male immediately fertilizes
    • Their strategy is to produce a vast number of off-spring with only a few expected to survive to maturity 

Significance of Class Agnatha

• This is the class of the jawless fish
  • Species include lampreys and hagfish
• Organisms in this class are significant because they may represent the ancestors of bony fish/sharks
• Scientists theorize that during the Cambrian period, the first of three gill arches on a jawless fish evolved into the first jaws
  • Having jaws allowed vertebrates to become very successful predators
  • Having jaws put organisms in class Chondrichthyes (sharks and rays) and class Osteichtyhes (bony fish) near the top of marine foods webs
• This is the only group of fish whose blood is isotonic with its environment

Agnatha (Jawless)

• Lamprey
  • Round mouths
  • Tube-like bodies covered with slimy skin (no scales)
  • Cartilage skeleton (tough, flexible tissue that is not hard like bone)
  • Cuts other fish and feeds off of their fluids
  • Lives in fresh-water
• Hagfish
  • Round mouths
  • Tube-like bodies with slimy skin (no scales)
  • Cartilage skeleton 
  • Lives in salt water

Hemichordates: "The Missing Link"

• Even though Echinoderms and Chordates share some characteristics, the hemichordates fall in between
  • Same developmental characteristics 
    • Larval phase as seen in Echinoderms
    • Same key characteristics as Chordates
• 90 species, mostly acorn worms or enteropneusts usually found in the Benthic regions

Key Characteristics

• Dorsal, hollow nerve cord
• Gill slits
• Notochord
• Bilateral symmetry, with reduced segmentation, complete digestive tract
• Respiratory exchange is through skin
• Hybrid circulatory system