Feeding and digestion

 

  1. Diversity of feeding strategy
    1. Classification of what is eaten
      1. Detritivores
      2. Herbivores
      3. Carnivores
      4. Omnivores
    2. Classification by how it eaten
      1. Oral manipulators
        1. scrapers: parrotfish
        2. bite off chunks: Piranha, Sharks
      2. Ram feeders: predator ‘swims through’ prey
        1. continuous swimmers
          1. strain food (small items)
          2. food not strained but still relatively small, tuna
        2. stalkers or sit&wait
          1. fins often arranged for acceleration: pike
      3. Inertial suction feeders
        1. majority of fishes use this method
        2. items smaller than mouth
        3. How it’s done (expanding cone model: drawing):
          1. increase volume of buccal cavity
          2. sharks, rays, and lower bony fish do it
          3. derived teleost version (Fig. 8.3, Helfman et al.):
            1. elevating neurocranium, epaxial muscles
            2. dropping the floor of the mouth, hypaxial muscles
            3. moving the sides of the mouth laterally
            4. opening the operculum (these two are levator operculi)
            5. depression of mandible moves mandible forward, which rotates premaxilla forward
        4. advantages of protrusion:
          1. decreasing pred/prey distance
          2. increases suction efficiency (circular mouth)
          3. aiming
          4. versatility. Can suck limpets off pilings! Anything floating by can be eaten. Potentially high diet breadth
    3. Specialization vs. generalization
      1. Few species are specialized on a single prey species.
      2. One possible exception: the cichlids (handout from Fryer and Iles).
        1. decoupling of maxilla and suspensorium: permit greater diversification of action patterns and hence more specialization? see pp 109-110 in text
      3. In environment, food items fluctuate in abundance. Fish will tend to change utilization patterns, day to day, over seasons. Aquatic systems may specialize less on food than in terrestrial systems.
    4. Ontogenetic prey shifts
      1. As size increases, diet changes. Often, shift from zooplankton to fish (e.g. bluefish)
      2. Herbivores: also begin life feeding on ciliates and zooplankton
  2. Feeding and digestive structures and function
    1. Mouth, teeth and pharynx
      1. Mouth: terminal, subterminal/inferior, superior (fig. 8.7, Helfman et al.)
      2. Teeth (handout)
        1. on jaws (advanced bony fish: lower jaw only)
        2. other head bones; lower part of mouth, on tongue; upper part, commonly vomer, palatine
        3. pharyngeal jaws, modified gill arch elements (ceratobranchials below, bite against pharyngobranchials above)
        4. shape of teeth varies with function (read pp 113-114)
        5. often, variation in function anterior to posterior; capturing anterior, processing posterior
      3. Gill rakers (handout)
        1. short and stubby in most piscivores
        2. long and thin, closely spaced sieve-type arrangement in planktivores; paddlefish, basking shark (which shed rakers each autumn, regrow in spring)
    2. Stomach (handout)
      1. Most fish have stomachs; some lack (no differentiable lining)
      2. Some, highly muscular, gizzard-like
      3. Usually U-shaped
      4. Acidic: HCl, and with digestive enzyme (pepsin) that acts at low pH
    3. Intestine (handout)
      1. Much diversity in length
      2. correlated with indigestibility; herbivores, v. long (20X body, vs. <1X in carnivores)
      3. Alkaline
      4. Spiral valve in sharks: another way of increasing absorptive surface
    4. Accessory digestive structures (handout)
      1. Pyloric caecae
        1. Blind sacs off intestine, just beyond stomach
        2. None, to thousands (tuna); may be highly branched or simple; etc.
        3. Absorptive and/or secretory.
      2. Pancreas
        1. several enzymes aiding in digestion: proteases, carbohydrases, lipases
        2. diffuse tissue in advanced bony fishes
      3. Liver
        1. storage, but also secretory: bile
  3. Bioenergetics
    1. Definitions
      1. Nutrition: what is makeup of diet, chemically?
      2. Bioenergetics: what is the rate of energy intake, and the fate of this energy?
    2. Important fields of study
      1. Aquaculture. Want to grow fish; have to know how much energy to supply and what the makeup of feed should be.
      2. Fisheries. Understanding of bioenergetics necessary to predict rates of production of new fish tissue.
      3. Ecology. Understanding what fish eat, how much they eat, and how much is turned into new fish tissue; trophic transfer, ecosystem dynamics.
    3. Energetics
      1. How much fish eat
        1. Depends on temperature
        2. Some rough amounts: generally maximum ration is less than 10% of body weight per day, but larvae can eat a great deal more per day
      2. What happens to the energy (handout)
        1. energy budget: consumption = metabolism + production + egestion (feces) + excretion (urine, nitrogenous waste). C = R + P + F + U
        2. losses to
          1. undigested (feces)
          2. excretion (urine, nitrogen through gills)
          3. heat of digestion
          4. activity
          5. standard metabolism (basal)
          6. production