I. Who are the nekton?
    A. Mostly vertebrates
        1. invertebrates -- cephalopods – (mollusca) -- squid
        2. fish
            a. sharks (Class Chondrichthyes, cartilaginous fishes)
            b. rays (Class Chondrichthyes, cartilaginous fishes)
            c. bony fish (Class Osteichthyes)
        3. reptiles
            a. turtles
            b. sea snakes
            c. in ancient seas (60 million years ago) were many marine reptiles (ichthyosaurs, mosasaurs, plesiosaurs)
        4. birds (example, penguins)
        5. mammals (next week)
            a. whales (Order Cetacea)
            b. seals and sea lions (Order Carnivora)
            c. manatees and dugongs (Order Sirenia)
            d. sea otters (Order Carnivora)

    B. Difficult to study

    C. Functional groups of pelagic fish
        1. holoepipelagic fish
            a. often lay floating eggs and have epipelagic larvae
            b. most abundant in tropics and subtropics
        2. meroepipelagic fish– only part of their life cycle in the epipelagic
            a. may spawn inshore
            b. may spawn in freshwater
            c. may only be in epipelagic at certain times -- migrate to surface to feed at night

II. Environmental Conditions and Adaptations
    A. Buoyancy
        1. most nekton are close to neutrally buoyant
        2. squid replace heavy ions in their body fluids with lighter ones
        3. many fish have a swim or gas bladder
            a. can regulate to change buoyancy
            c. two main types
                (1) physostome – has an open duct between gas bladder and esophagus

                (2) physoclist – no duct; rely on the gas gland and the rete mirabile – a network of small blood vessels

        4. lungs can also help to regulate buoyancy (marine mammals)
        5. birds have bones with air channels; can trap air under the feathers
        6. lipid reserves in fish without swim bladders (e.g., sharks, mackerels, bluefish, and bonito)
            a. distributed throughout the body
            b. localized
                (1) pelagic sharks  -- enlarged liver with lipids
                (2) marine mammals deposit lipid (blubber) below the skin
        7. many fast fish have no gas bladder and get lift from their body shape
            a. pectoral fins or flippers
            b. heterocercal tail (has a larger upper lobe)
            c. front (anterior) portion of the body can be inclined at an angle - provide lift
    B. Osmotic regulation –
        · most marine invertebrates are isosmotic with sea water
        · sharks regulate the amount of urea in their blood  - isosmotic or even hyperosmotic
        · most marine fishes have body fluids that are more dilute than seawater and tend to lose water by osmosis (hyposomotic)
            o have impermeable skin (integument)
            o drink seawater
            o produce a small amount of urine (kidneys) – conserve water
            o excrete excess salts in urine and gills (chloride cells on gills)
        · birds and turtles have lower concentrations of salts than seawater; eliminate excess salt through salt-secreting glands in the orbit of the eye

    C. Locomotion
        1. Mechanisms for creating propulsion
            a. S shaped contractions (undulations moving from head to tail) of body or tail
                (1) speed depends on speed of muscle contraction waves
                (2) other fins provide maneuverability
            b. undulating the fins
                (1) rays – wing like pectoral fins; some squid; ocean sunfish
                (2) some elongated fish that hold their body rigid and undulate the dorsal fin (oarfish and ribbonfish)
                (3) generally relatively slow motion
            c. paddling motion of forelimbs, hindlimbs or both
                marine turtles, seals, sea lions, penguins
            d. jet propulsion using water -- squids -- can move quickly

        2. Mechanisms for reducing resistance (air more dense than water, so there is more resistance)
            a. types of resistance
                (1) frictional resistance -- proportional to the amount of surface area in contact with the water (least for a sphere -- least SA/V);
                (2) form resistance -- drag while moving is proportional to the cross-sectional area of the object in contact with the water
                    (a) big for a sphere
                    (b) smaller if long and thin
                (3) Induced drag – turbulence (vs. smooth laminar flow) creates vortices (eddies) and increases the drag
            b. Generally advantageous to maximize laminar flow – smooth or absent scales; covered with slime --
                    streamlined body surface
            c. BUT, if the animal is large and fast, you can’t prevent turbulent flow – have controlled turbulence
                (1) swordfish:  rough sandpaper-like skin on their sword
                (2) tunas: scales behind the head (corselet)
            d. most streamlined bodies have a teardrop shape balance between frictional resistance, form resistance and induced drag -
                (1) gives the lowest resistance for the largest volume
                (2) ratio of largest diameter to length is about 0.22 (whales, dolphins and tunas)

            e. Fast swimmers outperform subs and torpedos by behavioral mechanisms

    D. Defense and Camouflage
        1. fast movement
        2. camouflage
            a. alteration of body shape -- presence of a ventral keel
            b. cryptic coloration -- countershading
        3. flying fish -- large pectoral fins -- escape predators

        4. schooling

    E. Sensory Systems
        1. vision
        2. hearing – inner ear; swim bladder amplifies in some fish
        3. olfaction –  olfactory sacs; taste buds
        4. lateral lines of fish – detect vibrations in the water
        5. ampullae of Lorenzini (sharks and rays) – sensitive to electric currents
        6. geomagnetic sensory system (marine mammals and some other organisms-- long distance migration)
        7. echolocation in marine mammals (more in mammal lecture)
    F. Reproduction and Life cycle
        1. holonektonic bony fish (e.g., tuna and marlin) tend to spawn eggs that float and undergo development in the open ocean
            a. are subject to large predation rates
            b. produce large numbers of eggs
            c. growth rate very rapid and life spans relatively short (5-10 years)
        2. pelagic sharks -- produce only a few large eggs or embryos and retain in the female for a long period; may live 20-30 years
        3. marine birds and turtles produce shelled eggs that are laid on land
            a. young are then inaccessible to terrestrial predators
            b. susceptible to human predation and pollution
    G. Migrations
        1. Fish --
            a. salmon  migrate back to freshwater to spawn (anadromous fish)
                (1) can return to the same stream in which they hatched
                (2) may use land features, currents, salinity, temperature, the sun or magnetic field to get close to land
                (3)sense of smell
                (4) die after spawning
                (5) young return to the sea
            b. other marine fish
            c. catadromous fish – breed at sea and migrate into rivers to grow and mature – freshwater eels
            d. tuna                 anatomical basis for magnetotaxis -- magnitite
        2. turtles
            migrate long distances to breeding beaches, sometimes on islands
            possible homing mechanisms
                a. ability to detect magnetic fields (some vary with latitude)
                b. orient to wave propagation or direction
                c. follow chemical cues
        3. sea birds  --  may have very long migrations; patchy prey (tubenoses)

III. Ecology of Nekton
    · Planktivorous -- consume zooplankton
            o-- e.g., some fish -- flying fish, sardines, herrings, menhaden, whale sharks, basking sharks and megamouth sharks and some marine mammals (baleen whales)
            o most abundant nekton
            o generally these are size-specific and do not specialize
            o many mesopelagic small fish migrate into the epipelagic at night to feed on plankton
    · Most species of nekton consume other nekton
            o generally depends on body size -- larger species take larger prey
            o sharks take bites from prey that can be larger than themselves
            o most other fish consume prey whole
            o pelagic birds feed on small fish and squid in the surface waters

Ocean Zone:



Trophic Level

Cold Temperate


Cold – lack lg. predatory fish (e.g., tuna, marlin)

more complex food webs; more links; more trophic levels, more spp.)

Important Primary Producers

Diatoms (net phytoplankton)


Dinoflagellates and coccolithophores


Misc. zooplankton

Copepods and euphausiids

Misc. zooplankton

Primary carnivores

Zooplanktivorous fish – salmon and migrating mesopelagic planktivores

Baleen whales and fish

Zooplanktivorous fish – flying fish and lantern fish (mesopelagic)

Top carnivores

Sharks, marine birds, mammals

Marine mammals

Predatory fish and squid

Marlin, swordfish, tuna

Large sharks (white and mako)


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