Anatomy, Physiology, and Ecology of Fishes I
Biology of Fishes
10.18.12
Overview
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Exam I – Return & Review next week
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Presentations & Other Assignments
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Introduction to Anatomy, Physiology, and Ecology of Fishes
Anatomy, Physiology, and Ecology
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Buoyancy and Locomotion
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Swimming
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Feeding Mechanisms
Buoyancy and Locomotion
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Movement in water
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Water ~800x denser than air
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High density provides upward force – force buoyancy
Buoyancy – major force supporting a fish
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Typical mean density of a fish carcass = 1075 kg/m3
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Density of freshwater = 1000 kg/m3
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Density of saltwater = 1025 kg/m3
Buoyancy and Locomotion
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Weight of fish slightly greater than buoyancy force – fish must
produce an upward force or life force that overcomes the
downward pull of gravity not compensated for by buoyancy
of water
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Mechanisms for generating lift
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Hydrodynamic lift
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Hydrostatic lift
Buoyancy and Locomotion
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Hydrodynamic lift
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Achieved using pectoral fins like airplane wings – generate lift as fish
swims; thrust applied via caudal fin
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Most common method for supporting weight of fish in water
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Also used by fishes that regulate buoyancy in other ways
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Costs increase as speed decreases – primarily due to increases in drag
Examples
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Sharks, tunas, mackerels (fast-swimming teleosts)
Hydrodynamic Lift
Buoyancy and Locomotion
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Hydrostatic lift
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Achieved by storing light or low-density materials in the body – same
mechanism as in submarines, hot air balloons, blimps
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These materials include gas, lipids, and low-density fluids
Gas
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Contained within the swim bladder – gas-filled sack just under spinal
column
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Recall characteristic of bony fishes is presence of lungs
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Lung in primitive actinopterygians
evolved into swim bladder
Hydrostatic Lift
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Gas – contained in the swim bladder
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Physostomous – bladder-gut connection – can gulp or burp air
(primitive condition)
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Physoclistous – bladder is sealed – must secrete into or diffuse gas out
(Paracanthopterygii and Acanthopterygii)
Gas provides greatest amount of hydrostatic life per unit
volume, but presents a few problems
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Unstable in roll – fish can easily tip side to side
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Gas changes volume with pressure; pressure increases with depth (1
atm pressure for every 10 m depth). Fish must continuously add or
remove gas to remain neutrally buoyant if fish changes depths.
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Doesn’t respond quickly to changes in position
Gas Bladder
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Most fishes have gas/swim bladders, but some have lost them
in favor of other strategies – benthic life, lipids, low-density
fluids.
Hydrostatic Lift
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Lipid (fats, oils, related molecules)
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Found in livers of sharks and in the swim bladder wall,
skeleton, dermis, and muscle of other fishes
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Most common in deep water fishes that live near the
bottom; also mid water fishes that make large vertical
migrations
Hydrostatic Lift
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Low-density fluids
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Water content of the fish is increased, bones are reduced,
decreases the density of body fluids and tissues
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Only possible for marine fishes
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Found in deep water fishes
Buoyancy and Locomotion
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Trade-offs of various buoyancy mechanisms
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Swimming speed
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Hydrodynamic lift is more economical at higher swimming speeds
– cost of drag increases at low speeds, also harder to steer
(maintain position) at slow speeds
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Hydrostatic lift is more economical at slow swimming speeds
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Gas is cheaper than lipids
Depth
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Gas becomes expensive at large depths – high pressure makes it
costly to fill, difficult to prevent diffusion into blood
Exceptions to trends – adaptations to specific habitats
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Sculpins, darters, etc.
Swimming
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Recall density of fish is close to density of water –
therefore fish do not have to use their skeletons and
muscles to support themselves (in contrast to
terrestrial organisms).
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As a result, all fins and the body can be used for
locomotion.
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To swim, fish must generate thrust and overcome
sources of resistance (drag, inertia).
Swimming
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Types of swimming (6 primary forms)
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Anguilliform locomotion
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Subcarangiform locomotion
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Carangiform locomotion
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Thunniform locomotion
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Ostraciiform locomotion
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Median or paired fins
Swimming
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Anguilliform Locomotion – “eel like”
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Successive waves of muscle contraction passed backward on
alternate sides of body – throws body into series of S-shaped
curves
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Amplitude increases toward tail
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Body wave pushes mass of water backward – inertia of water
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Nearly all of body participates in undulatory, side-to-side motion
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Inefficient mode of swimming – body is long, most of body
(especially anterior) participates. Tail wags the head, therefore
high drag
Swimming
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Anguilliform Locomotion – “eel like”
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Considered primitive mode of swimming – seen in hagfish,
lamprey, many sharks
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Also seen in some more advanced groups such as eels
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Mode also used by many larval fishes – flexible skeleton is poorly
developed, other muscles and fins aren’t yet available for use
Swimming
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Most fishes do not swim using anguilliform locomotion – most are “tail
waggers”
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Instead of using most of the body to push against water for forward
propulsion, most fishes rely on a much smaller portion
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If smaller portion of body undulates, side-to-side movement of head is
reduced
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Reduction of side-to-side movement also accomplished by tapering of the
body towards tail; large forward body mass increases inertia, making sideto-side movement difficult
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Evolutionary trend away from anguilliform, instead towards more caudal
type propulsion found in most bony fishes
Swimming
Swimming
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Subcarangiform Locomotion
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Two-thirds to one-half of the body is involved in producing the
propulsive wave responsible for forward motion
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Side-to-side movement of head greatly reduced compared to
anguilliform
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Fish using this method typically have large flexible caudal fins
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Most of swimming is accomplished by the waves passing down the
body
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Caudal fin probably evolved for use in fast turning, hovering, and
fast starts
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Examples: trout, salmon, minnows, cods
Swimming
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Carangiform Locomotion
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Side-to-side undulations are confined to the last third of the body
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Fish using this method typically have stiff caudal fins that are
deeply forked with elongated upper and lower lobes
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Fin design is easier to move through the water (less drag) but still
generates great force
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Two major evolutionary developments to counteract side-to-side
movement of the head:
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1 – trend towards deeper body with more weight concentrated towards head
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2 – caudal peduncle is greatly reduced
Examples: clupeids, mackerels, jacks
Swimming
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Thunniform Locomotion
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Carangiform locomotion developed to the extreme
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Represents the end-point in evolutionary trend toward greater
speed in underwater locomotion among fishes – burst swimming
speeds over 40 mph and cruising speeds ~10 mph
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Very little of the body is involved in producing forward movement
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Thrust generated almost entirely by tall, stiff, and deeply forked
caudal fin – easy to move, very powerful
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Drag is greatly reduced by extremely narrow caudal peduncle
Swimming
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Ostraciiform Locomotion
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Only seen in those fishes that are unable to move body side to side
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All propulsion comes from “wagging the tail”
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Slow-moving fishes, not streamlined
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Typically bodies of these fishes are encased in armor
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Example: boxfishes
Swimming
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Median or Paired fins Locomotion
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Wide variety of fishes that typically swim without using their body
or caudal fin
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These fishes use either their median (anal and/or dorsal) or paired
fins (pectoral) to move
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Generally tend to be slow-moving fishes
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Continuum of those that use undulation to those that use
oscillation
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Median fin undulation
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Paired fin undulation
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Intermediate
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Oscillation
Median or Paired fins locomotion
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Median fin undulation (bowfin, electric fishes)
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Paired fin undulation (rays, skates)
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Intermediate (triggerfishes, porcupine fishes)
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Oscillation (puffers)
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Highly maneuverable; exploit complex habitats (e.g. coral reefs, dense
vegetation)
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Most can also use caudal fin for propulsion
Swimming
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Important considerations in fish locomotion
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Many fishes have a specialized form of swimming
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Specialization for 1 function usually involves a tradeoff in another
function
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Tunas are specialized for high-speed cruising – great distances at
high speed, but not very maneuverable and poor swimmers at low
speeds
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Cichlids and reef fishes are specialized for high maneuverability,
but lower speed – deep bodies, high dorsal/anal fins, large paired
fins allow for precise movements in complex environments
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Pikes are specialized for accelerating – large caudal fin with
dorsal/anal fins set back on body
Swimming
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Important considerations in fish locomotion
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We can identify some fishes that are specialized for one trait,
however, most fishes use a variety of modes of swimming and are
locomotor generalists as opposed to locomotor specialists
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Most fishes must cruise to get from place to place, accelerate to
eat and avoid being eaten.
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Largemouth bass can raise dorsal/anal fins to gain thrust in a “fast
start” attack, and can depress fins to reduce drag while chasing
prey. Can also raise dorsal/anal fins to aid in maneuvering.
Not all fishes fit neatly into these categories. These specializations
are likely related to how fish feed…