14.1 How are marine organisms able to stay above the ocean floor?

some animals increase buoyancy to remain in near-surface waters, might have internal structures containing gas, which significantly reduces their average density, may have soft bodies void of hard, high-density parts. larger animals can often swim

Use of Gas Containers

air is almot 800x less dense than water at sea level so even small amt of air inside an organism can dramatically increase its buoyancy

generally, animals use either an internal, rigid gas container or a swim bladder to achieve neutral buoyancy, using amt of air in their bodies to regulate their density so they can remain at a particular depth without expending energy to do so.

Rigid Gas Containers

cephalopods have rigid gas containers

Ex. nautilus has external many-chambered shell

Ex. cuttlefish sepia and deepwater squid have internal chambered structure

pressure in their air chambers is always 1kg/cm² —> nautilus must stay above depth of approx 500m to prevent collapse of chambered shell —> rarely ventures below 250m

Swim Bladder

instead of using rigid gas containers, some slow-moving fish use an internal organ called swim bladder to achieve neutral buoyancy and determine position in water column

very active swimmers i.e. tuna or fish that live on the bottom don’t usually have a swim bladder bc they don’t have a problem maintaining their position in water column

change in depth either expands or contracts the swim bladder —> fish must remove or add gas to the bladder in order to maintain a constant volume.

• in some fish, a pneumatic duct connects the swim bladder to the esophagus, allowing these fish to rapidly add or remove gases through the duct

• in fish w/out a pneumatic duct, the gases of the swim bladder must be added or removed more slowly by an interchange w/ the blood —> can’t withstand rapid changes in depth

composition of gases in swim bladders of shallow-water fish = similar to that of the atmosphere

• at surface, conc. of ox in swim bladder = 20%, same as atmosphere

• as depth increases, ox conc. increases to 90% or more bc of chemical reactions within the fish at depth that cause oxygen to leave the blood, then diffusing into the swim bladder

Ability to Float

floating animals range in size from small to large-ish. floating organisms = zooplankton, comprise second largest biomass in ocean after phytoplankton

• microscopic forms of zooplankton usually have a hard shell (test)

• larger forms have soft, gelatinous bodies w/ little if any hard tissue, reducing their density and allowing them to float

most types of microscopic zooplankton have adaptations to increase the surface area of their bodies or shells so they can remain in the sunlit waters near their food source. others produce low-density fats or oils to stay afloat

• ex. many zooplankton produce tiny droplets of oil to help maintain neutral or nearly neutral buoyancy

RECAP

marine organisms use a variety of adaptations to stay w/in sunlit surface waters i.e. rigid gas containers, swim bladders, spines to increase their surface area, soft bodies, and the ability to swim

Ability to Swim

many larger pelagic animals i.e. fish and marine mammals can maintain their position in the water column by swimming and can also swim easily against currents. organisms = nekton and bc of their swimming, they can undertake long migrations

Diversity of Planktonic Animals

diversity in planktonic marine animals stems from a shifting balance between competition for food and avoidance of predators —> diversity in plankton

Examples of Microscopic Zooplankton

3 of the most important groups of microscopic zooplankton:

1. radiolarians

2. foraminifers

3. copepods

Radiolarians = single-celled, microscopic protozoans that build hard shells out of silica, tests have intricate ornamentation, including long projections

• spikes and spines appear to be a defense mechanism but actually increase test’s surface area so organism won’t sink through water

Foraminifers = microscopic to barely macroscopic single-celled protozoans

• most abundant types are planktonic, but the most diverse are actually benthic

• produce a hard test made of CaCO3 that is segmented or chambered, with a prominent opening in one end

Copepods = microscopic shrimp-like animals of subphylum crustacea, which also includes shrimps, crabs, and lobsters

• like other crustaceans, copepods have hard exoskeleton and a segmented body w/ jointed legs

• most copepods have forked tails as well as distinct and elaborate antennae

• more than 7500 species exist

• most have special adaptations for filtering tiny floating food particles from water

• some are herbivores that eat algae, others are carnivores that eat zooplankton, and others are parasitic

• all copepods lay eggs, sometimes carried in egg sacs attached to abdomen but generally simply released into seawater where they hatch in about a day

  • rapid reproduction allows great #s of copepods to occur wherever fabvorable conditions (generally abundance of food which is mostly algae) exist

  • copepods are extremely numerous, super abundant, one of the most numerically dominant types of multicelled organisms on the planet

  • comprise majority of ocean’s zooplankton biomass = vital link in many marine food webs between phytoplankton and plankton-eating fish

Examples of Macroscopic Zooplankton

many types of zooplankton are large enough to be seen w/out aid of a microscope but don’t swim well, two of the most important groups of macro zooplankton = krill and cnidarians

krill = resemble mini-shrimp or large copepods

• >1500 species of krill, most of which don’t get larger than 5cm

• abundant near antarctica and form a critical link in the food web there, supplying food for many organisms from sea birds to largest whales in the world

Cnidarians = soft bodies that are >95% water and tentacles armed w/ stinging cells called nematocysts. two categories of cnidarians:

(1) hydrozoan = represented in all oceans, have gas chambers called pneumatophores that serve as floats and sails that allow the wind to push them across the ocean surface. sometimes wind pushes large numbers toward a beach. in the living organism, an entire colony of other small organisms that rely on the hydrozoan for habitat can be found within and beneath the float

(2) scyphozoan = jellyfish, bell-shaped body w/ fringe of tentacles and a mouth at the end of a clapper-like extension hanging beneath the bell-shaped float,

• size = micro-2m in diameter

• move by muscular contraction of their bell: water enters cavity under bell and is forced out when muscles that circle the bell contract, slowly jetting the animals in random directions as they pulsate. motion swooshes water that contains tiny floating food particles toward their tentacles. to allow animal to orient itself generally in an upward direction, light-sensitive or gravity-sensitive organs exist around the outer edge of the bell

• the ability to orient is important bc jellies feed by swimming to surface and sinking slowly through rich surface waters

• considered plankton bc they can’t control where they move

other types of macroscopic zooplankton:

• tunicates

• salps

• ctenophores

• chaetognaths

Examples of Swimming Organisms

swimming/nektonic organisms = invertebrate squid, sea turtles, and marine mammals

swimming squid include common squid, flying squid, and giant squid

• all active predators of fish

• most squid have long, slender bodies paired w/ fins and must remain active to stay afloat

• a few species have hollow gas-filled chambers to help them be less active

• squid can swim about as fast as any fish their size:

  • draw water into body cavity

  • expel water out through siphon, propelling themselves backward

• to capture prey: use two long arms w/ pads containing suction cups at ends

  • eight shorter arms with suckers convey the prey to the mouth, where it’s crushed by a mouthpiece that resembles a parrot’s beak

most fish are good swimmers too

• fish’s lateral line contains network of sensors detecting changes in water pressure and allows fish to monitor vibrations in water around them

• myomeres = packages of muscle tissue attached to vertebrae fish uses for propulsion through water

fish use a variety of fins for swimming. most commonly, movement is achieved as a fish alternately ocntracts and relaxes its myomeres on either side of its body, generating a curvature that travels the length of its body, ending in the tail or caudal fin and generating thrust

• swimming motion = thunniform

• in some cases, fish sacrifice speed for maneuverability i.e. in crowded reef environments or for camouflage by moving only small transparent fins —> other types of swimming motions involve using various sets of fins i.e.

  • thunniform

  • amiiform

  • labirform

  • ostraciform

Types of Fins and Their Usage in Fish

most active swimming fish use two sets of paired fins — pelvic fins and pectoral fins to turn, brake, and balance

• when not in use, these fins can be folded against the body

• vertical fins , both dorsal and anal, serve primarily as stabilizers

fin most commonly used to propel high-speed fish = caudal fin

• caudal fins flare vertically to increase surface area available to develop thrust against water

  • larger the size of the caudal fin, the more thrust it provides

  • increased surface area = increased frictional drag

• caudal fins have different efficiences related to size and shape

  • Ex. shark caudal fins = asymmetrical w/ most mass of surface area in upper lobe to provide significant lift bc sharks have no swim bladder and sink when not moving

sharks have many adaptations to compensate for bad buoyancy”

• pectoral (chest) fins = large and flat, positioned like airplane wings to lift front of the body, balancing rear lift supplied by caudal fin

• shark gains a lot of lift but sacrifices maneuverability —> sharks swim in circles and can’t make wide turns

caudal fin shapes organized into 5 basic forms:

1. rounded: flexible, useful in accelerating and maneuvering at slow speeds

2. truncate: moderately flexible for good propulsion, good for maneuvering

3. forked: indicative of faster fish, moderately rigid for better propulsion, useful for maneuvering

4. lunate: fast-cruising fish, fin is very rigid, which makes it useless for maneuvering but very efficient for propulsion

5. heterocercal: very rigid, produces tremendous lift but sacrifices maneuverability

RECAP

fish use their fins for swiomming and staying afloat w/in the water column

fin that provides the most thrust is the caudal (tail) fins which can have a variety of shapes, depending on lifestyle of fish.

CONCEPT CHECK 14.1

(1) Discuss why the rigid gas chamber in cephalopods limits the depth to which they can descend. Why do fish with a swim bladder not have this limitation?

(2) Explain why hydrozoans and scyphozoans (jellies) are classified as macroscopic forms of zooplankton. Why are they not considered nekton?

(3) From memory, name and describe the different types of fins that fish exhibit. What are the five basic shapes of caudal fins, and what are their uses?

• rounded: very flexible, for slower acceleration and great maneuverability

• truncated: moderately flexible for good propulsion, decent for maneuvering

• forked: indicative of faster fish, moderately rigid for better propulsion, useful for maneuvering

• lunate: fast acceleration, very rigid, useless for manuevering, very efficient propulsion

• heterocercal: very rigid, produces tremendous lift but very bad maneuverability