nekton --- those organisms that have developed powers of locomotion so that they are not at the...
TRANSCRIPT
Nekton --- those organisms that have developed powers of locomotion so that they are not at the mercy of prevailing ocean currents or wind-induced water motion.
Major zones of life in a marine
ecosystem
General characteristics of nekton
Larger body size Greater swimming power Most nekton animals are vertebrates, and
most vertebrates are fish Only the squid and a few species of shrimps
are truly nektonic invertebrates Few reptiles (turtles and sea snakes), birds
(penguin) and mammals
Vertical distribution
Epipelagic countershadingCountershading: a nektonic organism is bicolored, dark above and light below
Holoepipelagic: shark, tuna
Meroepipelagic: herring, salmon
Light beam
Reflecting light makes fish to appear dark from above
Reflecting light makes fish to appear light from below
Countershading fish
MesopelagicSeldom exceed 10 cm Equipped with well developed teeth and large
mouthLarge light-sensitive eyes, uniformly blackPhotophores: light-producing organs
Abyssalpelagic Species-specific pattern of photophoresSmall with flabby, soft, nearly transparent
flesh supported by weak bones Oversized mouth
Morphological features of nekton at different vertical zones
Lipid: large amounts of lipid are present in many nektonic fishes, primarily those lack swim bladder, such as shark, mackerel etc. Deep-sea fish may have fat-filled swim bladder.
Fins : flat pectoral fin and heterocercal tail Replace heavy, chemical ions with lighter ones:
squid: replace sodium ions with lighter ammonium.
Question: Why does shark never stop swimming in the water?
Adaptations of oceanic nekton
Adaptations of oceanic nektonGetting Oxygen: breath-hold diving in marine mammals
Apneustic breathing pattern: several minutes of diving followed by a few seconds of breathing
Extensive elastic tissue in the lungs and diaphragms allowing taking up extra additional O2
Callapsing lungs during a deep dive to avoid N2 narcosis
Capability of storing O2 in blood: more red cells, more O2 on hemoglobin
Anaerobic tolerance of muscles
Bradycardia: slowing heart beating rate
Adaptations of oceanic nektonSpecific gravity:
Seawater: 1.02-1.03; muscle: 1.05; bone, scale and shell: 2.0; fat, wax and oil: 0.8-0.9
Gas inclusion: increase 1 atm per 10 m, 5000 m at pressure 501 atmRigid gas containers
Nautilus is the only living cephalopod with external shell
others may reduce to an internal chambered structure
vestige of "pen”, a thin chitinous structure-internal shell
Non-rigid gas inclusions lung for mammals swim bladder (5-10% body volume)
Pneumatic duct: the connection between esophagus and swim bladder is present during the larval and juvenile stages of all bony fishes
Physostomous: with pneumatic duct Physoclist: nearly half of the more than 20,000 species of
bony fish lose not only the pneumatic duct but also the swim bladder when they mature
Adaptations of oceanic nekton
The gas gland and associated countercurrent rete mirabile of some bony fish are capable at high pressures of concentrating gases from the blood into their swimming bladders.
Swim bladder are notably lacking in bottom fish and in active, continuously swimming fish. However in very fast swimming fishes, the gas baldder can not adjust fast enough to compensate for pressure changes and maintain neutral buoyancy.
Adaptations of oceanic nekton
Buoyancy adaptations of nektonic fish and mammals.
Swim Bladder
EsophagusPhysostomous Physoclistous
Pneumatic duct
Development and relative positions of physostomous and physoclistous swim bladders
Reproduction Nonseasonal reproduction –deep-sea marine
organisms Oviparity (r)- skipjack 2 million eggs;
albacore 2.6 in. striped marlin 13 in; ocean sunfish 300 in, egg case in skates, rays, and benthic sharks
Ovoviviparity (K) - thresher shark 2 embryos; blue shark up to 54 embryos
Viviparity (long longevity) - mammals
Sensory reception
Chemoreception Vision Echolocation (sound reception) - sperm
whale (melon) Electroreception - shark (ampullae of
Lorenzani) Geomagnetic reception - whales mass
stranding
Defence and camouflage
Camouflage Cryptic coloration -- a nektonic organism is
coloured dark blue or dark green on dorsal surface and white or silver on ventral side
Cryptic body shape -- the presence of a ventral keel
Transparency of the body, mainly for plankton or small fish
Alternation of body shape: develop a ventral keel the body to eliminate a conspicuous shadow on the belly.
Diagram showing how a keel on the ventral surface of an animal eliminates the dark shadow normally cast downward by an unkeeled animal. The presence of the shadow means that an animal living deeper and looking upward would see the unkeeled nektonic animal due to the shadow, but would not see the keeled animal, which would blend into the lighted background.
Cryptic coloring on the sides of a Pacific white-sided porpoise, Lagenorphynchus obliquidens, mimicking the wave-roughened surface of the water.
Contrasting color patterns on various nekton. (A) Dall’s porpoise (Phocoenoides dalli). (B) Manta ray (Manta ray (Manta hamiltoni). (C) Albacore (Thunnus alalunga).
Locomotion Create the propulsive forceReduce the resistance
Body shapeHydrodynamic mechanism for producing additional buoyancy during movement
FinsCaudal fins: rounded, truncate, forked, lunate, heterocereal Aspect ratio = (fin height)/fin area
High ratio fish are capable of long-distance, continuous swimming
Three views of a tuna showing the adaptations necessary for fast movement. (A) Front view. (B) Side view. (C) Top view.
Some characteristic meroepipelagic fishes. (A) Ribbon halfbeak, Euleptorhamphus viridis. (B) Herring, Clupea harengus, (C) Whale shark, Rhincodon typus. (D) Dolphin, Coryphaena hippurus. (E) Salmon, Oncorhynchus keta.
Fast-swimming fishes with the characteristic lunate-shaped tail and narrow caudal peduncle. (A) Tuna (Thunnus thynnus). (B) Sailfish (Istiophorus platypterus).
Speed killer whale --- 40-50 km/hr barracuda --- 40
m/hr . human -- 4 to 5 km/hr; yellow fin tuna 74.6
km/hr for 1.9 second Schooling
Provide a degree of protection May act as a drag-reducing behavior and allow
closely spaced individuals to captalize on the turbulence generated by their neighbors.
Ensure high proportion of egg fertilisation and greater larval survival
Migration Purposes of migration
Needed for successful reproduction or feeding The food available in spawning areas may be appropriate
for larval and the juvenile stage, but it might not support the mature members of population
Often exhibit a strong similarity to patterns of ocean surface currents
Orientation Biological clock operating on longer period rhythms Day length, water temperature, food availability, earth's
magnetic field - shark. Skates Keen sense of smell -- salmon for home stream