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