Oceanic Nekton 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.
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 countershading • Countershading: a nektonic organism is bicolored, dark above and light below • Holoepipelagic: shark, tuna • Meroepipelagic: herring, salmon
Countershading fish Light beam Reflecting light makes fish to appear dark from above Reflecting light makes fish to appear light from below
Morphological features of nekton at different vertical zones • Mesopelagic • Seldom exceed 10 cm • Equipped with well developed teeth and large mouth • Large light-sensitive eyes, uniformly black • Photophores: light-producing organs • Abyssalpelagic • Species-specific pattern of photophores • Small with flabby, soft, nearly transparent flesh supported by weak bones • Oversized mouth
Adaptations of oceanic nekton • 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 Getting 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 nekton • Specific 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 atm • Rigid 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
Adaptations of oceanic nekton • 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 mirabileof some bony fish are capable at high pressures of concentrating gases from the blood into their swimming bladders. • Swim bladderare 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.
Development and relative positions of physostomous and physoclistous swim bladders Pneumatic duct Swim Bladder Esophagus Physostomous Physoclistous
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 force • Reduce the resistance • Body shape • Hydrodynamic mechanism for producing additional buoyancy during movement • Fins • Caudal 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