Necton are organisms living in the water column with swimming abilities that allow them to actively move through the water and against currents.
Unlike plankton, whose distribution is mainly determined by currents, necton can determine their distribution through their swimming abilities.
There is a large and ambiguous border between plankton and necton.
It's a continuum ranging from tiny things that can't swim at all to massive things that can swim very fast for very long distances.
Taxonomic Composition
Necton is strictly an animal group, unlike plankton which includes a diverse range of organisms.
Mainly vertebrates due to their good mobility.
Broad range of fishes.
Some secondarily marine vertebrates.
Some invertebrates, such as pelagic cephalopods (pelagic squid).
Pelagic squid have really good swimming ability, with speeds up to 55 kilometers per hour and cruising speeds around 20 kilometers per hour.
Borderline Organisms
Certain invertebrates straddle the line between necton and plankton.
Examples include box jellyfish, which have limited swimming ability that can be overcome by strong water currents.
Life Cycle Stages
Mainly juveniles and adults are considered necton due to their larger size and greater mobility.
Secondarily Marine Vertebrates
Cetaceans are born in the marine environment and are capable swimmers from birth.
Others lay eggs on land and enter the water as good swimmers after early development.
Sharks
Pelagic sharks have live birth, with embryos nourished in the mother's uterus through egg yolk or direct nutrition from the mother.
At birth, they are already capable swimmers.
Size at birth can be quite large, such as 1.5 meters for great white sharks.
Teleosts and Other Invertebrates
They tend to have complex life cycles.
Early life stages are planktonic, with size and mobility increasing as they develop.
Early life stages being planktonic and later life stages in the nectar.
This is an example of a meroplanktonic organism.
Some meroplanktonic organisms have bed thick adults and some have nectonic adults.
Typical Life Cycle of a Nectonic Teleost
Eggs and larvae are planktonic.
They develop into small juveniles called micronecton, which have moderate swimming abilities.
Large juveniles and adults are truly nectonic.
Life Cycle of a Tuna
Adults are broadcast spawners, releasing gametes into the water.
Eggs are planktonic for a few days, developing into larval tuna.
The larval phase lasts about 20 days, followed by metamorphosis into small juveniles (micronecton).
They then grow larger and become truly nectonic.
General Characteristics of Necton
Trophic Role
Animals and consumers, mainly carnivores and predators.
They tend to be at the upper levels of the food web.
Small necton feed on zooplankton, while larger necton feed on smaller necton.
At the bottom of the food web, we have tiny phytoplankton, our tiny primary producers.
Small zooplankton feed on tiny primary producers, and then the nectar come in at the upper ends of the food chain.
Size
Wide size range, including very large animals.
Largest Invertebrate
Colossal squid (Mesonychoteuthis hamiltoni).
Known to be shorter but heavier than the giant squid.
Eyes can be about the size of a 30 centimeter ruler.
Largest Fish
Whale shark (Rhincodon typus), a large, slow-moving plankton feeder with a global distribution in tropical and subtropical waters.
Heaviest Bony Fish
Giant sunfish (Mola mola), which can approach 3,000 kilos.
They often sit at the surface of the water on their side sunning themselves.
They feed on gelatinous plankton.
Larval sunfish reveal their evolutionary relationship to pufferfish.
Largest Animal
Blue whale (Balaenoptera musculus), reaching 30 meters and 120,000 kilos.
The largest animal that ever lived.
Water's buoyancy helps to support their massive bodies.
Terrestrial animals require a massive skeleton to support their weight.
Depth Variation in Teleosts (Bony Fish)
The type and characteristics of fish vary with depth.
Factors Varying with Depth
Temperature: Warmer in surface waters, rapidly declines in the mesopelagic, and is cold beyond 1,000 meters.
Light: Sufficient for photosynthesis only in the upper levels, with twilight in the mesopelagic and no ambient light beyond 1,000 meters.
Plankton Abundance: More in surface waters, less in the mesopelagic, and even less below 1,000 meters.
Common Types of Fish
Epipelagic (0-200 meters)
Flying fish (Exocoetidae), with elongate pectoral fins for gliding through the air to avoid predators.
Tuna.
Dolphin fish (Coryphaena hippurus), easily recognized because of the sort of pronounced melon on the head of the males.
Billfishes (marlin, sailfish, swordfish).
Billfish Characteristics
Large predators with a projection of the upper jaw into a bill used to stun prey.
Swordfish use their swords to stun prey.
Blue marlin is the largest, reaching up to 5 meters and 900 kilos.
Black marlin or Indo-West Pacific sailfish are the fastest, reaching speeds up to 110 kilometers per hour.
They cruise at low speeds over large distances, searching for prey.
They have morphological adaptations for sustained swimming.
Adaptations for Sustained Swimming
Streamlined body shape.
Fins that can be folded close to the body to reduce turbulence.
High aspect ratio tail (tall but with little surface area) to reduce drag.
Narrow caudal peduncle to reduce drag.
Keels to reduce turbulence.
Visual predators with a heater organ to maintain visual acuity in cold water.
Heater Organ in Swordfish
Modified muscles filled with mitochondria to produce metabolic heat.
Maintains the eye and brain temperature between 19 and 28 degrees Celsius, which may be 15 degrees higher than the water temperature.
Allows the fish to distinguish different wavelengths of light up to 25-30 hertz.
Metabolic Expense
High-speed swimming over long distances requires lots of oxygen and food.
This lifestyle is not sustainable at deeper depths.
Mesopelagic (200-1,000 meters)
Common fish include hatchetfish, dragonfish, and lanternfish.
Lanternfish
Belong to the family Myctophyidae, with over 250 species.
Mostly small, less than 15 centimeters in length.
Contribute 65% of the mass of mesopelagic fish.
They have dark tops, silvery sides, and elaborate photophores.
The arrangement of photophores differs from species to species and may be used for mate recognition.
Many undertake daily vertical migrations, living in deeper water during the day and moving up at night to feed.
Bathypelagic (1,000-4,000 meters)
Common taxa include anglerfish, bristle mouse, gulper and swallow eels, and whalfish.
Whalfish
Named because they're said to look like whales.
They have really, really big mouth, and its eyes are vestigial.
They've a really well developed lateral line.
Gulper Eels
Gulper eels tend to have long, thin bodies, but they've got absolutely ginormous heads and huge mouths.
Anglerfish
They're all characterized by this fishing lure.
The fishing lure is a modification of the ray of the dorsal fin, which is greatly elongated, and it's got a little fleshy blob on top.
They tend to be ambush predators, and they use the lure to attract prey into the vicinity of their mouth.
Bristlemouths
Small fish with large mouths and numerous bristle-like teeth.
It's one of the most abundant types of fish in the mesopelagic.
Characteristics of Fish at Different Depths
Size
Epipelagic: Wide size range, including many large species.
Mesopelagic: Mainly small, most species are less than 15 centimeters.
Bathypelagic: Smallish, but generally a little bit larger than mesopelagic species (less than 100 centimeters).
Lifestyle
Epipelagic: Good swimmers, migrating over large horizontal distances.
Mesopelagic: Vertical migrators (good swimmers) and permanent residents (poor swimmers).
Bathypelagic: Poor swimmers, hovering in the water column.
Body Shape
Epipelegic fish: Streamlined.
Bathypelagic Fish: Globular.
Mesopelagic Migrators tend to be elongated, A lot of them are laterally compressed.
Musculature and Skeleton
Epipelagic: Well developed musculature and skeleton.
Bathypelagic: Reduced musculature and skeleton, watery tissue.
Mesopelagic: Vertical migrators similar to epipelagic, permanent residents similar to bathypelagic.
Vision
Epipelagic: Well-developed eyes on the side of the head.
Mesopelagic: Big eyes.
Bathypelagic: Small, sometimes vestigial eyes, other senses take over.
Bathypelagic anglerfish equipped with pressure sensors. The long hairs and all the long fin rays
The midwater fish have tubular eyes. So they're able to collect any light coming from above, but they're able to look to the sides. They might be able to pick up things that are bioluminescing to the side.
Color and Camouflage
Epipelagic: Silvery bodies, countershading (darker on top, lighter on the belly).
Mesopelagic: Silvery bodies, counter illumination (photophores on the ventral surface to break up the silhouette).
Bathypelagic: Reddy brown, black, or transparent.
Predator-Prey Interactions
Epipelagic: Driven by vision and active pursuits.
Bathypelagic: Ambush predators, using lures, and opportunistic feeders with big mouths and stomachs.
Reproductive Strategies
Epipelagic: Broadcast spawners with external fertilization, planktonic eggs and larvae, and very high fecundity (1-10 million eggs per spawning round).
Mesopelagic: Complex life cycles with planktonic eggs and larvae, and positively buoyant eggs. Larvae hatches out, there's a relatively large amount of food around.
Bathypelagic: Similar to mesopelagic, but with higher fecundity and males that specialize in finding females.
Adaptations for Reproduction in the Bathypelagic
Males have enhanced olfaction to find females emitting pheromones.
In some anglerfish species, males bite into the female and become permanently attached, feeding off the female's blood and donating sperm.