Marine Biology Terms Starting With S
Marine Biology Glossary: S
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Salinity
/ sah-LIN-ih-tee / · Latin salinus (salty) + -ity
Salinity is the concentration of dissolved salts in water, measured in practical salinity units roughly equivalent to grams of dissolved salt per kilogram of seawater, with open ocean water averaging about 35 PSU.
Ocean salinity is maintained by the balance between evaporation, which concentrates dissolved salts, and precipitation, river input, and ice melt, which dilute them. Saltier water is denser and tends to sink, a property that drives thermohaline circulation, the global system of deep ocean currents that redistributes heat and nutrients across ocean basins. Organisms are adapted to specific salinity ranges; stenohaline species tolerate only narrow variation, while euryhaline species such as Atlantic salmon (Salmo salar), European eels (Anguilla anguilla), and eastern oysters (Crassostrea virginica) can survive across a broad range by actively regulating their internal ion concentrations.
The Dead Sea, a landlocked hypersaline lake bordering Israel and Jordan, maintains a salinity of roughly 340 PSU, nearly ten times that of the open ocean. At that concentration, almost no multicellular organisms survive, though certain halophilic bacteria and archaea thrive in its waters.
Ocean salinity is the same everywhere. Evaporation, rainfall, river discharge, sea ice formation, and ocean currents all create regional salinity differences, ranging from below 10 PSU in some estuaries to above 40 PSU in enclosed seas like the Red Sea.
Estuaries experience salinity gradients because freshwater from rivers mixes with saltwater from the sea, and tidal cycles shift that boundary twice daily. Eastern oysters (Crassostrea virginica) tolerate salinities from about 5 to 35 PSU, allowing them to thrive across much of this gradient where many other bivalves cannot survive.
Sea Floor Spreading
/ see flor SPRED-ing / · Old English sae + flor + spredan
Sea Floor Spreading is the process by which new oceanic crust forms continuously at mid-ocean ridges as magma rises between diverging tectonic plates, solidifies, and pushes existing seafloor outward on both sides of the ridge.
Harry Hess proposed sea floor spreading in 1960, and the theory was confirmed by the discovery of symmetrical magnetic reversal stripes on either side of mid-ocean ridges, recorded as the seafloor cooled and locked in the orientation of Earth’s magnetic field at the time of formation. New basaltic crust forms at the ridge axis, cools, and moves laterally away from the spreading center at rates of 2 to 18 centimeters per year, with fast-spreading ridges like the East Pacific Rise producing broader, lower-relief structures than slow-spreading ridges like the Mid-Atlantic Ridge. Oceanic crust produced at spreading centers is eventually subducted at convergent margins, making it geologically young compared to continental crust, with no oceanic crust older than about 200 million years preserved anywhere on Earth.
The East Pacific Rise spreads at up to 18 centimeters per year, fast enough that researchers using seafloor mapping surveys conducted decades apart can measure the displacement directly. At that rate, the Pacific Ocean floor moves roughly the width of a human hand each year.
The seafloor is permanent and unchanging. Oceanic crust is continuously generated at mid-ocean ridges and consumed at subduction zones, completing a cycle that recycles all oceanic crust within roughly 200 million years.
At the Mid-Atlantic Ridge, magma rises and cools to form new basaltic seafloor at a spreading rate of about 2.5 centimeters per year. Hydrothermal vent fields such as those at Lost City, located about 15 kilometers from the ridge axis, develop where seawater circulates through the newly formed crust and exits superheated and laden with minerals.
Seafloor
/ SEE-flor / · Old English sae; Old English flor
Seafloor is the solid surface at the bottom of the ocean, covering approximately 70 percent of Earth's surface and encompassing wide abyssal plains, mid-ocean ridges, deep trenches, seamounts, submarine canyons, and hydrothermal vent fields.
The average depth of the seafloor is about 3,800 meters, placing most of it in permanent darkness under pressures exceeding 380 atmospheres. Far from lifeless, the deep seafloor supports bacteria, polychaete worms, sea cucumbers, brittle stars, and specialized fish adapted to cold temperatures and high pressure. The Mid-Atlantic Ridge forms a submarine mountain chain stretching more than 16,000 kilometers where tectonic plates diverge and new crust forms continuously from erupting magma.
At the opposite extreme, the Challenger Deep in the Mariana Trench descends to about 11,000 meters, the deepest point yet measured on Earth’s surface.
Less than 25 percent of the global seafloor has been mapped at high resolution using multibeam sonar, meaning more of Earth's ocean bottom remains uncharted than the surfaces of Mars or the Moon, both of which have been mapped in greater detail by orbiting spacecraft.
The seafloor is flat mud everywhere. The seafloor includes towering mid-ocean ridges rising more than 2,000 meters above the surrounding plain, trenches plunging beyond 11,000 meters, active volcanic vents, and submarine canyons that rival the Grand Canyon in scale.
On soft abyssal sediments of the Pacific Ocean, sea cucumbers (order Holothuroidea) can make up more than 90 percent of the visible animal biomass at depths below 4,000 meters. These animals ingest sediment continuously, processing several times their body weight each day and recycling organic matter that sinks from surface waters.
Seagrass
/ SEE-gras / · Old English sae + graes
Seagrass is one of the only true flowering plants adapted to live fully submerged in marine and estuarine environments, forming dense underwater meadows that provide habitat for seahorses, manatees, and juvenile fish.
Unlike seaweeds and algae, seagrasses have roots, rhizomes, leaves, flowers, and fruits and reproduce both sexually and vegetatively. Their meadows stabilize sediments, filter nutrients and particles from the water, produce oxygen, and store large quantities of carbon in slowly decomposing rhizome and root systems. Seagrass beds support nursery habitat for over 20 percent of the world?s commercially important fisheries and feeding grounds for dugongs (Dugong dugon), green sea turtles (Chelonia mydas), and seahorses.
Because much of their carbon is buried in low-oxygen sediment, some seagrass meadows store carbon at high rates per unit area compared with many terrestrial habitats.
A single hectare of seagrass meadow can support over 40,000 fish and 50 million small invertebrates, making these meadows among the most productive coastal habitats on Earth. Globally, seagrass meadows are estimated to cover between 300,000 and 600,000 square kilometers of seafloor.
Seagrass and seaweed are the same type of organism. Seagrass is a flowering plant with true roots, vascular tissue, and the ability to produce seeds, while seaweeds are algae that anchor with holdfasts and lack all of these structures.
Eelgrass (Zostera marina) meadows along the Pacific and Atlantic coasts provide nursery habitat for young Dungeness crabs and juvenile rockfish. A single square meter of healthy eelgrass can support hundreds of small invertebrates, and the dense root mats reduce sediment erosion by up to 70 percent compared to bare seafloor.
Seamount
/ SEE-mownt / · Old English sae + Latin mons (mountain)
Seamount is an underwater mountain that rises from the ocean floor but remains completely submerged, typically forming a habitat where upwelling currents concentrate nutrients and support dense communities of corals, fish, and invertebrates.
Seamounts divert deep currents upward through a process called Taylor column formation, bringing nutrient-rich water to shallower depths and fertilizing local waters. These features act as oases of biological richness in the open ocean, providing hard substrate for cold-water corals, sponges, and other sessile organisms while aggregating fish, sharks, and marine mammals. Over 100,000 seamounts taller than one kilometer exist in the world’s oceans, and many remain unexplored despite harboring species found nowhere else.
The New England Seamount Chain in the northwestern Atlantic, for example, supports dense populations of deep-sea corals and commercially targeted orange roughy (Hoplostethus atlanticus).
The Davidson Seamount off the California coast rises about 2,400 meters from the seafloor yet its summit remains more than 1,200 meters below the surface, and researchers have documented over 200 species of cold-water corals and sponges on its slopes, some colonies estimated to be over 1,000 years old.
Underwater mountains are barren rock piles with little biological activity. Seamounts concentrate prey through upwelling and provide hard substrate, making them among the most species-rich habitats in the open ocean.
The Coral Seamount in the southwestern Indian Ocean hosts dense thickets of black coral (Antipatharia spp.) and aggregations of pelagic fish including yellowfin tuna (Thunnus albacares). Current surveys have recorded more than 300 invertebrate species on its slopes, many of which are endemic to that seamount system.
Sublittoral Zone
/ sub-LIT-or-ul zohn / · Latin sub (below) + litoralis + zone
Sublittoral Zone is the region of the seafloor that remains permanently submerged, extending from the low-tide line to the edge of the continental shelf at approximately 200 meters depth.
This zone supports the most diverse and structurally complex benthic communities accessible by scuba diving and is the primary target for benthic fisheries worldwide. Subtidal rocky reefs within the sublittoral zone support kelp forests, coral reefs, sponge gardens, and diverse fish assemblages that depend on the stable, continuously submerged conditions. Light penetrates through much of the inner sublittoral zone, supporting photosynthetic organisms such as giant kelp (Macrocystis pyrifera), which can grow up to 60 centimeters per day under optimal conditions.
Deeper portions of the zone, where light diminishes, shift toward filter feeders and detritivores that depend on organic material sinking from above.
The sublittoral zone of the Gulf of Maine hosts some of the most studied benthic communities in the world, where researchers have tracked how the collapse of Atlantic cod (Gadus morhua) populations in the 1990s triggered cascading changes in sea urchin and kelp abundance across thousands of square kilometers of sublittoral seafloor.
Sublittoral habitats are exposed during low tide. Unlike the intertidal zone, the sublittoral zone remains submerged at all tidal stages, so its organisms never experience aerial exposure.
Giant kelp forests along the California coast grow entirely within the sublittoral zone, anchored to rocky substrate by holdfasts at depths of 5 to 30 meters. Canopy fronds at the surface can reach lengths exceeding 45 meters, and a single kelp forest patch may shelter over 800 species of fish and invertebrates.
What Do Otters Eat? →Surf Zone
/ SURF zohn / · Old Dutch schurft + zone (girdle)
Surf Zone is the nearshore area where incoming ocean waves break, generating turbulent white water, strong currents, and wave-driven sediment transport between the outermost breaking wave and the shoreline.
The surf zone is one of the most physically dynamic habitats on Earth, with waves generating turbulence, undertow, longshore currents, and rip currents that move enormous quantities of sand daily. Despite this physical intensity, specialized communities of sand crabs, sand-dwelling clams, and polychaete worms exploit the wave-driven nutrient flux. Pacific mole crabs (Emerita analoga) on California beaches, for example, burrow and re-emerge with each wave, filtering suspended diatoms and detritus through their feathery antennae.
Surf zone ecology links the nearshore marine environment directly to the adjacent beach, and shorebirds such as sanderlings (Calidris alba) forage along the swash line to pick off invertebrates exposed by receding waves.
On high-energy beaches in South Africa, the surf zone supports dense populations of the beach clam (Donax serra), with densities exceeding 1,000 individuals per square meter recorded in some surveys, making it one of the most productive invertebrate communities in any wave-swept habitat.
The surf zone is too rough to support much life. Many clams, crabs, fish, and polychaete worms are specifically adapted to moving water and shifting sand, and some species complete their entire life cycles within this turbulent strip.
Mole crabs (Emerita analoga) live in the sandy surf zone along the Pacific coast of North America and burrow rapidly as waves wash over them. A single square meter of surf-zone sand can contain more than 200 mole crabs during peak summer densities, and they migrate up and down the beach face with each tidal cycle.
Suspension Feeding
/ sus-PEN-shun FEE-ding / · Latin suspendere, to hang; Old English faedan
Suspension Feeding is a feeding strategy in which an animal captures small food particles, including plankton, bacteria, and organic detritus, directly from the surrounding water rather than pursuing individual prey.
Suspension feeders span an enormous size range, from microscopic larvaceans that build mucus houses to filter bacteria, to blue whales (Balaenoptera musculus) that gulp tonnes of krill-laden water and push it through baleen plates. Barnacles sweep feathery cirri through the water column to intercept passing particles, while bivalves such as oysters pump water across ciliated gills that trap food and transport it to the mouth. A single adult eastern oyster (Crassostrea virginica) can filter up to 190 liters of water per day, removing phytoplankton, bacteria, and suspended sediment in the process.
This feeding mode is especially common in sessile or slow-moving animals that cannot pursue prey and instead rely on water movement to deliver food.
The giant barrel sponge (Xestospongia testudinaria) of Indo-Pacific reefs can filter a volume of water equal to its own body volume every 5 seconds, and a reef covered in sponges may process the entire overlying water column multiple times each day, making sponges among the most productive suspension feeders on coral reefs.
Suspension feeders absorb dissolved nutrients directly through their body surfaces. They capture particulate food, including living plankton, detritus, and bacteria, by straining or trapping it from the water column.
Blue mussels (Mytilus edulis) suspension-feed by drawing water across their gills with cilia. A mussel bed covering one square meter can filter more than 1,000 liters of seawater per hour, removing phytoplankton at efficiencies exceeding 90 percent under low-flow conditions.