Marine Biology Terms Starting With R
Marine Biology Glossary: R
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Red Tide
/ RED tyd / · Old English read + tid
Red Tide is a colloquial term for a harmful algal bloom caused by the rapid proliferation of certain dinoflagellates or other microalgae that discolor the water and can produce toxins dangerous to marine animals and humans.
Red tides are not always red and are not caused by tides; they result from the rapid multiplication of specific phytoplankton species under warm, nutrient-rich, calm conditions. Some red tide organisms produce potent neurotoxins including saxitoxin and brevetoxin that accumulate in shellfish and cause paralytic shellfish poisoning, neurotoxic shellfish poisoning, and amnesic shellfish poisoning in humans. Dense blooms can also cause mass mortality of fish and marine mammals through oxygen depletion as decomposing cells consume dissolved oxygen near the seafloor.
Karenia brevis, the dinoflagellate responsible for most Gulf of Mexico red tides, produces brevetoxins that can become airborne in sea spray, causing respiratory irritation in beachgoers up to a kilometer from the water. Florida health authorities have documented beach closures and manatee die-offs linked to these blooms repeatedly since the 1940s.
Every algal bloom is a red tide. Red tide refers specifically to certain harmful blooms caused by toxin-producing species, while many algal blooms are non-toxic and cause no harm to marine life or humans.
Karenia brevis blooms in the Gulf of Mexico can reach cell densities of millions of cells per liter and produce brevetoxins that kill fish by disrupting their sodium channels. These blooms have caused the deaths of more than 100 bottlenose dolphins (Tursiops truncatus) during single bloom events recorded off the Florida coast.
Reef Building Coral
/ REEF BIL-ding KOR-ul / · Old English reef; Old English bold; Greek korallion
Reef Building Coral is a colonial cnidarian animal that maintains symbiotic photosynthetic algae inside its tissues and secretes calcium carbonate skeletons that accumulate over thousands of years into coral reef structures.
Each coral polyp is a small, anemone-like animal that secretes a hard calcium carbonate cup around itself, adding to its skeleton throughout its life. When a polyp dies, its skeleton remains as the foundation for new polyps growing on top, allowing reefs to grow upward and outward over centuries. Symbiotic dinoflagellate algae called zooxanthellae live within the coral’s tissues and supply up to 90 percent of the coral’s energy through photosynthesis, while the coral provides the algae with shelter and access to nutrients from its metabolic waste.
Without this partnership, reef growth slows dramatically, and sustained warming above about 1 degree Celsius above the seasonal maximum triggers bleaching, during which polyps expel their zooxanthellae and may die if the stress persists.
The Great Barrier Reef contains over 400 species of reef-building corals and took approximately 8,000 years to form its current structure, growing at rates of only 0.3 to 2 centimeters per year depending on species.
One coral reef is made by one giant coral animal. Reefs are built by colonies of genetically distinct coral polyps, each only a few millimeters wide, along with contributions from coralline algae, mollusks, and other calcium-carbonate-depositing organisms accumulated over thousands of years.
Staghorn coral (Acropora cervicornis) grows branching skeletons at rates of up to 10 centimeters per year, among the fastest of any reef-building species, and its dense thickets shelter dozens of reef fish species and juvenile invertebrates. Populations across the Caribbean have declined by more than 97 percent since the 1970s due to disease, bleaching, and storm damage.
Reef Flat
/ REEF flat / · Old Norse rif + Old English flaet
Reef Flat is the shallow, often wave-scoured platform of a coral reef that extends between the reef crest and the lagoon or shoreline, lying at or near sea level.
Reef flats form as the outer reef grows upward to sea level, leaving a platform subject to extreme physical conditions including intense solar radiation, temperatures that can exceed 35 degrees Celsius in shallow pools, and periodic aerial exposure during low tides. Despite these stresses, reef flats support diverse communities of encrusting coralline algae, sea urchins, small fish, and mollusks adapted to fluctuating conditions. Many juvenile fish and invertebrates use reef flats as nursery habitat before recruiting to the deeper reef crest and slope as they mature.
On Indo-Pacific reef flats, the crown-of-thorns sea star (Acanthaster planci) sometimes migrates from deeper reef slopes onto the flat during nighttime low tides to feed on encrusting organisms. Population outbreaks of this species have destroyed up to 90 percent of coral cover on affected reefs across the Great Barrier Reef since the 1960s.
Reef flats are bare, dead rock platforms. They support living communities of encrusting algae, seagrasses, mollusks, worms, sea urchins, and small fishes, and their biological activity contributes to reef sediment production and nutrient cycling.
On the reef flats of Palau, grazing rabbitfish (Siganus spp.) and sea urchins crop algae that would otherwise overgrow encrusting coralline surfaces. Studies on these flats have recorded algal removal rates exceeding 50 percent of available algal biomass over short experimental intervals, keeping coral recruitment surfaces clear even during periods of elevated nutrient input.
Riptide
/ RIP-tyd / · Old English ripan (to tear) + tid (tide)
Riptide is a common but less accurate name for a rip current, a narrow, fast-moving channel of water that flows away from shore through the surf zone.
Rip currents can flow at speeds exceeding 2 meters per second, faster than even competitive swimmers can sustain, and account for more than 80 percent of lifeguard rescues at surf beaches in the United States. They form most commonly in channels between sandbars, at breaks in sandbars, and alongside structures such as piers and jetties where wave energy concentrates water flow. Swimmers caught in a rip current should swim parallel to shore to exit the narrow channel rather than fighting the current directly toward the beach, which leads to exhaustion.
The United States Lifesaving Association estimates that rip currents cause an average of 100 drowning deaths per year in the United States alone. Researchers at the University of New South Wales have used dye tracers and GPS-equipped drifters to map rip current pathways and show that many follow a circular path, eventually returning swimmers toward shore if they remain calm and float.
A riptide pulls swimmers underwater toward the seafloor. Rip currents move water horizontally away from shore along the surface; the danger is exhaustion from fighting the current, not submersion by a downward pull.
Rip currents form where breaking waves push water toward shore and the accumulated water escapes seaward through a narrow gap between sandbars. At beaches along the Florida Atlantic coast, rip currents can reach widths of 10 to 30 meters and extend more than 100 meters beyond the surf zone before dissipating.