remiconnesson/oceanservice-noaa-facts huggingface dataset:
# What are barnacles?

![Image of gooseneck barnacles found in the rocky tide pools of Olympic Coast National Marine Sanctuary Photo credit: Elizabeth Weinberg, NOAA National Marine Sanctuaries](/facts/barnacle.jpg)

_Those aren't dragon claws — they're gooseneck barnacles! These filter feeders are found in the rocky tide pools of [Olympic Coast National Marine Sanctuary](http://olympiccoast.noaa.gov/). Their shells are made up of multiple white plates that help protect them from predators and from drying out._

Of the more than 1,400 species of barnacles found in the world’s waterways, the most common ones are called acorn barnacles. As anyone who’s ever maintained a vessel knows, removing barnacles requires some elbow grease (or a pressure washer). That's why some boaters call them by their slang name: "crusty foulers."

How do barnacles stick to the undersides of vessels, to other sea life, to each other, and to pretty much anything they come in contact with? They secrete a fast-curing cement that is among the most powerful natural glues known — the glue is so strong that researchers are trying to figure out how it can be used commercially.

Barnacles like places with lots of activity, like [underwater volcanos](/facts/volcanoes.html) and intertidal zones, where they reside on sturdy objects like rocks, pilings, and buoys. Moving objects like boat and ship hulls and [whales](http://sanctuaries.noaa.gov/pgallery/pgfarallones/living/living_4.html) are particularly vulnerable to the pesky critters. Large barnacle colonies cause ships to drag and burn more fuel, leading to significant economic and environmental costs. The U.S. Navy estimates that heavy barnacle growth on ships increases weight and drag by as much as 60 percent, resulting in as much as a 40 percent increase in fuel consumption!

Barnacles feed through feather-like appendages called cirri. As the cirri rapidly extend and retract through the opening at the top of the barnacle, they comb the water for microscopic organisms. They quickly withdraw into their protective shells if they sense a potential threat. Barnacles secrete hard calcium plates that completely encase them. A white cone made up of six calcium plates forms a circle around the crustacean. Four more plates form a "door" that the barnacle can open or close, depending on the tide. When the tide goes out, the barnacle closes up shop to conserve moisture. As the tide comes in, a muscle opens the door so the feathery cirri can sift for food.

Did you know? - *Most barnacles are hermaphrodites, meaning that they have both male and female sex organs. To create baby barnacles, they must be fertilized by a neighbor. A retractable tube containing sperm can reach outside the barnacle’s shell as far as 15 to 20 centimeters (six to eight inches) to fertilize a nearby barnacle. And reproduce they do: up to six broods a year. In this photo, barnacles encroach upon the blowholes of a California gray whale. Photo credit: Jan Roletto*
# What is the difference between local sea level and global sea level?

##### **Did you know?**

_[Global sea level](/facts/sealevel.html) has been rising over the past century, and the rate has increased in recent decades._

Global sea level trends and relative sea level trends are different measurements. Just as the surface of the Earth is not flat, the surface of the ocean is also not flat — in other words, the sea surface is not changing at the same rate at all points around the globe. [Sea level rise at specific locations](https://tidesandcurrents.noaa.gov/sltrends/sltrends.html) may be more or less than the global average due to many local factors: [subsidence](/facts/subsidence.html), ocean [currents](/facts/current.html), variations in land height, and whether the land is still rebounding from the [compressive weight of Ice Age glaciers](/facts/glacial-adjustment.html).

Sea level is primarily measured using [tide stations](https://www.climate.gov/news-features/climate-tech/reading-between-tides-200-years-measuring-global-sea-level) and [satellite laser altimeters](/facts/satellites-ocean.html). Tide stations around the globe tell us what is happening at a local level — the height of the water as measured along the coast relative to a specific point on land. Satellite measurements provide us with information on how the ocean surface is changing — at a particular location or on the average across the entire ocean. Taken together, these tools tell us how our ocean sea levels are changing over time.

Did you know? - *[Global sea level](https://oceanservice.noaa.gov/facts/sealevel.html) has been rising over the past century, and the rate has increased in recent decades.*
# What is a Portuguese Man o’ War?

![The Portuguese man o’ war is recognized by its balloon-like float.](/facts/portuguese.jpg "The Portuguese man o’ war is recognized by its balloon-like float.")

_The Portuguese man o’ war is recognized by its balloon-like float, which may be blue, violet, or pink and rises up to six inches above the water line. Image credit: Elizabeth Condon, [National Science Foundation](http://www.nsf.gov/discoveries/disc_summ.jsp?cntn_id=126630)_

The Portuguese man o’ war, (_Physalia physalis_) is often called a jellyfish, but is actually a species of siphonophore, a group of animals that are closely related to jellyfish. A siphonophore is unusual in that it is comprised of a colony of specialized, genetically identical individuals called zooids — clones — with various forms and functions, all working together as one. Each of the four specialized parts of a man o’ war is responsible for a specific task, such as floating, capturing prey, feeding, and reproduction. Found mostly in tropical and subtropical seas, men o' war are propelled by winds and ocean currents alone, and sometimes float in legions of 1,000 or more!

Resembling an 18th-century Portuguese warship under full sail, the man o’ war is recognized by its balloon-like float, which may be blue, violet, or pink and rises up to six inches above the waterline. Lurking below the float are long strands of tentacles and polyps that grow to an average of 10 meters (about 30 feet) and may extend by as much as 30 meters (about 100 feet). The tentacles contain stinging nematocysts, microscopic capsules loaded with coiled, barbed tubes that deliver venom capable of paralyzing and killing small fish and crustaceans. While the man o’ war’s sting is rarely deadly to people, it packs a painful punch and causes welts on exposed skin.

[Beachcombers be warned](/hazards/beach-dangers/sea-creatures-to-avoid.html): The stalwart man o’ war may still sting you even weeks after having washed ashore.
# What are the doldrums?

##### **Did you know?**

_Due to Earth’s rotation, the ITCZ shifts from season to season, causing both dry and rainy seasons in the northern and southern hemispheres._

_[Coral bleaching](/facts/coral_bleach.html) is prevalent in the ITCZ due to surface warming. NOAA’s [Coral Reef Conservation Program](https://coralreef.noaa.gov/) monitors environmental conditions in coral reefs to identify areas at risk for bleaching._

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Did you know? - *Due to Earth’s rotation, the ITCZ shifts from season to season, causing both dry and rainy seasons in the northern and southern hemispheres.*

- *[Coral bleaching](https://oceanservice.noaa.gov/facts/coral_bleach.html) is prevalent in the ITCZ due to surface warming. NOAA’s [Coral Reef Conservation Program](https://coralreef.noaa.gov/) monitors environmental conditions in coral reefs to identify areas at risk for bleaching.*

- [What are the trade winds?](https://oceanservice.noaa.gov/facts/tradewinds.html)

- [What are the horse latitudes?](https://oceanservice.noaa.gov/facts/horse-latitudes.html)

-
# What color is an iceberg?

## The colorful hues of icebergs result from the interplay of light with dense ice, ice crystals, and algal growth.

![Iceberg located in Ross Sea, Antarctica.](/facts/iceberg-color.jpg "Iceberg located in Ross Sea, Antarctica.")

An iceberg (and a few penguins) floating off the coast of Antarctica. Photo courtesy of Kathy Reitz.

Most people would say that icebergs are white—and most of them are. But did you know that icebergs can also appear in spectacular shades of blue and green? An iceberg looks white because compressed snow on its surface contains large numbers of tiny air bubbles and crystal edges that equally reflect all wavelengths of visible light.

As more and more heavy snow accumulates atop an iceberg, the air bubbles get compressed, forcing the smaller ice crystals to grow together and merge into larger grains. When the iceberg is underwater, the air bubbles are squeezed out and washed away. Then, when light encounters the dense, compressed ice, much of the light penetrates it. The ice absorbs longer wavelengths of colors, such as red and yellow. Colors of shorter wavelengths, like green and blue, reflect the light. This "leftover" blue-green light is what gives some icebergs their remarkable colors.

Additionally, algae often grow on the underwater sides of icebergs, producing beautiful green stripes in the ice. These are readily seen when an iceberg rolls over and sections that were previously underwater are exposed.

The [U.S. National Ice Center](https://usicecenter.gov/), operated by NOAA, the U.S. Navy, and the U.S. Coast Guard, monitors ice and snow worldwide.
# What is spat?

## Once **oyster larvae** permanently attach to a surface, they are known as **spat**.

![an oyster bed with an inset showing spat](/facts/spat.jpg "Oyster larvae typically settle on the shells of other oysters, forming dense, expansive clusters known as oyster reefs or beds. Once larvae “settle down” on such a surface, they are known as spat. ")

Once oyster larvae attach to a surface, such as other oyster shells, they are known as spat (shown in inset image). As generation after generation of spat grow into adult oysters, they form dense clusters known as oyster reefs or beds.

Oysters are a type of [shellfish](/facts/bivalve.html) that live in brackish and saltwater bays, [estuaries](/facts/estuary.html), and tidal creeks. When oysters reproduce, they spawn tiny larvae that freely navigate the water column until they find an appropriate habitat with a structure to settle on. Once the larvae permanently attach to a surface, they are known as spat.

Oysters are frequently cultivated for food and pearls. The successful farming of oysters and other shellfish relies upon successful settlement of larvae onto a selected substrate—typically other oyster shells or ceramic tiles—within a hatchery or wild setting. The tiles or shells that hold the spat are secured to frames or in cages and submerged along an [intertidal](/facts/saltmarsh.html) area or suspended from a long line. From this point forward, the oysters are self-sustaining, filtering all the nutrients they need directly from the water in their environment.

Because oysters are filter feeders, they help keep the water clean. This promotes the growth of underwater grasses, such as wild celery, which serve as important habitat for other species. In addition, oyster beds form large, complex structures where many aquatic species, such as fish and crabs, hunt for food and hide from predators.

Did you know? - 

- Oyster farming is a great way to [improve water quality](http://coastalscience.noaa.gov/news/ecosystem-management/oyster-gardening-improves-water-quality-featured-national-geographic/)! A NOAA [study found](http://coastalscience.noaa.gov/news/coastal-pollution/noaa-estimates-nitrogen-removal-rates-shellfish-farms-around-world/) that shellfish aquaculture helps to remove excess nutrients and reduce eutrophication from our waterways. [Eutrophication](https://oceanservice.noaa.gov/facts/nutpollution.html)—excessive nutrient input into estuarine and coastal environments—can lead to [algal blooms](https://oceanservice.noaa.gov/hazards/hab/), [oxygen depletion](https://oceanservice.noaa.gov/hazards/hypoxia/), fish kills, and a loss of key habitats.
# What is the Forchhammer's Principle?

## Forchhammer's Principle refers to the chemical composition of ocean water.

![A CTD. CTD stands for conductivity, temperature, and depth, and refers to a package of electronic instruments that measure these properties.](/facts/forchhammers960.jpg "CTD stands for conductivity, temperature, and depth, and refers to a package of electronic instruments that measure these properties.")

A CTD (conductivity, temperature, and depth) device. The primary function of this tool is to detect how the conductivity and temperature of the water column changes relative to depth. Conductivity is a measure of how well a solution conducts electricity. Conductivity is directly related to salinity, which is the concentration of salt and other inorganic compounds in seawater. Salinity is one of the most basic measurements used by ocean scientists. When combined with temperature data, salinity measurements can be used to determine seawater density, which is a primary driving force for major ocean currents.

In 1865, the Danish geologist and mineralogist Johan Georg Forchhammer, with the help of naval and civilian collaborators, collected numerous samples of seawater from the Northern Atlantic and the Arctic Ocean. He wanted to determine why the salinity (or "saltiness") of seawater varies in different areas of the ocean.

Forchhammer put the samples through a detailed series of chemical analyses and found that the proportions of the major salts in seawater stay about the same everywhere. This constant ratio is known as _Forchhammer's Principle_, or the _Principle of Constant Proportions_. In addition to this principle, Forchhammer is credited with defining the term _salinity_ to mean the concentration of major salts in seawater.

Forchhammer's discovery helped scientists understand that salinity levels in seawater vary due to the addition or removal of fresh water, rather than differing amounts of salt minerals in the water. The principle is still applied today in marine research, and provides a simple way to estimate salinity and trace the mixing of water masses in the global ocean.
# What is a platypus?

## The platypus is a **remarkable mammal** found only in Australia.

![the platypus](/facts/platypus.jpg "A platypus")

Sometimes known as a duck-billed platypus, this curious mammal combines the characteristics of many different species in one.

The platypus is a duck-billed, beaver-tailed, otter-footed, egg-laying aquatic creature native to Australia. If its appearance alone somehow fails to impress, the male of the species is also one of the world’s few venomous mammals! Equipped with sharp stingers on the heels of its hind feet, the male platypus can deliver a strong toxic blow to any approaching foe.

While the platypus generally inhabits freshwater rivers, wetlands, and [billabongs](http://en.wikipedia.org/wiki/Billabong) Down Under, it is also known to venture into brackish [estuaries](/facts/estuary.html) (the combined fresh-and saltwater areas where rivers meet the sea).

The platypus is a bottom-feeder that uses its beaver-like tail to steer and its webbed feet to propel itself through the water while hunting for insects, shellfish, and worms. The watertight nostrils on its bill remain sealed so that the animal can stay submerged for up to two minutes as it forages for food. The bill also comes equipped with specialized nerve endings, called electroreceptors, which detect tiny electrical currents generated by the muscular contractions of prey. It has no teeth, so the platypus stores its "catch" in its cheek pouches, returns to the surface, mashes up its meal with the help of gravel bits hoovered up enroute, then swallows it all down.

The female platypus lays her eggs in an underground burrow that she digs near the water’s edge. Baby platypuses hatch after 10 days and nurse for up to four months before they swim off and forage on their own.

The Commonwealth of Australia reveres this remarkable mammal so much that it honors the platypus with a place on its [20-cent coin](https://www.ramint.gov.au/twenty-cents).

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Did you know?

There is only one other egg-laying mammal in the world. The [echidna](https://parks.tas.gov.au/?base=4796), a distant cousin of the anteater, also lays eggs and is found in Australia and New Guinea. Both the platypus and the echnida are monotremes, meaning that they lay eggs and have a single opening (cloaca) for reproduction and elimination of wastes.

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**Last updated:** 06/16/24  
**Author:** NOAA  
[How to cite this article](/about/faq.html)

[Contact Us](/contact.html)

Did you know? - 

- There is only one other egg-laying mammal in the world. The [echidna](https://parks.tas.gov.au/?base=4796), a distant cousin of the anteater, also lays eggs and is found in Australia and New Guinea. Both the platypus and the echnida are monotremes, meaning that they lay eggs and have a single opening (cloaca) for reproduction and elimination of wastes.
# What is marine biogeography?

## Marine biogeography is the study of **marine species**, the **geographic distribution** of their habitats, and the **relationships between** living organisms and the environment.

 [](/caribbean-mapping/video/mapping.mp4) 

Creating a habitat ecosystem map of the seafloor—a key component of marine biogeography—is a tricky process. Learn how it works in this two-minute video. [Learn more](/caribbean-mapping/gallery-oceanglider.html).

By mapping [benthic habitats](/facts/benthic.html), studying what occurs on the bottom of a body of water, and assessing the relationships between the environment and the organisms that live there, biogeographers provide useful information to protect and conserve marine resources.

Marine biogeographers often use [Geographic Information Systems](/facts/gis.html), or GIS, to aid in their research of marine animals, plants, and habitats. Scientists and GIS specialists develop map-based data that describe the distribution and ecology of living marine resources and their connections to human communities. State and federal planners can apply these tools and information to position aquaculture sites and alternative energy facilities, and to protect fisheries and coral spawning areas. Information from biogeographers allows planners to consider possible scenarios, such as new development, that may, or may not, impact the environment.

NOAA's [National Centers for Coastal Ocean Science](https://coastalscience.noaa.gov/) conducts research on the distribution and ecology of marine plants and animals to predict future trends. Tools such as surveys, maps, and reports provide specific information to assess ecosystem conditions, anticipate changes in the environment, and evaluate how people's social and economic needs can be met.

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### Did you know?

Vast and diverse coral reefs surround the U.S. territory of American Samoa, an archipelago located in the central Pacific. In 2011, NOAA completed a [Biogeographic Assessment of the Samoan Archipelago](http://coastalscience.noaa.gov/news/ecosystem-management/biogeographic-assessment-of-the-samoan-archipelago-sheds-light-on-marine-life-and-biological-connectivity-among-islands/), the first broad-scale effort to characterize the islands' marine ecosystems in a comprehensive manner.

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**Last updated:** 06/16/24  
**Author:** NOAA  
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[Contact Us](/contact.html)

Did you know? - Vast and diverse coral reefs surround the U.S. territory of American Samoa, an archipelago located in the central Pacific. In 2011, NOAA completed a [Biogeographic Assessment of the Samoan Archipelago](http://coastalscience.noaa.gov/news/ecosystem-management/biogeographic-assessment-of-the-samoan-archipelago-sheds-light-on-marine-life-and-biological-connectivity-among-islands/), the first broad-scale effort to characterize the islands' marine ecosystems in a comprehensive manner.
# What is a wetland?

VIDEO: What is a wetland? Here's what you need to know in less than a minute. [Transcript](#transcript)

There are many different kinds of wetlands and many ways to categorize them. NOAA classifies wetlands into five general types: marine (ocean), estuarine (estuary), riverine (river), lacustrine (lake), and palustrine (marsh). Common names for wetlands include marshes, estuaries, mangroves, mudflats, mires, ponds, fens, swamps, deltas, coral reefs, billabongs, lagoons, shallow seas, bogs, lakes, and floodplains, to name just a few!

Often found alongside waterways and in floodplains, wetlands vary widely due to differences in soil, topography, climate, water chemistry, and vegetation. Large wetland areas may also be comprised of several smaller wetland types.

  Wetland habitats serve essential functions in an ecosystem, including acting as water filters, providing flood and erosion control, and furnishing food and homes for fish and wildlife. They do more than sustain plants and animals in the watershed, however. Many wetlands are not wet year-round because water levels change with the seasons. During periods of excessive rain, wetlands absorb and slow floodwaters, which helps to alleviate property damage and may even save lives.

Wetlands also absorb excess nutrients, sediments, and other pollutants before they reach rivers, lakes, and other waterbodies. They are also great spots for fishing, canoeing, hiking, and bird-watching, and are enjoyable outdoor "classrooms" for people of all ages.

### Video Transcript

Wetlands are found along waterways and in floodplains. They come in all shapes and sizes. Wetlands filter water, providing flood and erosion control. Wetlands were once thought of as useless swamps. But now, we know they are home to abundant fish and wildlife.
# What is a thermocline?

![illustration of the thermocline](/facts/thermocline.jpg "This illustration shows a typical seawater temperature profile. As depth increases, water temperature decreases.")

_The red line in this illustration shows a typical seawater temperature profile. In the thermocline, temperature decreases rapidly from the mixed upper layer of the ocean (called the [epipelagic](/facts/mesophotic.html) zone) to much colder deep water in the thermocline (mesopelagic zone). Below [3,300 feet](#) to a depth of about [13,100 feet](#), water temperature remains constant. At depths below 13,100 feet, the temperature ranges from near freezing to just above the freezing point of water as depth increases._

Bodies of water are made up of layers, determined by temperature. The top surface layer is called the epipelagic zone, and is sometimes referred to as the "ocean skin" or "sunlight zone." This layer interacts with the wind and waves, which mixes the water and distributes the warmth. At the base of this layer is the thermocline. A thermocline is the transition layer between the warmer mixed water at the surface and the cooler deep water below. It is relatively easy to tell when you have reached the thermocline in a body of water because there is a sudden change in temperature. In the thermocline, the temperature decreases rapidly from the mixed layer temperature to the much colder deep water temperature.

In the ocean, the depth and strength of the thermocline vary from season to season and year to year. It is semi-permanent in the tropics, variable in temperate regions (often deepest during the summer), and shallow to nonexistent in the polar regions, where the water column is cold from the surface to the bottom.

Thermoclines also play a role in meteorological forecasting. For example, hurricane forecasters must consider not just the temperature of the ocean's skin (the sea surface temperature), but also the depth of warm water above the thermocline. Water vapor evaporated from the ocean is a hurricane's primary fuel. The depth of the thermocline is the measure of the size of the "fuel tank" and helps to predict the risk of hurricane formation.