terms and definintions

Common Marine Aquarium and Reef Tank Terms

A glossary of words and their definitions you may come across as you read literature, online forums, and other reef keeping and marine tank related material. We are continually adding to the list so if you have a term you think should be included, check our acronym list first, and then let us know so we can add it in!

shortcut: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z


actinian: sea anemone.
aerobic: relating to, involving, or requiring free oxygen.
allelopathy: biological phenomenon where and organism produces one or more biochemicals that then influence the growth, survival, and reproduction of other organisms.
ambulacrum: name for any of the five radial bands to which the tube feet are attached on the underside (oral) of echinoderms such as star fish and sea urchins.
ambush predator: species that lies in wait for prey to come within striking distance.
amoeboid: similar in form, shape, and movement to a single-celled amoeba.
anaerobic: relating to, involving, or requiring the absence of free oxygen.
anchialine: having both salt and brackish water.
analog: creating an artificial environment that maintains similar conditions to a specific area in nature.
anterior: nearer the front; situated in the front of the body; nearer to the head or forepart.
antipatharians: black corals mostly found at depths over 328 ft (100m).
appendage: projecting part of an organism, with a distinct appearance or function.
aragonite: of the the two crystalline forms of calcium carbonate (CaCO3). Used by stony corals and other marine organisms to grow shells and skeletons.
artemia: genus that includes eight species of marine crustaceans known as brine shrimp. Naturally found in saltwater lakes world wide They are a primary food used in aquaculture and aquariums.
artemia cyst: the dormant egg of brine shrimp. It is encased in a tough membrane and able to exist in an inactive state in dry conditions for several years.
arthropod: invertebrate animal with an exoskeleton, segmented body, and jointed appendages. Arthropoda phylum includes crustaceans (shrimp, barnacles, lobsters, crabs, copepods), spiders and sea spiders, insects, and others.
aspergillosis: fungal disease that infects corals.
assemblage: fishes of different species and sizes coming together for a common purpose such as resting or feeding.
autotroph: organisms that produce complex organic compounds from simple inorganic compounds using the sun’s energy.
axial skeleton: bones of the head and main body in vertebrates.
azooxanthellate coral: coral with no zooxanthellae and get nourishment from capturing plankton forms.


bacterioplankton: bacterial part of plankton found free-floating in the water column. Serve as a food source from some corals.
bacterial aggregates: bacterial adhered to each other (clumped) or adhered / embedded in some other organic or inorganic material (detrius) and part of bacterioplankton. Biofilms dislodged from a solid surface are sometimes considered bacterial aggregates.
bacterial guild: group of bacteria sharing certain habits or characteristics.
benthic: bottom-dwelling.
bleaching: event on coral reef where weather prompts an increase or decrease in water temperature over several weeks causing corals and other zooxanthellae to expel their symbiotic algae and thus their loose color.
bio-piracy: taking plants and animals from a country or indigenous entity without permission and / or compensation; biological theft.
biofilms: microbial communities that form on surfaces. The are made from many different organisms (bacteria, protists, and unicellular algae) that embed themselves in an extracellular matrix of proteins, polysaccharides, and other organic molecules secreted by the bacteria.
bioload: density of fishes, corals, and other animals in an aquarium; nitrogen waste processing demands placed on filtration system.
biotic: relating to living organisms.
biotope: area in nature having physical conditions inhabited by a specific assemblage of plants and animals. When used in aquaria it refers to tank aquascaped to mimics a specific area and inhabited by the fish and invertebrates found together in that area in the wild.
brown jelly disease: protozoan that attacks and feeds on live coral tissue.
buccal: pertaining to the mouth or oral cavity of an animal.


calcification: fusing of calcium and carbonate ions to form shells and skeletons.
calcite: of the the two crystalline forms of calcium carbonate (CaCO3). Used by benthic snails and other marine organisms to grow shells and skeletons.
captive-bred: describes a fish or invertebrate that was hatched, spawned, settled, and grown to the juvenile or adult stage in an enclosed system that is not connected to wild habitat.
catch collagen: compound in the skin of echinoderms that allows skin to go from liquid to solid at will.
caudal: pertaining to the tail of an animal.
caudal fin: the tail of a fish.
cheliped: one of the leg pairs that has the large pincer of a decapod crustacean.
chitin: a tough, protective, semitransparent polysaccharide and the principal component of arthropod exoskeletons.
chitinous: made of, resembling or pertaining to chitin.
choanocytes: cells in the internal cavities of sponge active in digestion and moving water through it. Thought to shed prolifically and serve as food source for filter-feeding invertebrates.
chloroplasts: organelles found in plants and other eukaryotes that conduct photosynthesis.
chromodorid: typically colorful sea slug in family Chromodoridae, often eats live sponges.
cirritulid: free moving polychaete worm having tentacles that arise from different parts of its body; commonly called hair worms.
cladoceran: member of order Cladocera or water fleas; used as fish food.
cnidarian: members of the animal phylum Cnidaria, characterized the presence of stinging-cell cnidocytes used for prey capture and territorial disputes. Includes corals, sea anemones, sea pens, jellyfish, and sea wasps.
cnidocyst: stinging structure found in cnidarians, also called nematocyst
cnidocyte: cells in corals and sea anemones that can erupt and fire a sharp, toxin-bearing structure (cnidocyst). Used to capture prey, for defense or in territorial struggles.
coccolithophores: single celled marine algae with microscopic calcite plates, part of phytoplankton.
color rendition index: Abbreviated as CRI, it is the measure of the accuracy of a light source of a given color in creating the true colors of the item being illuminated. CRI of 100 is perfect, above 80 is considered good.
color temperature: describing the color of light by comparing it to the color of a standard black body at a certain temperature and expressed in Kelvin (K). Natural daylight ranges from 5,000-6,500K.
congeners: members of the same taxonomic genus.
cone snails: 600 species of venomous marine mollusks of the genus Conus; also known as “cones” or “cone shells.”
conotoxin: name of venom produced by cone snails. A very complex mix of toxins that can cause rapid paralysis and death having no known antidote.
conservation breeding: practice of maintaining a wild threatened, endangered, or extinct species in captive-bred populations with careful management practices to preserve genetic diversity and viability.
convergence / convergent evolution: when unrelated organisms develop similar anatomical features while evolving in different ecosystems.
coral bommie: an isolated piece of reef or distinct mound found in a lagoon or apart from a reef; distinct cluster of coral forming an isolated micro habitat.
coral triangle: area in north Australia that encompasses tropical marine waters of Indonesia, Malaysia, Papua New Guinea, Philippines, Solomon Islands, and Timor Leste.
cryptic: describing animal behavior where camouflage and / or hiding tactics are used to avoid predators.
ctenophore: commonly called comb jellies, sea gooseberries, sea walnuts, or Venus’s girdles, are animals in the phylum Ctenophora. They have eight “comb rows” of fused cilia arranged along the sides of the animal and lack stinging cells found in cnidarians.
cuticle: arthropod’s outermost protective layer of the exoskeleton.
cultivar: short for cultivated variety, term borrowed from plant breeders to describe a specimen selected for its desirable traits.
cyanide: highly toxic compound. Used in the form of sodium cyanide, it is sprayed into the water and used to stun reef fish making them easy to collect for either food or aquarium trade.
cyanobacteria: photosynthetic, microscopic organisms that can form slimy red or blue-green biofilms on aquarium substrates under favorable conditions.
cyclop-eeze: brand of bioengineered decapod crustacean cultured in an Arctic lake and used as food for corals and small marine fishes.


demersal spawners: fish spawning on sea bottom or in nests on or in the reef structure or substrate; such as clownfish.
dentary: the lower jaw supporting the teeth in fishes.
diandric: juvenile fish females can transform into adult, terminal phase males.
diode: an electrical device that allows electricity to flow only in one direction; similar to a plumbing check valve.
diurnal: active during daylight hours.
dorsal: pertaining to the back of an animal.
dorsal fin: one to three fins on the back in fishes that are used primarily for stability and making sudden movements. Frogfishes have a modified anterior dorsal fin that mimics a fishing rod and lure above the mouth.


echinoderm: radially symmetrical marine invertebrates of the phylum Echinodermata having an internal calcareous skeleton and often covered with spines. Includes starfishes, sea urchins, and sea cucumbers.
ecosystem: integrated unit composed of a biotic community and its physical environment. Can range from very small to very large.
elasmobranchs: fish possessing a cartilaginous skeleton without bones such as rays and sharks.
endangered: as defined by the U.S. Fish and Wildlife Service: “The classification provided to an animal or plant in danger of extinction within the foreseeable future throughout all or a significant portion of its range” (from the Endangered Species Act).
endocrine disruptor: chemicals that may interfere with the hormone systems of fish, marine invertebrates, humans, and other animals.
envenomation: injection of a poison via a bite, sting, or puncture wound.
esca: the anatomical appendage resembling a small fish, worm, or crustacean, used by frogfishes to attract prey. It is attached to the illicium.
eukaryote: organisms composed of cells that possess a membrane-bound nucleus (holding genetic material) as well as membrane-bound organelles.
euphotic zone: also called photic zone, it is the upper region of a body of water in which enough sunlight penetrates to drive photosynthesis. Often stated as the upper 206 feet (80 meters) of the seas.
exoskeleton: an external skeleton that protects the body and gives support for various body parts; it is the “shell” in crustaceans, copepods, and mollusks.


fluorescence: in aquaria, light emitted from animal tissue that has absorbed light or other electromagnetic radiation; luminescence.
fragging: cutting or breaking apart a living coral to create new clones.
fungal hypha: long, branching filamentous structure of a fungus.


gall: the purple lesion appearing on Gorgonian infected by Aspirigillus sydowi fungus.
gastrodermis: the epithelial lining of the digestive cavity (gastrovascular cavity) in cnidarians and ctenophores.
gastropod: invertebrate class in phylum Mollusca that includes snails, slugs, nudibranchs, and sea hares.
glucose isomer: one of the multiple different structures of glucose such as D-glucose (dextrose) and L-glucose.
gravid: pregnant, carrying young or full of eggs.
gorgonian: a sessile cnidarian animal in the order Alcyonacea (formerly Gorgonacea) having a flexible, horn-like skeleton. Some are photosynthetic, others filter feeders. Includes a number of suborders and families that include sea fans, sea whips, sea fingers, sea rods, and others.


H2S: Hydrogen sulfide, a gass with noxious rotten egg smell produced by anaerobic decomposition of protein.
HQI: Hydrargyrum Quartz Iodide lighting, a high intensity type metal halide bulb used in aquarium and reef tank lighting.
heat sink: substance that absorbs heat, often used to keep heat away from its emitting source. In LED lighting it is the diffusion of heat away from the emitters often by convection using aluminum as the heat sink.
heterotroph: organism that obtains energy by consuming other organisms.
hybrid: offspring from mating of sexual reproduction of two different species or genera.
hydrologic cycle: water cycle, describes the movement of water on Earth through evaporation, precipitation, and flow from land to sea.
hypurals: bones that support the caudal fin rays and bony fishes.


illicium: flexible rod-like first dorsal spine on frogfishes to which the bait like esca is attached.
inbreeding: mating of closely related individuals such as sibling to sibling or sibling to parent.
initial phase or IP: he first adult color phase in wrasses after which the fish can trans form into a functioning female or male. Also called the primary phase.
infauna: animals adapted to sediment living conditions.
integument: the outer covering of a body; shell, husk, or skin.
intertidal zone: the shore area that is under water at high tide and emerged at low tide. Also known as the littoral zone, seashore, or foreshore.
invertebrate: animal lacking a backbone.
ionization: adding or removing charged particles to an atom.



kalkwasser: solution of calcium hydroxide used to supply calcium needs of calcifying organisms; also called limewater.

kelvin: either a unit of measure of temperature on the thermodynamic (absolute) temperature scale or to a lighting rating where color temperature is a measurement in degrees Kelvin that indicates the hue of a specific type of light source.


light emitting diode: LED; a form of lighting used in aquarium and reef tanks.
LPS: informal term used for large-polyp stony corals that includes the genera Acanthastrea, Blastomussa, Catalahyllia, Caulastrea, Cynarina, Euphyllia, Favia, Favites, Fungia, Galaxia, Mycedium, Nemenzophyllia, Scolymia, and others.
labrid: referring to a member of the family Labridae which are the wrasses.
larval drift: period of time when larval invertebrates and fish become part of plankton and are carried by currents before settling.
led driver: device that converts AC current to DC and regulates the flow of current through an LED.
limewater: solution of calcium hydroxide used to supply calcium needs of calcifying organisms; also called kalkwasser.
liquid organic carbon: supplement, such as vodka, vinegar, glucose, patent blend) used in aquariums to promote the growth of beneficial nitrate and phosphate consuming bacteria.
littoral zone: the shore area that is under water at high tide and emerged at low tide. Also known as the intertidal zone, seashore, or foreshore.
lm/W: lumens per watt, how the light output efficiency of a light emitting device is measured.
lugols solution: solution of potassium iodide and iodine in water used as an antiseptic and stimulate for corals.
luminous efficacy: measure of the efficiency of a white light source in converting energy into watts and lumens.
lunate: having a shape like a crescent moon; in fish anatomy a tail with a crescent shape


MPA: marine protected area. Designated areas of water where fishing and other human activities are forbidden to encourage re-population of over fished or threatened species.
madreporite: circular ossicle that is the inlet to the water vascular system and found on the arboral surface of echinoderms.
mariculture: farming marine organisms in ocean environments for the aquarium hobby.
marine ornamental: describes species collected or bred for the aquarium trade as opposed to species classified as food or game fishes.
maxilla: the upper jaw supporting the teeth in fishes.
mesoglea: layer of collagen like fibers and mucopolysaccharides occurring as a layer of jelly like material between external and internal cellular layers in the body of sponges or cnidarians.
mimeric camouflage: using mimicry or imitation for camouflage.
misbar: aberrant or incomplete barring of pigments; missing bars or incompletely formed bars. Thought to result from dietary or environmental factors.
monophyletic: organisms that have shared characteristics and are thought to have a common ancestor.
morphs: each of several variant forms of an animal or plant.
motile: organisms that can move actively and spontaneously in their environment, such as a fish or crab; opposite of sessile.
mutualism: when two organisms of different species exist in a relationship that benefits each individual.


nM: nanometer; 1 billionth of a meter.
nano: prefix meaning a billionth but in general refers to something being very small. Nano aquariums are usually under 30 gallons (114 L); for fish those usually under 2 inches (5 cm).
nauplius, nauplii (plural): larval stage in the life cycle of a crustacean.
nematocyst: stinging structure found in cnidarians, also called cnidocyst.
nocturnal: active during nigh time hours.


ocean acidification: process of lowing the pH of global seas; attributed to absorption of excess carbon dioxide from the atmosphere.
ocelli: false eyespots or rounded areas of dark pigment that resemble eyes, often serve to confuse predators.
octocorals: Cnidarians in which each polyp has eight tentacles, feather-like in shape, with numerous side-branches, or pinnules but no stony skeleton. This subclass includes blue coral, soft corals, sea pens, and gorgonians.
operculum: In bony fishes it is the hard bony flap covering and protecting the gills. In some marine and freshwater snails, and minority of terrestrial snails it is a structure attached to the foot that serves as a sort of door to close off the opening of the shell when the soft body parts are retracted inside.
ossicle(s): small piece(s) of calcified material forming part of the skeleton in invertebrates; found embedded beneath the epidermis and sever as skeletal support in echinoderms.
outcrossting: mating unrelated individuals or strains to increase vigor in offspring.


PAR: measurement of light intesity.
PIT: passive integrated transmitter. Implantable microchip, each with an unique number that is used for identification. Also called RFID for radio frequency identification.
palytoxin: one of the deadliest natural toxins known, found especially in the genera Palythoa and Protopalythoa, and less commonly in the genus Zoanthus. Handle all zoanthid type polyps with proper protective hand and eye gear.
pantopods: referred to as sea spiders or pycnogonids that are not true spiders; they lack the venomous bite of true spiders. Seen in aquariums as hitchhikers on live rock or with zoanthids and other cnidarians.
pectoral: pertaining to the chest or breast.
pectoral fins: fins on fishes located usually just behind the operculum on each side.
pedicellariae: one of the minute pincer like structures common to starfish and sea urchins; modified spines that resemble harmless flower blossoms but can inflict painful and potentially fatal sting wounds. Used for cleaning and to capture tiny prey.
pelagic: living in the open sea.
pelagic spawners: fishes mating in mid-water whose eggs are dispersed by tides and currents.
pelvic fins: also called ventral fins, are the paired fins on the ventral surface of a fish. Gobie pelvic fins are often fused in a single sucker-like organ.
penes: paired sex organs on a male Mantis shrimp seen near the last pair of walking legs.
pH: measure of the acidity or alkalinity of a solution commonly based on a scale of 0 (most acidic) to 14 (most alkaline) with 7 representing neutral (neither acid or alkali). Pure fresh water has a pH of 7.
phenotype: the composite of an organism’s observable characteristics or traits and products of behavior.
phoenix effect: term coined by Dr. David Krupp to describe coral regeneration from a seemingly lifeless skeleton. Thought to be due to deep tissue surviving and waiting for favorable conditions to trigger new polyp growth.
photic zone: also called euphotic zone, it is the upper region of a body of water in which enough sunlight penetrates to drive photosynthesis. Often stated as the upper 206 feet (80 meters) of the seas.
photon: basic unit of light energy or other form of electromagnetic radiation.
photosynthesis: the synthesis of organic compounds from carbon dioxide and water using light energy absorbed by chlorophyll and releasing oxygen.
phytoplankton: green plant component of plankton, mostly microscopic, they are responsible for most of the photosynthetic activity in oceans.
pinnule: side branch structure on the tentacle of soft coral giving it a feathery appearance.
pipehorse: member of family Sygnathidae; intermediate form resembling seahorses and pipefish.
piscine (adj): characteristic of or pertaining to fish.
piscivorous: fish eating, consuming fish.
plankton: the different types of organisms (zooplankton, phytoplankton, bacterioplankton) drifting in bodies of water; a crucial part of ocean and reef food webs.
planula: free swimming larval form of coral or other cnidarian.
polycentric: when a stony coral colony has more than one mouth or center.
polychaete: member of annelid worm class Polychaeta and often found in marine environments; often called bristleworms.
polymer: large molecule made from repeating units.
polymorphism: when different morphs or phenotypes of a species occur in the same population or habitat.
prehensile: animal appendages, such as a tail, claw, or foot, that are able to grasp or hold.
probiotics: practice of stimulating growth of or introducing beneficial bacteria.
propodium: anterior portion of a mollusk foot.
protandric, protandrous hermaphrodites: animals having male organs when young and mature into functional females.
protogynous hermaphrodites: animals that have female organs but are capable of becoming functional males.
pyrams: snails in the family Pyramidellidae that parisitize Tridacna clams and other mollusks.



radula: scraping organ for chewing (mastication) in mollusks. Many gastropods have a radula made of a variable number of chitinous teeth found on the anterior of the pharynx. In cone shell family members this has been reduced to a sharp, grooved tooth to harpoon and inject venom into prey.
rays: anatomically they are the structural ribs in the fins of bony fishes and can be either soft or spiny.
reticulate evolution: characterized by occasional hybridization and combination of two species.
rissoid: small harmless snail sometimes confused with predatory Pyramidellid snails.


SPS: informal term used for small- polyp stony corals that includes genera as Acropora, Hydnophora, Montipora, Pavona, Pectinia, Pocillopora, Seriatopora, Stylophora, and others.
sanguinivory: feeds on blood.
school: group formed by same species and size of fishes that keep and equal distance between them and move in unison.
scleractinian: having a skeleton of calcium carbonate (aragonite). Another term for a stony coral.
sea spiderr: not true spiders but members of the marine arthropod class Pycnogonida.
sessile: organism that is fixed in one place and not free to move about, like corals. The opposite of motile.
settlement: the crucial stage in the development of invertebrates and larval fish when it sinks out of the water column to the bottom to undergo metamorphosis from larva to juvenile.
shoal: group formed by same species of that don’t stay a uniform distance from each other and don’t move in unison.
silversides: small, silvery minnow like fish used as food for carnivorous fishes and aquaculture.
settlement cues: chemical signals that trigger larval corals, fishes, and other life forms to drop out of the water column and attach to appropriate substrates and develop.
smasher: mantis shrimp with club-like raptorial appendages allowing it to smash open shells of snails, clams, oysters, shrimp, and other crustaceans.
Solid organic carbon: a bacterial produced biopolymer material (biopellets) serving as a carbon source for beneficial nitrate and phosphate consuming bacteria.
spearer: mantis shrimp with sharp tipped raptorial appendages allowing it to snag fish, octopi, and other swimming prey.
species tank: aquarium set up to primarily house one species of invertebrate or fish.
stolon: the horizontal connection between individual polyps in corals.
stomatopod: manits shrimp, marine crustacean of order Stomatopoda.
symbiont: organism in a symbiotic relationship.
symbiotic: relationship between two organisms where one or both benefit.
syngameon: a single speciesor a cluster of different species which have variable genetic links (genetic flow through cross-fertilization) with other members of the syngameon.


T5: fluorescent lighting tubes with 5/8″ diameter.
T-Iso: brown colored motile microalgae (Tahitian Isocrysis) used in aquaculture as food for copepods. Easily cultured live algae source with high levels of the fatty acid DHA.
tanaids: small, shrimp-like crustaceans found living in association with marine substrates.
tank-raised: describes a fish or invertebrate that was spawned in the wild and then raised in captivity from collected eggs, larvae, or small juveniles.
taxa: taxonomic groups such as species or genus; single is taxon.
teleosts: belonging to the Teleostei or Teleostomi, a large group of bony skeleton fishes.
telson: last body segment on a crustacean, forms tail fan on lobsters and shrimp.
tetrodotoxin: deadly neurotoxin produce by certain bacteria found in pufferfish, tobies, and Mola Mola fish tissues and in the bite of Blue-ringed Octopus. There is no antitoxin.
terebellid: polychaete worm that lives in a permanent burrow or buried tube; commonly called spaghetti worms.
tterminal phase or TP: adult male fishes with color patterns distinctive for the species as seen in wrasses.
thorax: the division of an animal’s body that lies between the head and the abdomen; thoracic is relating to the thorax.
threatened: defined by the U.S. Fish and Wildlife Service: “Any species which is likely to become an endangered species within the foreseeable future throughout all or a significant portion of its range” (from the Endangered Species Act).
trace element: elements in seawater found in concentrations between 1µM and 1nM.
truncate: ending abruptly as if cut off across the base or tip; in fish refers to a tail with an even or square end.


µM: Micrometer or micron; 1 millionth of a meter.
ultra trace element: elements in seawater found in concentrations less than 1nM.


vegetative reproduction: form of asexual reproduction where new individuals are created by budding, cutting, and tissue culture.
ventral: referring to the abdomen or underside area of an animal.
ventral fins: also called pelvic fins, are the paired fins on the ventral surface of a fish. Gobie pelvic fins are often fused in a single sucker-like organ.
vibrio: genus of gram negative bacteria found in marine environments.
visible light: energy release as light in the wavelengths of 400nm-450nm.
vodka dosing: addition of vodka to an aquarium as a carbon source to decrease nitrate and phosphate molecules, clarify water, and increase coral color.


white light: light composed by combining a wide spectrum of colors.




zeolites: naturally formed rocklike aluminosilicate minerals often used as adsorbents. Used by some for removing ammonia and other dissolved nitrogenous dissolved compounds in aquariums.
zoanthid: anemone members of family Zoanthidae. Generally found in intertidal areas and coral reefs.
zoea: marine invertebrate larval stage where propulsion is by thoracic appendages.
zooplankter, zooplankton (plural): small to microscopic animal living in the water column and feeding on phytoplankton (single-celled algae). An essential food element for most marine fish species.
zooxanthellae: golden- brown photosynthetic algae that live in coral tissues, sea anemones, and other marine invertebrates in a mutualistic relationship. Algae gain protection while producing sugars and carbon the host uses for energy.
zooxanthellate corals: corals that contain zooxanthellae in their tissues.

One comment

Leave a Reply