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Glossary of Terms for Aquariums

Aquatic Science Terms for Freshwater and Marine Animals

Glossary of terms for freshwater fish, saltwater fish, and coral reef animals

Get in the know, all sorts of glossary terms used by ichthyologists and aquarists alike!

Aquarium Glossary

This aquarium glossary comes in real handy for both beginners and advanced hobbyists.

Have you ever been stumped by the terms used in the aquatic world, like dorsal fin, barbel, or how about gonopodium? That's where this dictionary of scientific terms, related to freshwater aquatic and marine animals, comes in very handy.

There are lots of unique words used to describe parts of fish, plants or other water critters. These terms are not only unusual, but actually quite interesting. And the more you read, the more questions that will come up. But beware, with the list of aquarium glossary terms, you'll find answers and then you'll start thinking like an aquatic expert!

Find all sort of aquatic animals in these encyclopedias:
Freshwater Fish - Saltwater Fish - Coral Reef Animals


A - C

adipose fin - The small fin located between the dorsal fin and the caudal fin. It seems to serve no purpose.

asexual reproduction - Asexual means having no sex or sex organs, therefore asexual reproduction would be reproducing by means other than sex.

amyloodinium- Amyloodinium ocellateum is commonly called Oodinium, Marine Velvet, or Saltwater Ick.  Possible signs of the disease are cloudy eyes, gasping for breath, listlessness, and white spots.  Positive signs of the disease are gold or brown spots, rough skin, and rubbing against rocks, etc.  Treatment can be done by a freshwater dip and copper (as long as inverts are not in the tank).

anal fin - Single fin mounted vertically below the fish.

anthostele - The lower part of the polyp, often stiffened, into which the distal portion of the polyp, the anthocodia (which includes the mouth and the eight tenacles) is withdrawn. The calyx.

ascidians - Also known as sea squirts. Filter feeding animals found exclusively in marine environments that come in a variety of shapes and sizes. Due to their appearance they are frequently confused with sponges or even rocks. Most are sessile, meaning they don't move, and are attached to the substrate, but a few deep water species can move and are predatory.

aufwuchs - the German term "aufwuchs" refers to microscopic food consisting of small animals, algae and other plant matter that grown on and encrusts hard surfaces, such as rocks or other substrate. Fish that graze on this type of food are called Limnivores.

autozooid - A coral's primary feeding polyp with 8 tentacles

barbels - Barbels are the whisker-like appendages found on both sides of the mouth of all catfish.

benthic - Benthos and benthic refers to living on or under the substrate at the bottom of aquatic environments including lakes, ponds, rivers and streams, and oceans.

  • Sessile - means the organism is attached to the substrate.
  • Pelagic- refers to living in the water above the bottom. Pelagic organisms usually have some ability to move around.

brackish - water that is neither fresh nor saltwater, but is somewhere in between. In nature this occurs at the mouths of rivers and swamps near the sea.

  • Brackish water should have enough salt added to reach a specific gravity in the range of 1.002 to 1.008. Since brackish water is generally a mixture of freshwater and saltwater, a high quality saltwater mix works best but pure rock salt will work in a pinch. NEVER USE IODIZED SALT!
  • A hydrometer is needed to measure salinity but be sure that the one used will measure the necessary range. Most hydrometers found in aquarium stores are made for measuring pure saltwater so they will not measure the lower salinity needed for brackish. A couple direct reading of specific gravity and ppt. that will work for these lower levels are: Instant Ocean Salinity Hydrometer manufactured by Marineland Labs, and SeaTest Full Range Specific Gravity Meter manufactured by Aquarium Systems.

   Some fish live in salt water but are spawned in brackish or fresh water and vice versa.  There are several brackish species available in the aquarium hobby, see Freshwater Fish for descriptions.

byssus gland- The structure in clams that produces fibrous threads (byssus) that attach the clam to substrate. Sometimes permanent although more usually temporary attachment of tough organic threads secreted from a gland in the foot of the clam.

calcyes - plural for calyx. See anthostele.

capitulum - broad flat upper surface where the polyps reside on some Octocorals, also known as polyparium or more commonly the "head".

carnivore - Animals who hunt and eat other animals; meat eater. Sharks are a good example.

carapace - A bony or hard shell that covers part or all of an animal. Turtles, crabs,  and boxfish are good examples.

caudal - The single fin mounted vertically at the rear of the fish. The tail fin.

caudal peduncle - The part of the body which attaches the caudal (tailfin) to the body. The surgeonfish's spines are located on the caudal peduncle.

coelenterate - an aquatic animal of the Phylum Coelenterata which is characterized by a central mouth usually surrounded by tentacles bearing stinging cells, and no anus; includes sea anemones, corals, and jellyfishes.

coenenchyme - a stiff gelatinous matrix found on certain soft corals that anchors and supports the polyps, and may be embedded with sclerites or other particulate matter. The tissue of a zoanthid that surrounds the polyps, consists of mesoglea and may have sand imbedded in it.

coenosarc - the soft tissue that lies over the stony skeleton and usually links the tissue between polyps

comensal, comensalism - one of several types of symbiotic relationships between the individuals of two (or more) different species In a comensal relationship one species benefits while  the other does not (see symbiosis).

Competition - one of several types of symbiotic relationships between the individuals of two (or more) different species. In a competitive relationship neither species benefits (see symbiosis).

conspecific - refers to animals of the same species.

corallivores - species that feed on corals/ coral polyps

cryptocaryon - Cryptocaryon irritans is a parasitic infection where white spots appear on the body and fins. Fish will scratch themselves against rocks and breathing may become rapid if gills are affected. Treatment can be done by copper or other anti-parasite remedies, but this is incompatible with inverts. Cleaner shrimps and wrasses will remove the parasites, but may not keep up with a major infestation. Cryptocaryon is often referred to as the marine equivalent of the freshwater white spot disease, Ichthyophthirius, or Ick.

cyanobacteria - Cyanobacteria is commonly referred to as red slime algae by hobbyists although it is not really an algae. It is a bacteria. Poor water quality with excessive nutrients are the usual causes.  To combat, do frequent water changes, siphon out detritus, and use a good protein skimmer.

D - H

demersal - Sinking to or lying on the bottom; living on or near the bottom and feeding on benthic organisms.

detritus - grayish piles of organic compounds that accumulate in the aquarium. Commonly will contain fish wastes, fragments of rock, leftover food, among other things. Usually detritus will accumulate in low water flow areas, sumps, etc.

detrivores - Animals that eat detritus. Common detritvores are urchins, stars, hermits, etc.

diatom - A diatom is any of a class of microscopic one-celled algae having walls of silica consisting of two interlocking valves.

dinoflagellates - Any of numerous minute, chiefly marine protozoans.

diurnal - An organism that is active during the day, and sleeping at night.

dorsal fin - the fin directly on the top of the body (it's the fin that sticks out of the water when you see a shark). Some fish have two dorsal fins one directly behind the other.

endemic - restricted to a particular place. Means the animal lives in a certain place and nowhere else.

facultative - capable of living in varying conditions, ie. facultative cleaners do not rely strictly on parasites for food (compare to obligatory).

foraminiferans, forams - shelled ameboid protozoans, very small one-celled animals. Primarily marine although a few live in freshwater or in brackish conditions

gametes - haploid reproductive cells that unite during sexual reproduction to form a diploid zygote. Male gametes are sperm and female gametes are eggs

genus - In the taxonomy classification, the genus is the category ranking below a family and above a species.

gonopodium - particular to the Live-bearing tooth carps, the gonopodium is the pelvic fins of male fish that have been converted into genital organs.

gonochoristic - Species with sexes separate, each individual has either male or female reproductive organs. See also hermaphrodite and gynogenetic.

gorgonian - a tropical or subtropical octocoral with upright branchy plant-like or fan-like growths and a skeleton made of a horny organic material.  For example, the Sea Fans (Scientific names:  Gorgonia ventalina, G. flabellum, G. mariae, Pacifigorgia spp.)

gynogenetic - the consistent production of only female offspring where the paternal chromosomes are not incorporated in the embryo.

head and lateral line erosion - also known as lateral line erosion, hole-in-head disease, and lateral line disease. A fish with this condition will develop holes in its' head and sometimes along its' lateral line. The main cause is nutritional deficiency, especially vitamin C. Stress and poor water quality also play a role. Untreated cases will cause disfiguring or death. To combat and cure, ensure good water quality and provide vitamin enriched foods, especially vitamin C.

herbivore, herbivorous - animals whose diet consists mainly of vegetable matter.

hermatypic - refers to organisms that contain zooxanthellae. This usually means they need strong light to thrive.

hermaphrodite - refers to both male and female in the same organism. This occurs either at the same time (synchronous hermaphrodite) or at different times (successive hermaphrodite). When the female form occurs first, this is referred to as protogynous hermaphroditism, otherwise protandrous hermaphroditism.

heterospecific - refers to animals of different species. If two or more animals are heterospecific, it means they are not the same species.

hydrometer - an instrument used to determine the specific gravity of a fluid. Hobbyist grade hydrometers are temperature corrected to read the specific gravity at around 77F (25C) because specific gravity is temperature dependent.

I - O

ichthyophthirius, Ick: see cryptocaryon.

insectivore - insectivores are fish that eat insects and the larvae of insects.

invertebrate, inverts - commonly called inverts by many in the hobby, invertebrates are animals without backbones like anemones, corals, shrimps, snails, and crabs.

kalkwasser - German word meaning 'calcium water', kalkwasser is a mixture of calcium hydroxide in water. The ph is very high (around 12.0) and it is used as makeup water to replace calcium used by hard corals and clams to build calcerous skeletons.

larvae - the first stage of development after hatching for many fish and invertebrates.

lateral line - The lateral line is a line of perforated scales along the flanks of a fish which lead to a pressure-sensitive nervous system. This enables the fish to detect vibrations in the surrounding water caused by other fish and their own reflected vibrations against obstacles.

limnivore - limnivores are also known as "mud-eaters" or "bio-film grazers". Liminores are fish that eat algae, microorganisms and other things it can find at the bottom of its habitat. This is what is meant by the German term "aufwuchs" that has gained popular use.near the shore.

littoral - pertaining to the edge of the lake, near the shore.

mantle - large, pigmented fleshy portion of tridacnid clams that is exposed to the light by gaping of the shell valves. Also called siphonal tissue. Also, the coral tissue in fleshy polyps (e.g. Catalaphyllia).

mimicry - when an animal 'mimics', or 'copies' the appearance of another animal in order to gain an advantage, like camouflage or a better defense. A good example is the marine fish, Mimic tang.

mutualism - one of several types of symbiotic relationships between the individuals of two (or more) different species. In a mutualistic relationship both species benefit, see symbiosis.

nacreous - resembling mother-of-pearl; lustrous. Usually used to decribe the appearance of pearlscale goldfish.

nauplii - the larval stage, of a crustacean such as the brine shrimp, Artemia salina or brine shrimp. "Artemia nauplii" are commonly used as a first food for fish.

necrosis - slow decaying and rotting of coral flesh.

nematocysts - the cells at the tip of an anemones' (corals) tentacles which "sting" when touched.

neutralism - one of several types of symbiotic relationships between the individuals of two (or more) different species. In a neutral relationship both species are unaffected (see symbiosis).

obligatory - obligate or required: ie. an obligatory cleaner fish relies entirely on this feeding mode to obtain nutrients (compared to facultative).

octocoral - octocorals have eight tentacles on each polyp.  There are many different forms which may be soft, leathery, or even those producing hard skeletons.

omnivore - animals who eat both meat and vegetables like marine angelfish.

ocellus - a marking that resembles an eye, eye-like colored spot; an eyespot.

oviparous - producing eggs which are fertilized, develop and hatch outside the body.

oviviparous, ovoviviparity, ovovivipary, ovivipary - producing eggs (usually with yolk) that are fertilized internally.  Hatching may occur internally or external to the mother.

P - R

parasitism - one of several types of symbiotic relationships between the individuals of two (or more) different species In a parasitic relationship one species benefits, the other is harmed (see symbiosis).

pinnate - feather like. The 'pinnules' which are found on many octocorals are small side branches of the polyp tentacle which give it a 'pinnate' appearance.

pelagic - refers to living in the water of the ocean above the bottom. Pelagic organisms have the ability to swim around or move in some fashion.  "Pelagic" is also used to refer (usually) to eggs that are basically at the mercy of the ocean currents. Benthos and benthic refers to living on or under substrate at the bottom of the ocean. Sessile means the organism is attached to the substrate.

pectoral fins - the anterior or dorsalmost paired fins of fishes. They correspond to the anterior limbs of the higher vertebrates.

pharyngeal teeth, throat teeth - tooth-like structures in the back part of the throat derived from gill supports. Primarily used for grinding, may be used to produce sound.

pharyngeal jaws - specialized bony plates in the throat that bear teeth

photosynthetic, photosynthesis - the process by which organisms, usually plants, use the energy contained in light, usually sunlight.

phytoplankton - microscopic free-floating aquatic plants, mainly algae. It lives suspended in bodies of water and drifts about. See plankton.

piscivore - habitually feeding on fish; fish-eating

planktivore - a general term to describe an organism adapted to feed and survive from eating mostly plankton.

plankton - plankton are the drifters of the sea. Although they may have some form of locomotion they are mostly carried by water currents. Plankton is divided into macroplankton (jellyfish, sargassum weed) and microplankton, organisms that can only be seen by a microscope.  The microplankton is divided into zooplankton, tiny marine animals, and phytoplankton, or plants. Most fish start their lives as small animals in the plankton.

polyp - a polyp is the living unit of a coral.

predaceous - this means an animal will hunt and eat other animals. The old rule comes to mind, big fish eat little fish!

protogynous hermaphrodites - begin life as a female but able to change sex to a male if the need arises.

protozoan - A protozoan is a single-celled, microscopic (usually) organism.  For example, an amoeba.

red slime - see cyanobacteria.

S - Z

septa, plural of septum - thin skeletal plates in stony corals that protrude from the corallite wall inward, toward the center where the polyps reside.

septum - a thin skeletal plate in stony corals that protrudes from the corallite wall inward, toward the center where the polyps reside

sessile - sessile means the organism is attached to the substrate at the bottom of the ocean and therefore cannot move around.   Pelagic refers to living in the water of the ocean above the bottom. Pelagic organisms have the ability to move around. Benthos and benthic refers to living near or under substrate at the bottom of the ocean.

setae - a bristle or stiff hairlike structures.

siphonozooid - a coral's smaller secondary polyp used to move water around the colony.

specific gravity - as a hobbyist definition, specific gravity is the amount of salt in the water.  See the hydrometer definition above for more information.  Specific Gravity of seawater ranges from 1.022 to 1.030.

siphon - the inhalent and exhalent siphons of tridacnid clams are used to allow for gas exchange and to expel wastes.

sweeper tentacle/polyp - a coral tentacle or polyp that has an increased number of nematocysts and elongates in order to 'sting' neighboring corals and sessile invertebrates. See the Frogspawn  coral, Euphyllia divisa for a picture of sweeper tentacles.

symbiosis - a close ecological relationship between the individuals of two (or more) different species.   Sometimes a symbiotic relationship benefits both species, sometimes one species benefits at the other's expense, and in other cases neither species benefits.

symbiotic - a relationship where two or more different kinds of animals live together and both benefit in some way from the other's company. The most famous example is the clownfish and the anemone.

taxonomy - the classification of organisms in an ordered system that indicates natural relationships.

ultraviolet (UV) light - Ultraviolet is a high energy, short wavelength of light.  It is shorter than violet in the visible spectrum and on the border of the x-ray region.

umbo / umbones (plural) - The apparent "apex" or "beak" of each valve around which radial growth has proceeded.

UV sterilizer - a UV Sterilizer is a device that uses ultraviolet light to make the water treated with it free of microorganisms which may cause disease.

Weber's apparatus - described in 1820 by E. H. Weber, the Weber's apparatus connects the hearing organ with the swim bladder where it operates as a sounding board thus amplifying sounds.

wood eater - wood eaters are fish, like some types of plecostumus, that can digest wood. They use "friendly bacteria" to break down the wood to simpler forms of carbohydrates that are then digested by the fish.

zoobenthos - Invertebrate animals that live in or on seabed habitats, including the intertidal zone.

zooplankton - small, usually microscopic animals; includes tiny waterborne crustaceans and fish larvae, also includes corals, rotifers, sea anemones, and jellyfish. - See plankton.

zooxanthellae algae - pronounced 'zo-zan-thel-ee', zooxanthellae algae are tiny plants called dinoflagellates (single-celled microscopic organisms which belong to the Protista kingdom) that live symbiotically with corals, tridacnid clams, and some sponges.  The algae provides food for the host and in return gets the nitrogen, phosphorous, and carbon dioxide it needs to grow.  The scientific name is Symbiodinium spp.


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