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Habitat: The place or kind of place in which it is natural for an animal or plant to live or grow.

Halocline: Depth zone within which salinity changes maximally.

Halogen Lights: Lights that have a very yellow light not appropriate for aquarium use. Do not confuse these with metal halide lights.

Hap: Short for Haplochromine. See Haplochromine.

Haplochromine: Both a genus of Cichlids as well as an informal group of Cichlids that are generically (although not precisely) described as a non-Mbuna flock because many of these fish once belonged to the broad genus Haplochromis Hilgendorf. There are a total of eighteen Lake Malawi genera that belong to this informal group of Cichlids. Aristochromis, Bucchochromis, Champsochromis, Chilotilapia, Copadichromis, Cytocara, Dimidiochromis, Exochochromis, Fossrochromis, Maravichomis, Nimbochromis, Nyassachromis, Otopharynx, Placidochromis, Protomelas, Sciaenochromis, Taeniochromis, and Tyrannochromis. There are also eight Cichlid genera from Lake Victoria that are also commonly called "Victorian Haps."

Hard Coral: A group of coral species known as stony coral that forms the hard, calcium carbonate skeleton in several shapes; other include the brain corals, fungus or mushroom corals, staghorn and table corals, flower pot corals, bubble corals and lettuce corals.

Hard Water: Water with a high concentration of dissolved salts.

Hardness: The hardness of a water is a measure of the concentration of the multivalent cations (positively charged particles) in the water, but primarily it is equivalent to the calcium and magnesium concentration of the water. Hardness is typically reported as mg /L as CaCO3 (calcium carbonate), but it may also be reported as grains per gallon (1 gpg (US) = 17.12 mg CaCO3/L ). Hardness Classification: Soft: 0 to 17 mg CaCO3/L; Slightly Hard: 17 to 60 mg/L; Moderately Hard 60 to 120 mg/L; Hard 120 to 180 mg/L; and Very Hard > 180 mg/L. For more information visit the Hardness Website.

Hardy-Weinberg Law: The law that states that the frequencies of genotypes in a population at a locus are determined by random mating and allele frequency.

Hatchery: A place for hatching eggs of fish and allowing them a place to grow until they are better able to survive on their own.

Head and Lateral Line Erosion (HLLE): Also known as lateral line disease. A fish with this condition may develop holes in its head and along its lateral line. The main cause is usually due to the aquarium not being grounded properly.

Health Advisory Level (HAL): A non-regulatory health-based chemical concentration in drinking water that results in no adverse health risks when a given amount of water is ingested over exposure periods ranging from one day to a lifetime.

Heater: A device to warm the aquarium water. They are available as submersible heaters, over-the-side heaters, and under tank coils.

Heavy Metals: A classification of elements, many of which are necessary for animal nutrition in trace quantities but which are also toxic to plants and animals in low concentrations.

Hemimetabolous: Undergoing incomplete metamorphosis.

Herbivore: Animal whose diet consists only of vegetable matter.

Heritable Character: A morphological character whose given state can be explained partially in terms of the genotype of the individual.

Hermaphrodite: An individual capable of producing both eggs and sperm during its lifetime.

Hermatypic: Refers to organisms that contain zooxanthellae. This usually means they need strong light to thrive.

Heterophyllous: Having leaves of different shapes on the same plant.

Heterostylic: Having flowers which differ in the relative length of their styles and stamens, such that any one flower is very rarely, if ever, self pollinated.

Heterotrophic: Pertaining to organisms requiring preformed organic compounds for food; unable to manufacture food from inorganic compounds.

Heterotrophic Algae: Algae that take up organic molecules as a primary source of nutrition.

Heterotrophic Bacteria: Bacteria that require preformed organic compounds for food because they are unable to manufacture food from inorganic compounds. Heterotrophic plate count is a procedure for estimating the number of live heterotrophic bacteria in the water. Colonies may form in pairs, colonies, clusters or single cells, which can be termed as "colony-forming units". The colonies are relatively small/compact and do not encroach on each other. This procedure can accommodate volumes of sample or diluted

Heterozygote: With respect to a given genetic locus, a diploid individual carrying two different alleles.

Highly Stratified Estuary: An estuary having a distinct surface layer of fresh or very-low-salinity water, capping a deeper layer of higher salinity, more oceanic water.

Hinge: A joint in between the shells of a bivalve.

Histogram: A multiple-bar diagram representing the frequency distribution of a group as a function of some variable. The frequency of each class is proportional to the length of its associated bar.

HO Lighting: High Output fluorescent lighting.

Holdfast: An organ of a benthic alga that attaches the alga to the seafloor.

Holometabolous: Undergoing complete metamorphosis.

Holoplankton: Organisms that live their entire life cycle in the floating (planktonic) state and not on or in the sea bed.

Homeostasis: A state of equilibrium of the internal body environment. The body's equilibrium is maintained by dynamic processes of feedback and regulation of hormone levels, salt balance, immune status, chemical metabolic capacity, etc.

Homeotherm: An organism that regulates its body temperature despite changes in the external environmental temperature.

Homozygote: With respect to a given genetic locus, a diploid individual carrying two identical alleles.

Horny Coral: A group of coral specics that embeds calcium carbonate in a semi-soft, horn-like material. This allows the formation of flexible sea fans and sea whips that bend with ocean currents; also known as Gorgonians.

Hospital Tank: See Quarantine Tank.

Humpback: The "nickname" for a pink salmon. The male pink salmon develops a pronounced hump on his back as he ages.

Hydraulic Conductivity: A term used to describe the ease with which water moves through soil or a saturated geologic material. Hydraulic conductivity is influenced by the type of material comprising the formation (sand, gravel, rock, limestone, sandstone, clay), the slope of the water table, the type of fluid, and the degree to which existing pores are interconnected.

Hydraulic Gradient: The slope of the water surface in an aquifer. The hydraulic gradient indicates the direction groundwater will flow. Water always flows from higher water table elevations to lower water table elevations. All other factors being equal, flow is greater when the hydraulic gradient is steeper.

Hydrogen Sulfide: A molecule composed of a hydrogen and sulfur atom. It is a toxic compound which has a rotten egg odor. It is synthesized anaerobically by unwanted bacteria.

Hydrographic: Referring to the arrangement and movement of bodies of water, such as currents and water masses.

Hydrologic Cycle: Describes the constant movement of water above, on, and below the earth's surface. Processes such as precipitation, evaporation, condensation, infiltration and runoff comprise the cycle. Within the cycle, water changes forms in response to the Earth's climatic conditions.

Hydrometer: A Hydrometer is an instrument used to determine the specific gravity of a fluid, or the salinity of water. Hobbyist grade hydrometers are temperature corrected to read the specific gravity at around 77F (25C) because specific gravity is temperature dependent.

Hydrophilous Pollination: The transference of pollen from the Anthers of the Stamens to the Stigmas on the surface of the water or under water.

Hydrothermal Vents: Sites in the deep ocean floor where hot, sulfur-rich water is released from geothermally heated rock.

Hypocotyl: The part of the Stem of a seedling below the cotyledons.

Hypothesis: A refutable statement about one or a series of phenomena.
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