habitat - the place or environment where a particular organism, population, or species lives

habitat affinity index - defines habitat affinity based on the relative concentration of a species in a particular habitat, compared with the availability of that habitat in a given study area

habitat complexity - the areal extent and number and diversity of habitat types and distinct ecological zones within a specified area

habitat distribution - the structure and spatial characterization of all habitat types in a specified area

habitat diversity - the number of different types of habitats within a given area

habitat equivalency analysis (HEA) - an analysis designed to determine the compensation the public is due to reconcile injuries to the ecosystem and the lost services the ecosystem provides to the biotic component

habitat fragmentation - the breaking up of a habitat into unconnected patches interspersed with other habitats which may not be inhabitable by species occupying the habitat that was disrupted. The breaking up is usually by human action

habitat patch - any discrete area that is used by a species for reproduction or obtaining other resources, such as food and shelter; an area of distinct habitat type; patchiness occurs when a habitat is divided (fragmented)into usable areas, called "patches," which are separated from one another by nonusable habitat

habitat sharing - a situation in which species occupy the same habitat without competition, either through requiring different resources or being present at different times

habituation - in animal behavior, the temporary waning or disappearance of an innate response when it is elicited many times in succession

hair cell - a mechanoreceptor cell having hair-like processes (stereocilia and kinocilia). Vibrations or pressure deform these processes which relay the induced nervous impulses to the central nervous system. They are responsive to sound and other mechanical stimuli

half-life - the time required for a radioactive substance to lose half of its nuclei

Halimeda - an important genus of calcareous green algae in coral reef environments

hallucinogenic fish poisoning - see" ichthyoallyeinotoxism"

halo - in animal coloration, a circle of color around a spot of another color; a ring of sand surrounding a patch reef

halocline - the boundary where there is a marked change in salinity between surface fresh water and underlying saltwater in a stratified coastal environment

Halofolliculina - a genus of sessile heterotrich ciliates, of which one species (H.corallasia), causes skeletal eroding band (SEB) disease in corals in the Indian and Pacific Oceans and the Red Sea, and a congeneric species causes a similar disease in Caribbean corals (Caribbean ciliate infection)

halophile - an organism which lives in an environment of high salt concentration. Halophiles have special adaptations to permit them to survive under these conditions

A black mangrove is an example of a halophyte, a plant that thrives in a saline environment.

halophyte - a plant that grows in soils that have a high content of various salts

hamular - hook-shaped

hamulus - a small hook-like projection

handline - a fishing line with attached hook(s) that is tended directly by hand

haplogroup - a population descended from a common ancestor; a group of similar haplotypes that share a common ancestor with a single nucleotide polymorphism (SNP) mutation

haploid - a haploid cell contains a nucleus with a single complete set of chromosomes. The haploid condition is often abbreviated as n. Most fungi, protists, and algae are haploid, as are some insects, bryophytes, and the gametes of all organisms

haplotype - a set of closely linked genes that tends to be inherited together as a unit; a particular set of alleles at linked loci that are found together on a single homolog; a genetic type; a unique DNA sequence

hapteron - a single branch within a holdfast

hard bottom - a substrate formed by the deposition of calcium carbonate by reef building corals and other organisms or existing as bedrock or volcanic rock usually of minimal relief

Skeleton of a hard coral colony.

hard coral - a coral in the anthozoan order Scleractinia. Also known as the stony corals, these organisms possess a hard external calcareous skeleton. A synonym of stony coral

hard coral forms - may be conveniently summarized as: encrusting (lichen-like); branched (staghorn-like); massive (rock-like); sub-massive (pillar-like); tabulate (table-like); foliose (scroll-like); and solitary

Hardy-Weinberg Law - the frequency of a given genotype will reach equilibrium in a randomly mating population and will stay constant over many generations in the absence of selection pressures

harmonics - pertaining to sound, harmonics are multiples of the fundamental frequency (a single sine wave)

harvest - to capture, catch, collect or harvest organisms by any means

hastate - spear-shaped

hatch - the process of an embryo leaving the egg envelopes

hatchling - a fish larva just out of the egg

Hawaiian Archipelago - the Hawaiian Archipelago consists of eight large islands and 124 small islands, reefs, and shoals. It stretches for over 2,400 km from 19 degrees - 28 degrees N to 155 degrees -178 degrees E. It can be divided into two distinct regions: the Northwestern Hawaiian Islands (NWHI), primarily uninhabited atolls, islands, and banks accounting for the majority of U.S. reefs, and the Main Hawaiian Islands (MHI) largely made up of populated, high, volcanic islands with non-structural reef communities, fringing reefs, and two barrier reefs

hawser - a large rope for mooring or towing a ship

hazardous waste - a discarded material which contains substances known to be toxic, mutagenic, carcinogenic, or teratogenic to humans or other life; ignitable, corrosive, explosive, or highly reactive alone or with other materials

heading - the direction in which a vessel is pointed at any given moment

heat shock proteins (HSPs) - a group of proteins that are present in the cells of all living organisms. They are induced when a cell is exposed to various types of environmental stresses, e.g.,heat, cold and oxygen deprivation. Heat shock proteins are also present in cells under normal conditions, assisting in other cellular protein functions and behavior. They also trigger immune responses through both intracellular and extracellular activities; also called stress proteins

heavy metal - a metal having a specific gravity of 5.0 or greater. Heavy metals are generally toxic to organisms in relatively low concentrations, and tend to accumulate in the food web. Examples include arsenic, cadmium, chromium, lead, and mercury

hectare - a metric measure equal to 10,000 square meters or 2.47 acres

heiau - a traditional Hawaiian place of worship; a religious shrine

helicoid - spiral-shaped

heliophilous - having an attraction for sunny places

heliotropism - an orienting response to the sun

heliox - a breathing gas mixture for scuba divers that contains only oxygen and helium, used for deep diving to remove the narcotic effect of nitrogen

helix - a structure with a spiral shape

helophilous - having an affinity for marshes

heme - a complex red organic pigment containing iron and other atoms to which oxygen binds

An acorn worm in the phylum Hemichordata. (Photo: BIODIDAC)

hemichordate - any of various worm-like marine animals belonging to the phylum Hemichordata, having a primitive notochord and gill slits

hemocoel - the extensive spaces of an arthropod's body through which the hemolymph (blood) circulates

hemocyanin - a bluish, copper-containing respiratory pigment with an oxygen-carrying function similar to that of hemoglobin that is present in the blood of certain mollusks and arthropods

hemoglobin - the iron-containing protein (pigment) found in red blood cells of vertebrates. Hemoglobin transports oxygen from the respiratory surface (gills, lungs) to the body's tissues. It is red when oxidized

hemolymph - the circulating and tissue-bathing fluid of the arthropod open circulatory system. It is composed of cells and plasma and often loosely termed as blood

Henry's Law - the amount of gas that will dissolve in a liquid at a given temperature is almost directly proportional to the partial pressure of that gas

hepato- - pertaining to the liver

herbivore - an animal that feeds on plants

heredity - the transfer of genetic information from parent cells to progeny

hermaphrodite - an animal or plant which is equipped with both male and female reproductive organs

hermatypic coral - a reef-building coral with zooxanthellae in its tissues

Herpetologists study the biology of amphibians and reptiles, such as this Indopacific sea snake, Laticauda colubrina. This species lays its eggs on land. Other species are livebearers and give birth to their young in the ocean. (Photo: Sohan Shetty)

herpetology - the scientific study of amphibians and reptiles

hertz - unit of frequency equal to one cycle per second

heterocercal - a caudal fin where the upper lobe is larger than the lower lobe. Most sharks have heterocercal caudal fins

heterochrony - a change in the timing of ontogenetic events between two species. These can be the result of relatively small genetic changes between an ancestor and its descendant species

heterodont - a type of dentition where the teeth are not all similar. For example, a mixture for special functions, such as canines, incisors and molars for piercing and tearing, snipping and grinding

heterogeneous - consisting of dissimilar elements, parts or forms; having non-uniform structure or composition

heteroscedasticity - in statistics: time-dependent variance; the property of a series of random variables of not every variable having the same finite variance; a sequence of random variables is heteroscedastic if the random variables have different variances

heterosis - a situation where crossing two inbred lines yields offspring that are more healthy/vigorous than their parents; hybrid vigor

Heterotrichea - a class of unicellular ciliates in the phylum Ciliophora. Heterotrichs are among the largest protozoan forms. Some sessile species cause coral diseases in the Pacific and Indian Oceans, the Red Sea, and the wider Caribbean

heterotroph - an organism that cannot manufacture its own food, and therefore requires external sources of energy

heterotypic school - a well-defined group or school of several kinds of fishes

heterozooid - a specialized non-feeding bryozoan zooid. Heterozooids include forms specialized for producing and brooding eggs, or, more rarely, spermatozoa. Others are specialized to protect the colony, or have a cleaning function, or strengthen and support the colony

heterozygote - an individual having a heterozygous gene pair. A diploid or polyploid with different alleles at a particular locus

heterozygous gene pair - a gene pair having different alleles in the two chromosome sets of the diploid individual, for example, Aa

heuristic - problem analysis based on informal judgment or experience versus data manipulation; a guideline or rule of thumb that is normally effective in dealing with a given situation

hexamerous - having six parts, or parts in multiples of six arranged radially, as found in anthozoans in which the tentacles and mesenteries are in multiples of six

hexaxon - in sponges, a spicule with six rays

hiatus - a gap, e.g., unoccupied space between the distributions of two species or populations; a space between teeth

hierarchy - in animal behavior, any social rank-order established through physical or ritualistic combat, threat, passive submission or some combination of these behavioral patterns

A small island at the mouth of the Amazon River at high tide. See low tide for contrast. (Photo: Alessandra and Michael)

high tide - the tide at its fullest extent, when the water reaches its highest level

high-resolution satellite SST climatology - 9 km monthly or yearly averages of satellite-derived (see AVHRR) sea surface temperatures obtained over periods of 10 years or longer

high-throughput sequencing - a rapid method of determining the sequence of bases in a DNA molecule

High-throughput sequencing (Next-Generation Sequencing) - technologies that parallelize the nucleotide sequencing process, producing thousands or millions of sequences at once. Using these technologies may lower the cost of DNA sequencing beyond what is possible with standard methods

hinge - the elastic part of a bivalve (Mollusca) shell that unites the valves along the top of the shell

histogram - a bar graph in which the area over each class interval is proportional to the relative frequency of data within this interval

histology - the branch of biology that studies the microscopic structure of animal or plant tissues. The four basic types of animal tissues are: epithelial tissue, nervous tissue, muscular tissue and connective tissue (bone, cartilage, blood, fat, and areolar (fibrous). The three basic plant tissues are: dermal tissue, ground tissue, and vascular tissue

histone - a type of protein present in the nucleus of eukaryotic cells that helps to compact DNA into tightly packed chromosomes

histone H2B - a small, highly conserved nuclear protein that, together with 2 molecules each of histones H2A, H3 and H4, forms the eukaryotic nucleosome core

historical data - data sets from previous studies

histosol - soil with a high organic content

hoa - a channel or pass connecting the atoll lagoon with the open ocean

holdfast - a root-like structure for attachment that anchors attached seaweeds and other algae to the substrate; also applied to the structure that attaches octocorals to the substrate

holobenthic - pertaining to fauna that are confined to a benthic existence throughout their entire life cycle

holoblastic cleavage - complete cleavage of the zygote. The cleavage furrows pass all the way through the zygote; typical of isolecithal and mesolecithal eggs

Holocene epoch - an epoch of the Quaternary period dating from the end of the Pleistocene approximately 8,000 years ago until the present

holoeuryhaline - pertains to organisms that inhabit fresh, brackish and marine waters

holoplankton - planktonic organisms that spend their entire life cycle in the floating state, as contrasted with organisms, such as fishes, which spend only a portion of their life cycle (eggs, larvae) as members of the planktonic community

A holothurian (sea cucumber), Holothuria atra.

holothurian - a sea cucumber in the echinoderm class Holothuroidea

holothurin - a toxic substance released by some holothuroids (sea cucumbers)

holotype - in taxonomy, a single specimen designated or indicated the type specimen by the original author at the time of publication of the original description

home page - the first page that a browser opens when accessing a Web address (URL). The home page generally serves as a gateway to the rest of the Web site by providing links to the other pages

home range - the area over which an animal normally travels in its daily activities

homeobox - family of genes that regulates activities of other genes (turns genes 'on' and 'off' )

homeostasis - the ability to maintain a relatively constant internal environment

homeotic gene - a gene that controls the activity of other genes involved in the development of a body plan

homeotic mutation - a mutation that causes a body part of an organism to develop in an inappropriate position

homing behavior - a type of behavior where the adult organism returns to its place of origin

homocercal - a caudal fin with upper and lower lobes that are approximately equal in size; characteristic of most bony fishes

homodont - type of dentition where the teeth are all similar, indicative of a uniform diet

homogeneous - of similar or uniform structure or composition throughout; refers to anything which displays a uniform or consistent composition

homolog - in genetics, one member of a chromosome pair. Homologous chromosomes have corresponding DNA sequences and come from separate parents, i.e., one homolog comes from the maternal parent and the other comes from the paternal parent; in evolution, a characteristic that is similar in different species because it evolved from a common ancestor

homologous chromosomes - the pair of chromosomes in a diploid individual that have the same overall genetic content. One member of each homologous pair of chromosomes is inherited from each parent

homology - the relationship of any two characters that have descended from a common ancestor. The term can apply to a morphological structure, a chromosome, an individual gene, or a DNA sequence

homonym - in taxonomy, each of two or more identical but independently proposed names for the same or different taxa. A junior homonym is the later published of two homonyms. A senior homonym is the earlier published of two homonyms

Homonymy, Law of (Principle of) - any name that is a junior homonym of an available name must be rejected and replaced; the principle that the name of each taxon must be unique

homoscedasticity - in statistics: time-independent variance; the standard error of estimate is the same for all fitted values of the dependent variable; a property of a set of random variables where each variable has the same finite variance

homotypic school - a well-defined school of fish composed of one species with individuals all of similar size

homozygote - an individual having a homozygous gene pair. A diploid or a polyploid with identical alleles at a locus

homozygous gene pair - a diploid gene pair having identical alleles in both copies, for example, AA or aa

hookah - "hookah" refers to diving where the diver is supplied with breathable air from the surface via an air compressor and an airline (hose). This type of diving is ideal for shallow water commercial applications

horizontal classification - in taxonomy, classification which stresses grouping together taxa in a similar stage of evolution, rather than location on the same phyletic line

horn - a bony projection, e.g. the "horns" on the heads of cowfishes

horse latitudes - two belts or regions of subtropical high-pressure areas, located between 30-35 degrees north and south, in which the wind is light and varied and weather is hot and dry

host - an organism which serves as the habitat for a parasite or symbiont. The host may provide nutrition to the parasite or symbiont, or simply a place in which to live

hotspot - an environmentally endangered region that is both rich in species and found nowhere else

HotSpot animation - animations from HotSpot imagery denoting coral reef bleaching events over time

HotSpot charts - charted regions that highlight sea surface temperature (SST) anomalies that are greater than 1 deg C above the maximum monthly climatological SST. See also coral bleaching and coral bleaching hotspot

hump - the raised area behind the head in certain fish species, usually mature adults or breeding males

Infrared image of Hurricane Hugo making landfall September 22, 1989. (Image: NOAA)

hurricane - an intense tropical cyclone in which winds tend to spiral inward toward a core of low pressure, with maximum surface wind velocities that equal or exceed 33.5 m/sec (75 mph or 65 knots) for several minutes or longer at some point

Surge from 1969's Hurricane Carol swamps a yacht club. (Photo: Providence Journal Co., NOAA/NWS Historic Collection)

hurricane surge - a rise in the sea surface on an open coast, often resulting from a hurricane

hyaline - translucent or transparent

hyalinization - a form of tissue degeneration in which the tissues develop a homogeneous and glassy appearance

hybrid - an individual with parents of different species

hybrid name - in taxonomy, names of progeny of two individuals belonging to different taxa. Names given to hybrids are not normally available, as they are individuals, not populations, and hence not taxa

hybridization - the production of offspring (hybrids) from genetically dissimilar parents

Hydranth of the hydrozoan, Obelia. 1=coenosarc; 2=hydrotheca; 3=tentacles. (Photo: Rick Gillis, Ph.D., Biology Dept., University of Wisconsin-La Crosse)

hydranth - a feeding polyp of a colonial hydrozoan. It bears tentacles armed with nematocysts, a mouth and a thin outer covering, the hydrotheca

hydrocarbon - an organic molecule, such as methane (CH₄), which consists only of carbon and hydrogen atoms

hydrocarbon seep - an area where hydrocarbons seep slowly from the sea floor

Hydrocaulus of the hydrozoan, Obelia. (Photo credit: Rick Gillis, Ph.D., Biology Dept., University of Wisconsin-La Crosse)

hydrocaulus - the main stem of a colonial hydrozoan which consists of a cylindrical tube of living tissue (coenosarc) covered by a thin outer membrane (perisarc)

hydrochory - spread of plant seeds, spores or propagules by water

hydrogen bond - a relatively weak chemical bond consisting of a hydrogen atom between two electronegative atoms (e.g., oxygen or nitrogen), with one side being a covalent bond and the other being an ionic bond

hydrogen ion - an individual atom of hydrogen which is not attached to a molecule and therefore has a positive (+) charge

hydrological cycle - the movement of water in all of its phases (gas, liquid, solid) from the Earth to the atmosphere and back to the Earth

hydrology - the science dealing with the properties, distribution and circulation of water and snow

hydrolysis - the breaking down of a compound into fragments by the addition of a molecule of water. The hydroxyl group is incorporated in one fragment and the hydrogen atom in the other

hydrophilic - having a strong affinity for water; tending to dissolve in, mix with, or be wetted by water -

hydrophobic - refers to molecules that do not interact with water and are not soluble in water. Hydrophobic compounds do not dissolve easily in water, and are usually non-polar. Oils and other long hydrocarbons are hydrophobic

hydrophone - an underwater microphone

hydrophyte - a plant that grows partly or wholly in water whether rooted in the substrate or floating without anchorage, as the water hyacinth; an aquatic plant

hydropic - containing an excess of water or of watery fluid; swelling of cells, usually associated with cell injury

hydrosere - an ecological succession beginning in a habitat with abundant water, typically on the submerged sediments of a standing water body (e.g. lake or lagoon), and ending on dry land

hydrostatic skeleton - a type of skeleton found in many soft-bodied invertebrates which consists of a turgid column of liquid within one of the body spaces that provides support or rigidity to the organism or to one of its parts. Hydrostatic skeletons are found in echinoderms (starfish, sea urchins), annelids (worms), nematodes (roundworms), and a number of other wormlike invertebrate phyla

hydrotheca - a thin outer covering of a hydranth, the feeding polyp, of a colonial hydrozoan

hydrothermal plume - a cloud of hot, mineral-rich water that flows out of a hydrothermal vent and disperses into the ocean, usually several hundred meters above the seafloor vent site. Rock particles and minerals in the plume water often make the plume look smoky

hydrothermal vent - a sea floor fissure from which a spring of geothermally heated mineral and gas rich seawater issues. Hydrothermal vents are found on some oceanic ridges in zones of active seafloor spreading (plate tectonics)

hydroxyl group - an [-OH] or alcohol group on a larger molecule. The oxygen is single-bonded to the hydrogen and has one free bond to the rest of the molecule

White-tipped mustard-colored fire coral in the Flower Garden Banks.

Hydrozoa - a class within the phylum Cnidaria. The Hydrozoa contains five orders that include: small medusae with no polyp generation; colonial forms with alternating polyp and medusa stages and a chitinous exoskeleton; solitary polyps that lack a medusoid stage; colonial forms with massive aragonite skeletons (e.g., fire coral); and complex colonial forms, with individual polyps specialized for feeding, swimming, prey capture, and reproduction. Some, but not all, float by means of a large pneumatophore, or gas bag

hyoid - having a "U" shape

hyper- - above, exceeding, higher, more than, abnormally increased

hypereosinophilia - a disease characterised by a marked increase in the eosinophil (a type of white blood cell) count in the blood

hyperlink - text or images on a Web page that, when clicked with a mouse, causes the browser to load another page of HTML. Because a simple mouse click allows the user to easily go from one page of hypertext to another, these pages are said to be "hyperlinked." Text links are usually, but not always underlined in blue, while hyperlinks that are images often take the form of "buttons"

hyperparasite - a parasite whose host is also a parasite

hyperplasia - abnormal increase in the size/volume of a tissue due to multiplication of cells

hypersaline - referring to water with a salinity higher than that of natural seawater

Hyperspectral AVIRIS (Airborne Visible/Infrared Imaging Spectrometer) - a hyperspectral image is a very high resolution image which was acquired with a hyperspectral scanner. These instruments acquire data in 224 contiguous channels of approximately 10nm bandwidth. They are spanning the visible, near-infrared and mid-infrared portion of the electromagnetic spectrum

hypertrophy - the over-development of a structure

hypertrophy - the excessive enlargement or development of an organ or tissue, with increase in cell size but without increased cell division

hypha - one of the long, branching filaments that forms the mycelium of a fungus

hypo- - under, below, beneath, lower than, small size, deficiency

hypobranchial gland - a gland in the mantle cavity of mollusks that secretes mucus

hypolithic - living on lower surfaces of rocks

hyposaline - referring to water with a salinity lower than that of natural seawater

hypostome - in cnidarians, the circular raised area of a hydrozoan polyp that lies between the tentacles and the mouth. The term is also used to describe mouthparts in other phyla

hypothermia - a condition when the body temperature is colder than normal (37 degrees C/98.6 degrees F in humans)

hypothesis - a tentative assumption made for the purpose of empirical scientific testing. A hypothesis becomes a scientific theory when repeated testing and the great body of evidence suggests that the hypothesis has a stong probability of being correct

hypoxia - a condition that may occur in aquatic environments as the concentration of dissolved oxygen becomes reduced to a level harmful to aquatic organisms; in physiology, hypoxia is the condition where oxygen concentrations are below normal physiological limits in a specific tissue

hypural plate - in fishes, the flattened bony plate at the posterior end of the vertebral column