dacriform - tear-drop shaped

dactyloid - finger-shaped

dactylozooid - a colonial hydrozoan polyp that possesses a large, nematocyst-bearing fishing tentacle, and functions in defense and in food capture

Dalton's Law - the total pressure exerted by a mixture of gases is the sum of the pressures that would be exerted by each of the gases if it alone were present and occupied the total volume

damselfish - a large family (Pomacentridae) of bony fishes which are abundant and common inhabitants of coral reefs. They possess robust, deep, and laterally compressed bodies. The majority of damselfishes do not have particularly brilliant markings or coloration. Exceptions are the brilliantly colored anemone fishes, the banded sergant major, and the bright orange garibaldi, Many species of damselfishes are highly territorial

dark-field microscope - a microscope that has a special condenser and objective with a diaphragm that scatters light from the observed object. The object appears bright on a dark background

Dark spots disease infecting Stephanocoenia intersepta. (Photo: NOAA; image copyrighted)

dark-spots disease - a coral disease characterized by darkly pigmented areas of tissue on stony corals.-At present, there is no known pathogen. The coral tissue remains intact, although at times lesions and coral tissue death are observed in the centers of the pigmented areas. Tissue loss is minimal, if present. This disease is widespread throughout the Caribbean.

Darwin Core - a body of data and metadata standards that consists of a vocabulary of terms to facilitate the discovery, electronic retrieval, and integration of information about organisms, their spatiotemporal occurrence, and supporting evidence in natural history specimen collections and species observation databases. It provides a stable standard reference for sharing information on biological diversity

Darwin Mounds - two areas of hundreds of sand and cold-water coral mounds at depths of about 1,000 m, in the northeast corner of the Rockall Trough, approximately 185 km northwest of the northwest tip of Scotland. The Darwin Mounds cover an area of approximately 100 sq. km. The tops of the mounds are covered with Lophelia pertusa corals and coral rubble

Darwin point - the latitude at which reef growth just equals reef destruction by various physical forces

Darwinian evolution - evolution of life forms by the process of natural selection acting on random genetic variations

data - multiple facts (usually but not necessarily empirical) used as a basis for inference, testing, models, etc.; the word is plural (sing. datum) and takes a plural verb

data management - the act, process, or means by which data are managed. This includes the planning, collection, compilation, archival, safe-guarding, listing, organization, extraction, retrieval, manipulation, and dissemination of data

data mining - an information extraction activity whose goal is to discover hidden facts contained in databases. Using a combination of machine learning, pattern recognition, statistical analysis, modeling techniques and database technology, data mining finds patterns and subtle relationships in data and infers rules that allow the prediction of future results

data warehouse - a database, frequently very large, that can access vast arrays of heterogeneous data, stored within a single logical data repository, that are accessible to different querying and manipulation methods . While the warehouse can be distributed over several computers and may contain several databases and information from numerous sources in a variety of formats, it should be accessible through a server. Thus, access to the warehouse is transparent to the user, who can use simple commands to retrieve and analyze all the information. The data warehouse also contains data about how the warehouse is organized, where the information can be found, and any connections between data. Frequently used for decision support within an organization, the data warehouse also allows the organization to organize its data, coordinate updates, and see relationships between information gathered from different parts of the organization

database - a structured file of information or a set of logically related data stored and retrieved using computer-based means

database management system (DBMS) - a set of computer programs for organizing the information in a database. A DBMS supports the structuring of the database in a standard format and provides tools for data input, verification, storage, retrieval, query, and manipulation

daughter cell - one of the two cells formed by the division of a parent cell

1985-2000 average sea surface temperature from AVHRR Pathfinder.

Day/Night SST - observations of sea surface temperature obtained during both daytime and nighttime orbits from the Advanced Very High Resolution Radiometer (AVHRR) on NOAA's polar satellite

de facto - in fact; in reality; existing but not officially recognized or legally established

dead ahead - a position directly in front of a vessel

dead zone - hypoxic (low-oxygen) areas in the world's oceans, usually in coastal zones, where little marine life is present, mostly because of eutrophication with subsequent algal blooms. Notable dead zones in the United States include the Gulf of Mexico area surrounding the outfall of the Mississippi River, and coastal regions of the Pacific Northwest

decadal - refers to a climatic process that re-occurs every decade or once every few decades

A spider crab is a decapod crustacean.

decapod crustacean - a members of the Order Decapoda, Class Malacostraca, Superclass Crustacea, Phylum Arthropoda; has five pairs of thoracic legs. Examples are shrimps, lobsters, crabs, and hermit crabs

decibel - a logarithmic scale used to denote the intensity (loudness), of a sound relative to the threshold of human hearing. A step of 10 dB is a 10-fold increase in intensity or sound energy

decibel - unit for measuring sound intensity

deciduous - periodically shed

decomposer - a heterotrophic organism that breaks down dead biological matter and uses some of the products and releases others for use by consumer organisms

decomposition - the breakdown of organic matter by bacteria and fungi

decompression - a change from one ambient pressure to a lower ambient pressure as the scuba diver ascends. Decompression also occurs in a decompression chamber. Decompression results in a reduction of gas pressures within the body

A NOAA decompression chamber.

decompression chamber - a hyperbaric steel enclosure used to treat victims of decompression sickness (the "bends") in which the air pressure is first gradually increased and then gradually decreased. This shrinks the nitrogen bubbles and allows the nitrogen to safely diffuse out of the victim's tissues

decompression dive - any dive where the scuba diver is exposed to a higher pressure than when the dive began. Decompression occurs as the diver ascends

decompression diving - scuba diving that requires in-water stops during ascent to the surface to allow off-gassing of nitrogen

decompression sickness (the bends) - a dangerous and potentially lethal condition of divers precipitated by rapid changes in ambient atmospheric pressure, mostly in rapid ascent from underwater, but can also result from flying in an aircraft too soon after a dive. It occurs because at high pressures (such as SCUBA divers experience while underwater) the blood can contain more dissolved nitrogen than at lower pressures. When the diver ascends too rapidly, the blood can no longer contain this dissolved nitrogen and tiny gas bubbles begin to form in the blood. Symptoms include: body pain (mainly in the joints), headache, confusion, itchy skin rash, visual disturbances, weakness or paralysis, dizziness, or vertigo. Treatment involves the administration of oxygen and placing the patient into a decompression chamber until the nitrogen bubbles shrink and safely diffuse from the tissues

These NOAA divers are making a decompression stop to allow nitrogen to escape from their tissues. (Photo: NOAA/National Marine Fisheries Service)

decompression stop - a specified time spent at a specific depth as a scuba diver ascends from a dive for purposes of releasing nitrogen gas from the tissues (nitrogen off-gassing)

deductive reasoning - an inference in which the conclusion about particulars follows necessarily from general theory. In science, deductive reasoning would involve stating an hypothesis first, and then trying to find facts that reject the hypothesis

deep fore reef - the deepest seaward part of a coral reef; a vertical cliff beginning at a depth of about 60 m

deep scattering layer - a thin sound-reflecting layer of zooplankton and nekton that ascends toward the surface at night and descends each day (diurnal vertical migration) in response to changing levels of light

deep water - the water beneath the permanent thermocline that usually has a low and uniform temperature

Lophelia pertusa, a deep-sea stony coral.

deep-sea corals - stony, soft, gorgonian, black, and horny corals that inhabit the colder deep waters of continental shelves and offshore canyons, ranging from 50 -1000m+ depths. They lack zooxanthellae and may build reef-like structures or occur solitarily

deep-sea sponge community - an assemblage of structure-forming deep-sea sponges and other associated species below 50 m

defensome - genes encoding protein complexes that participate in defense mechanisms to environmental chemical stressors; also called "chemical defensome"

definitive host - in a parasite's life cycle, it is the host organism in which the parasite reproduces sexually

deforestation - the removal of trees from a habitat dominated by forest

degeneracy - in relation to the genetic code, more than one codon can code for the same amino acid

degenerate character - a character, trait or structure that has evolved to a less developed state from its ancestral form or function

degeneration - a process by which tissue deteriorates, loses functional activity, and may become converted into or replaced by other kinds of tissue; deterioration which causes some degree of loss of original function; the process of declining from a higher to a lower l

Degree Heating Week (DHW) - the NOAA satellite-derived Degree Heating Week (DHW) is an experimental product designed to indicate the accumulated thermal stress that coral reefs experience. A DHW is equivalent to one week of sea surface temperature 1 deg C above the expected summertime maximum. For example, 2 DHWs indicate one week of 2 deg C above the expected summertime maximum

Degree Heating Week accumulation - accumulated thermal stress that coral reefs experience over a typical 12-week period

degrees of freedom - in statistics, the number of independent comparisons that can be made between the members of a sample; in a contingency table it is one less than the number of row categories multiplied by one less than the number of column categories. The number of degrees of freedom is defined as the number of observations that can be chosen freely, i.e., an estimate of the number of independent categories in a particular statistical test or experiment

deimatic behavior - defensive postures or other visual displays, including color changes, that function to intimidate or frighten another animal

delayed fertilization - when fertilization of an egg does not occur immediately following introduction of spermatozoa into the female reproductive tract, but may be delayed for weeks or months

delta - the fan-shaped area at the mouth or lower end of a river formed by eroded material that has been carried downstream and dropped in quantities larger than can be carried off by tides or currents

delta notation - the absolute abundance of an isotope is difficult to measure with accuracy. Therefore, we compare isotopic ratios in a sample with those in a standard resulting in the delta-notation: d(x) = [{Rx - Rst}/Rst] x 103, where d(x) is the delta-value of a sample, Rx and Rst are the isotopic ratios in sample (Rx) & standard (Rst). The d-value is the relative difference in the isotopic ratio of the sample and the standard. It is expressed in part per mille (o/oo); that is why the right-hand side of the equation is multiplied by 103 (1000). Carbon and oxygen data from carbonates are usually referred to the PDB standard (a belemnite, Belemnitella americana, from the Late Cretaceous PeeDee Formation in South Carolina)

deme - a local interbreeding population of a species

demersal - pertains to an organism that is essentially bottom living but may feed and swim in the water column

demography - the rate of growth and the age structure of populations, and the processes that determine these properties

denaturation - the inducing of structural alterations that disrupt the biological activity of a molecule. It often refers to breaking hydrogen bonds between base pairs (by heat) in double-stranded nucleic acid molecules to produce single-stranded polynucleotides, or altering the secondary and tertiary structure of a protein, destroying its activity

denatured alcohol - ethyl alcohol (ethanol) to which a poisonous substance, such as acetone or methanol, has been added to make it unfit for consumption

dendriform - having a structure that resembles a tree or shrub

dendrite - a sensory branch of a neuron that carries a nervous inpulse to the cell body

Black coral is dendritic in shape. (Photo: Waikiki Aquarium)

dendritic - branched like a tree

dendrogram - a branching tree-like diagram used to represent phylogenetic paths of evolution

denitrification - the formation of gaseous nitrogen and/or nitrogen oxides from nitrate or nitrite by denitrifying bacteria during anaerobic respiration

denitrify - to remove nitrogen from any substance or chemical compound

denitrifying bacteria - anaerobic bacteria in soil or water that use the nitrate ion as a substitute for molecular oxygen during their metabolism. The nitrate is reduced to nitrogen gas (N₂), which is lost to the environment during the process

dentate - having teeth or tooth-like points; serrate

denticle - a little tooth

denticulate - having an edge with small projecting teeth

deoxyribonucleic triphosphates - unreactive nucleotides that closely resemble the nucleotides that make up DNA. They are 'dummy' nucleotides that act as placeholders when DNA is sequenced

depauperate - an area poor in species richness and/or biodiversity; an impoverished habitat

dependent species - a species dependent on another for survival, e.g., a predator on a prey, a commensal or other kind of symbiont

dependent variable - the variable being measured

deposit - material left in a new position by a natural transporting agent, such as water, wind, ice, or gravity, or by human activity

deposit feeder - an animal that feeds on nutrients in the sediments

depressed - a body shape which is flattened dorso-ventrally, e.g., a ray, skate, monkfish

depth contour - a line on a nautical chart connecting points of equal depth

derived character - in evolution, an advanced trait which only appears in some members of a taxonomic group. For example,a derived character for some mammals would be the loss of the tail, which occurs in the great apes and man. Another derived character is the presence of feathers in birds. Scales are the ancestral feature. Derived characters are also called apomorphies

dermal - pertaining to or affecting the skin

dermal flap - a small skin flap

dermis - the layer of the skin beneath the epidermis. The dermis is largely fibrous and contains collagen and elastin which are the proteins responsible for the support and elasticity of the skin. Depending upon the species, the dermis also contains tiny sensory nerve endings, blood and lymph vessels, and sweat and sebaceous glands

designated Use - classification specified in water quality standards for each waterbody or segment describing the level of protection from perturbation afforded by the regulatory programs. The designated aquatic life uses established by the state or authorized tribes set forth the goals for restoration and/or baseline conditions for maintenance and prevention from future degradation of the aquatic life in specific waterbodies

desmocyte - a cell specialized in binding soft tissues onto skeletal structures in scleractinian corals

determinate cleavage - cleavage resulting in blastomeres each capable of developing only into a particular embryonic structure, not into a complete organism

Low tide along South Carolina shoreline with bits of sea shells and other detritus (Photo: Richard B. Mieremet, NOAA)

detritus - the particulate decomposition or disintegration products of plankton, including dead cells, cell fragments, fecal pellets, shells, and skeletons, and sometimes mineral particles in coastal waters

detrivore - an animal that eats detritus

deuterostome - one of two distinct evolutionary lines of coelomates, consisting of the echinoderms and chordates and characterized by radial cleavage of the early embryo. The cleaving cells are indeterminate (if early embryonic cells are separated, each one develops into a complete organism). The anus develops from the blastopore

developed country - describes nations or countries with social, cultural, industrial and technological advancement

developing country - describes regions and countries that are still in the process of acquiring modern technology and becoming economically productive. These regions are sometimes called the “Third World”

development - the chronological series of changes, from a lower to a higher state of organization, which multicellular organisms undergo from the fertilized egg (zygote) to maturity

developmental response - morphological and physiological characteristics an organism developed in response to prolonged exposure to environmental conditions

deviation - in statistics, the difference between an actual observation and the mean of all observations

dewlap - a fold of loose skin

dextral - right, as opposed to sinistral, or left

diadromous species - a species which undertakes a spawning migration from ocean to river or vice versa

diagenesis - all of the changes that occur to a deposited sediment during its conversion to rock; includes changes that result from chemical, physical as well as biological processes

diagnosis - with reference to disease, the determination of the nature and cause of a disease; in taxonomy. a description of those characteristics that distinguish one taxon from another

diagnostic characters - in taxonomy, the characters, or most important characters, which distinguish a taxon from other similar or closely related taxa

diapause - a state of arrested development or growth, accompanied by greatly decreased metabolism

diaphanous - thin and translucent; semi-transparent

diastema - a space; a gap

A living diatom (Pleurosigma angulatum) from Arctic seas.

diatom - a unicellular alga that consists of two interlocking valves composed of silica

diatomaceous - pertaining to diatoms or their fossil remains

dichopatric - pertains to allopatric populations with non-contiguous ranges

dichopatric speciation - a type of speciation in which a formerly contiguous population is split by the rise of some geographical barrier, e.g., a mountain range

dichotomous key - a tool to help identify taxa. It is made up of pairs of choices. Each choice is between statements describing specific traits of the taxa under consideration. Only one statement will be true for each choice. Each choice points to another set of choices until finally only one choice remains

diel - pertaining to the day-night cycle

diffusion - the movement of particles from an area of higher concentration to an area of lower concentration

digestion - the breakdown, by hydrolysis, of complex ingested nutrient compounds (carbohydrates, fats, proteins) into their building blocks, i.e., the conversion of food, in the alimentary canal, into soluble and diffusible products, capable of being absorbed into the circulating fluid and the cells

Digital Versatile Disc (DVD) - an advanced type of CD-ROM that holds a minimum of 4.7 gigabytes (unit of storage) to a maximum of 17 gigabytes of information. They are compressed using the MPEG codec, which stores only the changes from one frame to another instead of the entire frame

digitiform - finger-shaped

dimorphism - having two different distinct forms of individuals within the same species or two different distinct forms of parts within the same organism. It could refer to different colors or color patterns, sizes, anatomical parts, etc. Sexual dimorphism is a common case, where the two sexes have different shapes, sizes, etc.

dinghy - a small open boat

dioecious - having separate sexes. Individuals within the species contain only one or the other of male and female reproductive systems

diphycercal - a caudal fin shape which is primitively symmetrical and pointed, and with the vertebral column or notochord extending to the tip, as found in primitive fishes, such as lampreys and chimaeras

diploid - the condition in which a cell contains a nucleus with two complete sets of chromosomes, one set inherited from each parent. The diploid condition is often abbreviated as 2n. Most plants and animals are diploid. The term also represents the number of chromosomes in most cells except the gametes, which are haploid in chromosome number

directional selection - a type of natural selection that removes individuals from one end of a phenotypic distribution and thus causes a shift in the distribution. The frequency of an allele is changed in a constant direction, either toward or away from fixation for that allele. Directional selection occurs when individuals at one phenotypic extreme have an advantage over individuals with more common phenotypes

disaccharide - a sugar (carbohydrate) formed by the covalent bonding of two monosaccharides. Table sugar, sucrose, is a disaccharide

disciform - round or oval-shaped

discoidal - disc-shaped; flat and round shape

discrete random variable - a random variable whose range of possible values is finite or countably infinite

discrimination - differential response to different stimuli

Coral with yellow band disease, which results in serious losses of coral tissue.

disease - any impairment of an organism's vital functions or systems, including interruption, cessation, proliferation, or other malfunction

disease vector - an organism which transmits infective organisms from one host to another

disjunct - distinctly separate; disjunct populations are populations separated from other potentially interbreeding populations by a distance large enough to prevent exchange of genetic materials

disjunct distribution - the discontinuous or separated geographical distribution of a species or other taxonomic unit

dispersal - the spread of a species to a new location. In many organisms, this happens at a particular stage in the life cycle, and is often critical for the species' survival. Organisms may disperse as spores, seeds, eggs, larvae, juveniles, or adults; to diffuse, spread or scatter as in oil spills

dispersant - a toxic liquid used to place oil in suspension in the water mass and promote its dispersal in order to accelerate break down by the environmen,t including bacterial decomposition. Dispersants are mixtures of solvents, surfactants, and other additives

displacement behavior - a behavioral response that is appropriate for one situation appears in another situation, for which it is inappropriate

display - in animal behavior, visual messages or body language, used by animals primarily to communicate anger, fear, and other basic emotions. Displays are strong indications of an animal's emotional state

The color pattern of the juvenile spotted drum, Equetus punctatus, is an example of disruptive coloration. (Photo: Dr. Tom Doeppner, Brown University)

disruptive coloration - a color pattern that breaks up the outline of an organism

dissociation - the temporary or reversible chemical process in which a molecule or ion is broken down into smaller molecules or ions

dissolved organic carbon (DOC) - a measure of the organic compounds that are dissolved in water

dissolved oxygen - the concentration of oxygen dissolved in water, expressed in mg/l or as percent saturation, where saturation is the maximum amount of oxygen that can theoretically be dissolved in water at a given temperature and pressure

distal - the direction away from the midline of the body; the opposite of proximal

distinct - clearly defined and easily recognized

distinct population segment - "population," or "distinct population segment," are terms with specific meaning when used for listing, delisting, and reclassification purposes to describe a discrete vertebrate stock that may be added or deleted from the list of endangered and threatened

disulfide bond - a chemical bond between the sulfur atoms of two different amino acids in a protein

diurnal - active during the day light hours

dive computer - a small electronic sensor and calculator, carried by the scuba diver, that calculates and displays the basic information needed during a dive, i.e., depth, time, decompression status and tank pressure. By constantly monitoring depth and bottom time, dive computers automatically recalculate the diver's no-decompression status, giving longer dive times while still keeping the diver within a safe envelope of no-decompression time. Computers also monitor ascent rates, logs dives, and measures time intervals between dives

dive computer algorithm - a suite of equations that compute nitrogen uptake and elimination in tissues from changes in the diver's depth and elapsed time underwater

The PADI (Professional Association of Diving Instructors) recreational dive planner (dive table) has three tables. Table 1 gives the maximum amount of time the diver can stay at a certain depth on the first dive, and it also indicates how much nitrogen the diver has in in the tissues after a dive. Table 2 is concerned with the diver's surface interval time (how long a diver must remain at the surface before the next dive), and Table 3 allows the diver to determine safe diving limits on the next dive. (Photo: PADI)

dive table - dive tables present dive times for specific depths, adherence to which, the scuba diver can avoid contracting decompression sickness (the bends). The theory behind dive tables is based on our understanding of how nitrogen is taken up on compression (descent) and given off on decompression (ascent). The first dive tables were devised by John S. Haldane in the period 1906-1908

divergent evolution - the evolution from one species of organism into a number of different species. As the original population increases in size, it spreads out from its center of origin to exploit other habitats and ecological niches. In time, this results in a number of populations, each adapted to its particular habitat. Eventually these populations, genetically may differ from each other sufficiently to become new species. Divergent evolution has also been termed "adaptive radiation"

diversity index - a mathematical index of species diversity within a community

diverticulum - a blind sac branching off a cavity or canal

The SAT system diving bell is raised to the surface after an eight hour dive on the wreck of the USS Monitor. The bell is the divers' "taxi" between their topside saturation living quarters and their work site, some 240 ft below the surface. (Photo: official U.S. Navy photo by Photographer’s Mate Chief Petty Officer (DV/SW) Andrew McKaskle)

diving bell - a hollow, usually inverted vessel, such as one used for diving deep in a body of water. It is open on the bottom and supplied with air under pressure. During the Monitor 2001 Expedition, navy divers utilized a 12-person, two-chamber saturation system with a two-person closed diving bell. The system can operate as deep as 1,500 ft—considerably deeper than the Monitor, which rests on the sea floor at a depth of 235 ft. Saturation systems are often used in deep-water situations (below 200 ft) in order to reduce the time lost to decompression during the slow ascent to the surface required for preventing decompression sickness

division - in botanical nomenclature, "division" is used instead of "phylum", and is equal in taxonomic status to the phylum

Graphic of DNA shows the spiral double helix structure of the molecule.

DNA (deoxyribosenucleic acid) - also termed deoxyribonucleic acid. The molecule that encodes genetic information in the cells. It resembles a double helix held together by weak bonds of four nucleotides (adenine, guanine, cytosine, and thymine) that are repeated ad infinitum in various sequences. These sequences combine into genes that govern the production of proteins. The DNA located within the nuclear membrane of eukaryotic cells is sometimes referred to as nDNA

DNA annealling - the reformation of double stranded DNA from thermally denatured DNA. The rate of reassociation depends upon the degree of repetition and is slowest for unique sequences

DNA chip - a small piece of glass or silicon that has small pieces of DNA arrayed on its surface

DNA hybridization - the process of joining two complementary strands of DNA, or one each of DNA and RNA, to form a double-stranded molecule; a technique in which single stranded nucleic acids are allowed to interact so that complexes or hybrids are formed by molecules with sufficiently similar, complementary sequences. By this means the degree of sequence identity can be assessed and specific sequences detected

DNA library - a collection of cloned DNA fragments that collectively represent the genome of an organism

DNA marker - segments of chromosomal DNA known to be linked with heritable traits or diseases. Although the markers themselves do not produce the conditions, they exist in concert with the genes responsible and are passed on with them

DNA polymerase - an enzyme that replicates DNA. DNA polymerase is the basis of PCR ( polymerase chain reaction)

DNA probe - in genomics, the DNA affixed to a microarray; a small piece of nucleic acid that has been labeled with a radioactive isotope, dye, or enzyme that is used to locate a particular nucleotide sequence or gene on a DNA molecule

DNA replication - DNA replication or DNA synthesis is the process of copying the double-stranded DNA prior to cell division. The two resulting double strands are identical (occasionally errors (mutation) in replication can result in a less than perfect copy) and each of them consists of one original and one newly synthesized strand

dolioform - barrel-shaped

doliolaria larva - the larval stage of sea cucumbers immediately following the auricularia stage. It is cylindrical in shape and possesses five transverse bands of cilia

dome shaped - a form that resembles half of a sphere

dominant - having the ruling or controlling power over; to predominate over

dominant species - a species which make up a large proportion of a community in terms of its biomass or numbers of individuals

domoic acid - an acidic cyanotoxin found associated with certain diatom blooms. Domoic acid can bioaccumulate in marine organisms that feed on the phytoplankton, such as shellfish and some fishes. In mammals, including humans, domoic acid is a neurotoxin responsible for Amnesic Shellfish Poisoning (ASP) , causing short term memory loss, brain damage, and in severe cases, death

doppler radar - radar that can measure radial velocity, the instantaneous component of motion parallel to the radar beam (i.e., toward or away from the radar antenna)

doppler shift - the change in the tone of a sound caused by the sound source moving away or towards the listener

dorid nudibranch - a type of nudibranch (order Nudibranchia) possessing a feather-like external gill on the back and a rhinophoral sheath. The mantle is thick and extends over the foot. The surface of the mantle may bear tubercles which vary in size, shape and number, and are often a taxonomic diagnostic character

dormancy - a period of suspended growth and metabolic activity. Many plants, seeds, spores, cysts, and some invertebrates become dormant during unfavorable conditions

The upper or back surface of this queen angelfish is the dorsal surface, as opposed to the opposite belly surface, which is the ventral surface. (Photo: Chris Huss, Florida Keys National Marine Sanctuary)

dorsal - refers to the upper or back surface of an animal

Dragonets, such as Callionymus lyra, exhibit strong sexual dimorphism in the dorsal fin (male [top] and female [bottom]). (Image: NOAA)

dorsal fin - in fishes, one or more fins situated on the midline of the back, having spines or rays, sometimes both; excludes the adipose fin found in some fishes, such as catfishes and salmon

dorsoventral - an axis extending from the dorsal to ventral surface of an animal body

dot grid - a technique used to analyze a photograph of a quadrat (photo-quadrat), in which a grid of random dots is placed over an image of the photo-quadrat. It assumes that the proportion of dots that lies on a substrate is equal to the proportional area of the substrate

double helix - the normal structural configuration of DNA consisting of two helices winding about the same axis. The structure of DNA was first proposed by Watson and Crick (1953) with two interlocking helices joined by hydrogen bonds between paired bases

double stranded RNA (dsRNA) - long double-stranded RNAs (dsRNAs; typically >200 nt) can be used to silence the expression of target genes in a variety of organisms and cell types

download - transferring data (usually a file) from one computer to another. The opposite of "upload"

downstream - in the direction of the water movement

downwelling - a downward current of surface water in the ocean, usually caused by differences in the density of seawater

dredge - a metal collar with an attached collecting bag that is dragged along the bottom to obtain samples of rock, sediment, or benthic organisms

dredging - a method for deepening streams, swamps or coastal waters by scraping and removing solids from the bottom. The resulting mud is usually deposited in marshes in a process called filling. Dredging and filling can disturb natural ecological cycles. For example, dredging can destroy coral reefs and other aquatic life; filling can destroy the feeding and breeding grounds for many fish and invertebrate species

A marine turtle is caught in a drift net. (Photo: NOAA)

drift net - a fishing net, often miles in extent, arranged to drift with the tide or current and buoyed-up by floats or attached to a boat

drop root - an adventitious root in mangroves that originates from the branches, and roots in the surface-sediments

drowned river - a former river inundated by a rise in sea level in past times

dry weight - the moisture-free weight of a biological sample obtained by drying at high (oven-drying) or low (freeze-drying) temperatures for an time sufficient to remove all water

Dublin Core - a set of metadata elements used in digital libraries, primarily to describe digital objects and for collections management, and for exchange of metadata; a set of metadata elements used to describe electronic information and data resources

duct - any tubular structure

duplex DNA - double-stranded DNA

DVD-ram - a high-capacity, high-performance optical disk that allows data to be read, written, and erased. It is comparable to a rewritable CD, and can hold up to 2.6 gigabytes of information per side

dynamic optical demarcation - in animal behavior, a special signalling device used by an animal in a stereotypical movement, as for example, the waving of a fiddler crab's claw to attract the attention of other members of its species

dyne - unit of force to accelerate 1 gram to 1 cm per second per second

dystrophy - a degenerative disorder that mostly affects muscles; some lakes have dystrophic waters which have organically-rich waters from decomposing plants