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Glossary of Terminology: E

The glossary is organized alphabetically by page -- A through Z. Literature sources are listed on the References Page.

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Number of records found : 238


early life history - the stages from egg to juvenile in fish development

ebb current (ebb) - the movement of a tidal current away from shore or down a tidal river or estuary

ebb strength - phase of the ebb tidal current at the time of maximum speed; also, the speed at this time

ebb tide - that period of tide between a high water and the succeeding low water; falling tide

ecad - a non-inheritable plant or animal form produced in response to particular habitat environmental factors; an environmentally induced form

ecdysone - a molting hormone of arthropods. It stimulates growth and ecdysis (molting)

Ecdysozoa - the ecdyzoans comprise one of the major and largest protostome groups within the animal kingdom. It includes both the arthropods and the nematodes, as well as lesser groups such as rotifers, cephalorhynchs (which include priapulids, kinorhynchs, and loriciferans), and onychophorans. Ecdysozoans build a cuticle, an outer layer of organic material that functions as a lightweight flexible exoskeleton. The name Ecdysozoa refers to the fact that many members of this group regularly shed their cuticle, a process called ecdysis

Image of pencil urchin (Echinodermata)

A pencil urchin of the phylum Echinodermata.


Echinodermata - an animal phylum that contains starfishes, sea cucumbers, sand dollars, brittlestars, basket stars, sea lilies, feather stars, and sea urchins. Adults exhibit pentamerous radial symmetry, secondarily derived from a bilateral ancestor. They are not at all related to the other radiate phyla, such as the Cnidaria

Image of echinopluteus

Free-swimming echinopluteus sea urchin (Echinocardium cordatum) larva feeds on microplankton captured with its ciliated arms. (Photo: Jan Parmentier)


echinopluteus larva - a larval form of some echinoderms

Echiura - an animal phylum that contains the echiurans or spoonworms. Some species inhabit coral crevices. Echiurans are deposit feeders

echolocation - the sonar-like ability used by bats, dolphins, some whales, and two groups of cave-dwelling birds to detect objects in their environment. Using echolocation, the animal emits high-frequency sounds that reflect off of an object and return to the ears or other sensory receptors

ecocline - a gradual and continuous change in environmental conditions of an ecosystem or community; gradual transition between ecotypes; a continuous change in form as a response to a continuous change in an environment

ecodeme - an intrabreeding population occupying any specified ecological habitat

ecological equivalent - a different species that occupies a similar ecological niche in a similar ecosystem in a different part of the world

ecological isolation - a form of reproductive isolation in which two closely-related species are separated by what is often a slight difference in the niches they occupy

ecological restoration - the process whereby an entire ecosystem is brought back to healthy condition

ecological species - a concept of species, in which a species is a set of organisms adapted to a particular ecological niche

ecology - the study of the interrelationships between organisms and their environment, including the biotic and abiotic components

ecomorph - a local population or group whose appearance is determined by ecological factors

ecophenotype - a phenotype showing non-genetic adaptations associated with the habitat or environment

ecoregion - a geographic area of relative homogeneity in ecological systems or in relationships between organisms and their environment; a relatively large area of land or water that contains a geographically distinct assemblage of natural communities

ecospecies - an assemblage of ecotypes which are separated by incomplete sterility barriers

ecosystem - an ecological community considered together with the non-living factors of its environment as a unit

ecosystem (or environmental) services - these are “benefits people obtain from ecosystems.” These benefits are obtained from both direct use of ecosystems, and when ecosystems are not used directly, their services still play an important socioeconomic role

ecosystem based management (EBM) - EBM integrates knowledge of ecological interrelationships to manage impacts within an ecosystem. Effective implementation of EBM should: (1) consider ecological processes that operate both inside and outside ecosystem boundaries, (2) recognize the importance of species and habitat diversity, and (3) accommodate human uses and associated benefits within the context of conservation requirements

ecosystem integrity - the ability to support and maintain a balanced, integrated, adaptive biological community having a species composition, diversity and functional organization comparable to that of a natural habitat in the region

Image of coral restoration

Two divers work to reattach a large fragment of elkhorn coral, Acropora palmata, at Mona Island, Puerto Rico. The coral in the background has been attached to the reef framework using stainless steel wires. The wire binds the coral fragment to the living coral where it is expected to reattach to the live coral. (Photo: Erik Zobrist, NOAA Restoration Center)


ecosystem restoration - actions taken to modify an ecosystem for the purpose of re-establishing and maintaining desired ecological structures and processes

ecotone - a transitional area between two adjacent ecological communities

ecotope - the abiotic or non-living part of an ecosystem

ecotourism - travel undertaken to experience sites or regions of unique natural or ecological quality, or the provision of services to facilitate such travel

ecotype - a genetically differentiated subpopulation that is restricted to a specific habitat

ecotype - a population in a given ecosystem that is adapted to a particular set of environmental conditions

ecto- - a prefix meaning 'outside'

ectoderm - the outer germ layer of cells in an embryo that gives rise to the outer layer of skin (epidermis) and neural tissue

ectodermis - the outer cellular covering of a polyp

Image of ectoparasite in fish

A marine ectoparasite lodged in a fish's mouth. (Photo: NOAA Ocean Explorer)


ectoparasite - a parasite which lives on the surface of its host

ectoplasm - the clear, nongranular portion of the cell cytoplasm just inside the cell membrane

Ectoprocta - an animal phylum synonymous with the phylum Bryozoa, the "moss animals"

ectosarc - the ectoplasm of of some unicellular forms; the thin outer layer of cytoplasm adjacent to the cell membrane

ectothermic - having an internal body temperature that is dependent on the surrounding temperature. Most aquatic animals are ectotherms; also caled poikilothermal or "cold blooded"

Image of eddy in Gulf of Mexico

AVHRR image of eddy in the Gulf of Mexico. (Image: NOAA-14 Polar Orbiting Satellite/LSU Earth Scan Lab, Coastal Studies Institute)


eddy - a circular movement of water formed on the side of a main current

edentate - toothless

edge effect - habitat conditions created at or near the more-or-less well-defined boundary between ecosystems (ecotone). Typically there is an increased richness of organisms resulting from the mixing of two communities where they join

Image of eelgrass

Eel grass meadows. (Photo: NOAA)


eelgrass - a common seagrass (Zostera marina) distributed from Greenland to Florida that serves as an important habitat for fishes and shellfish. Unfortunately, approximately 90 percent of all eelgrass throughout its range along the Atlantic coast has been destroyed

effluent - a discharge of pollutants into the environment, partially or completely treated or in its natural state. Generally used in regard to discharges into waters; in contrast to an emmission, which is generally used in regard to discharges of pollutants into the air

effort - the amount of time and fishing power used to harvest fish. Fishing power includes gear size, boat size, and horsepower

egestion - the elimination of undigested food materials from an organism

egg - a female sex cell or gamete with the haploid number of chromosomes. It may be fertilized by a sperm cell to produce a zygote with the diploid number of chromosomes for that particular species. The eggs of some species may develop into multicellular individuals without being fertilized by a sperm cell. This is the process of parthenogenesis

egg case - the leathery case which contains the developing embryo in hagfishes, sharks, rays, skates and chimaeras; also called a "mermaid's purse"

eiloid - coil-like

Ekman layer - the thin horizontal layer of water riding on top of the ocean that is affected by wind

Image of 1997 sea surface temperatures showing El Niño

Graphic of sea surface temperatures show El Niño, May 1997.


El Niño - an irregular variation of ocean current that from January to March flows off the west coast of South America, carrying warm, low-salinity, nutrient-poor water to the south. It is associated with the Southern Oscillation. These two effects are known as the El Niño Southern Oscillation (ENSO). El Niño usually occurs during the Christmas season when the equatorial counter current strengthens and flows eastward to overlie the cold, nutrient-rich waters of the Peru current. It does not usually extend farther than a few degrees south of the equator, but occasionally it does penetrate beyond 12 deg S, displacing the relatively cold Peru Current. The effects of this phenomenon are generally short-lived, and fishing is only slightly disrupted. Occasionally, the effects are major and prolonged

Image of manta ray

The manta ray is a large, graceful, mostly plankton-feeding elasmobranch. They also feed on small fishes. (Photo: Jackie Reid/Flower Garden Banks National Marine Sanctuary)


elasmobranch - a member of the Subclass Elasmobranchii of cartilaginous fishes. These include sharks, rays, and skates

elastic - capable of returning to an initial form or shape after deformation; resilient

electrocyte - a cell that generates electricity. The electric currents produced by some species of fishes, such as torpedo rays and electric eels, are generated in stacks or columns of electrocytes.The electrocytes of most 'electric fishes' are modified muscle cells. They are usually arranged in columns within electric organs. When an electrocyte is stimulated, a movement of ions (electrically charged atoms) across the cell membrane results in an electric discharge

electrogenic - capable of generating electric currents and a painful electric shock, as in electric rays

electromagnetic radiation - energy that travels through space in the form of waves. The highest frequencies in the spectrum of electromagnetic radiation are gamma-rays; the lowest frequencies are radio waves. All electromagnetic radiation travels at the speed of light. Shorter wavelength radiation (eg, ultraviolet) carries more energy and is likely to be more harmful to living tissue

electromagnetic receptor - a neurological receptor that responds to light, electricity, and magnetism. Photoreceptors respond to light and electroreceptors detect electrical energy

electron microscope - a microscope which beams electrons, instead of light beams, at and through the object of interest. This type of microscope provides the greatest resolution of extremely small details in the nanometer size range. Magnification may be up to x300,000

electrophoresis - a technique for separating different types of molecules based on their patterns of movement in an electrical field

electroporation - the creation of reversible small holes in a cell wall or membrane through which foreign DNA can pass. This DNA can then integrate into the cell's genome

electroreceptor - a receptor organ that senses changes in an electrical current in the surrounding water, for example, the ampullae of Lorenzini in sharks

electrotaxis - the movement of organisms in response to an electric field

ELF (eliminate lionfish) tool - a short, hand-held spring propelled spear (patent pending, Reef Protection Inc.) used by scuba divers and snorkelers to spear and kill lionfish without causing damage to the coral reef

Image of trout

The caudal fin of this trout has an emarginate shape. (Image: U.S. Fish and Wildlife Service)


emarginate - a notched margin, but not so deeply as to be forked. As an example, a trout possesses an emarginate caudal fin

embayment - an indentation in the shoreline that forms a bay

embryo - the stage of early growth and differentiation of tissues and the formation of primitive organs and organ systems of a multicellular organism, from fertilization until birth or hatching. In seed plants, it is the young sporophyte that resulted from the union of the egg and one of the two tube nuclei

emergent vegetation - plants growing in water but emerging from it, usually at the water's edge

emersed - rising above the water surface

emigration - the movement of individuals out of a population or from one area to another

empirical - based on experience or observations, as opposed to theory or conjecture

emulsion - a suspension of small globules of one liquid in a second liquid with which the first will not mix

enantiomer - a molecule that is not superimposable on its mirror image; an enantiomorph is either one of a pair of compounds that are mirror images on each other but are not identical

encrustation - a crustlike deposit or growth over a substratum

Image of encrusting tunicate colony

Encrusting tunicates at Gray's Reef off Sapelo Island, Georgia (Photo: Karen Angle).


encrusting colony - a thin colony which adheres closely and is attached to the substrate

Endangered Species Act (ESA) - an Act of Congress passed in 1966 that establishes a federal program to protect species whose survival is threatened by habitat destruction, overutilization, disease, etc.

endangered taxa - taxa in danger of extinction and whose survival is unlikely if causal factors continue operating. Included are taxa whose numbers have been drastically reduced to a critical level or whole habitats have been so drastically impaired that they are deemed to be in immediate danger of extinction. Also included are those that possibly are already extinct, in so far as they have not been seen in the wild in the past 50 years

endemic species - a species whose distribution is restricted to a particular area

endo- - a prefix meaning 'inside'

endobenthic - refers to meiofaunal-sized organisms that move within the sediments

endocoel - in coral and anemone polyps, the space between two mesenteries belonging to one and the same pair.

endocoelic tentacle - in anemones, a tentacle whose tenticular cavity communicates with the endocoel

endocrine gland - a gland that manufactures hormones and secretes them directly into the circulatory system to act at distant sites in the body

endocrine system - the system of ductless glands in animals that secrete hormones

endocytosis - uptake of material into a cell by the formation of a membrane-bound vesicle

endoderm - the inner germ layer of diploblastic and triploblastic embryos that gives rise to internal tissues such as the gut and gut derivatives, e.g., air bladder, lungs, and the lining of the digestive tract

endogenous - having its origin, or produced within the organism or one of its parts

endogenous rhythm - a metabolic or behavioral rhythm that originates within the organism and persists regardless of external conditions

endolithic - growing within a rock or any other hard inorganic substratum

endolithic algae - algae which burrow into calcareous rocks or corals

endonuclease - a nuclease that cleaves nucleic acids at specific internal sites

endoparasite - a parasite which lives in the internal organs of its host

endophytic - living within the tissues of a host plant or alga

endopinacocyte - in sponges, a pinacocyte lining the incurrent and excurrent canals

endopinacoderm - in sponges, a surface lined with endopinacocytes

endoplasm - the granular portion of the cell cytoplasm between the ectoplasm and nuclear membrane

Image of whale skeleton

Endoskeleton of a 35-ton, 13-m gray whale on the floor of the Santa Cruz Basin.


endoskeleton - skeleton, or support structure, which is on the inside of the organism's body. All vertebrates possess an endoskeleton that is made of either bone and/or cartilage

endosymbiont - an organism which lives within the body of another organism as part of a symbiotic relationship. The relationship may be mutualistic or commensalistic; also called an 'endobiont'

endosymbiosis - a symbiotic association in which one organism lives inside another

endosymbiotic - being symbiotic and living within the body of an individual of the associated species

endothecal dissepiment - one of many horizontal partitions across the corallite within the corallite wall

Image of humpback whales

Whale fat (blubber) aids these humpback whales in maintaining a relatively constant body temperature. (Photo: Dave Matilla, NOAA/NOS National Marine Sanctuaries)


endothermic - an animal that is able to maintain a body temperature that varies only within narrow limits by means of internal mechanisms. Most birds and mammals are considered endothermic; also called homeothermal or "warm blooded"

endothermic reaction - a reaction which absorbs heat

endozoochory - the dispersal of plant seeds or spores within the body of an animal, as passing through the animal's digestive system; also called "endochory"

endozooic - living inside an animal

energy flow - the movement of energy through a community via feeding webs

Enriched Air Nitrox (EANx) - a breathing gas mixture for scuba divers which has a percentage of oxygen greater than 21 percent as is found in normal air. This mixture allows longer bottom times at limited depths

enrichment - the addition of nitrogen, phosphorus and carbon compounds or other nutrients into a lake or other waterway that greatly increases the growth potential for algae and other aquatic plants. Most frequently, enrichment results from the inflow of sewage effluent or from agricultural runoff

ENSO (El Niño Southern Oscillation) - see El Niño and Southern Oscillation

enterocoelous - the mesoderm and coelom initially develop as pouches off of the primitive digestive tract (archenteron) of an embryo. Enterocoelous development of the coelom occurs in deuterostomes

enteron - the alimentary canal or the gut of an embryo. Some structures of the enteron may not be completely developed or differentiated in early embryonic growth

entrainment - the synchronisation of one biological rhythm to another or to a zeitgeber cycle, e.g. circadian rhythms are often entrained to the light-dark cycle; also, the process of small organisms being captured in the cooling water of a power plant

entropy - the measure of the disorder or randomness of energy and matter in a system

environment - everything external to the organism

environmental impact assessment - detailed studies which predict the effects of a development project on the environment. They also provide plans for the mitigation of adverse impacts

environmental sensitivity index (ESI) map - ESI maps serve as quick references for oil and chemical spill responders and coastal zone managers. They contain three kinds of information: (1) Shorelines (color-coded to indicate their sensitivity to oiling); (2) Sensitive biological resources (such as seabird colonies and marine mammal hauling grounds. They are depicted by special symbols on the maps); and (3) Sensitive human-use resources (such as water intakes, marinas, and swimming beaches). Project scientists have created collections of ESI maps, called ESI atlases, for most coastal areas in the US, including Alaska, Hawaii, the Great Lakes, and the Trust Territories. Each atlas includes maps and additional data and information. Starting in 1995 with the outer coast of California, ESI project members began using a Geographic Information System (GIS) to produce updated ESI maps that are higher in quality and can be more readily reproduced

Image of the <i>Exxon Valdez</i>

A catastrophic environmental stress-the 1989 grounding of the Exxon Valdez, which spilled an estimated 11 million gallons of crude oil over 1,300 miles of coastline.


environmental stress - severe environmental effects on the natural ecosystem

environmentalism - advocacy for, or work toward, protecting the natural environment from destruction by human activities

enzootic - an infectious disease constantly present in an animal population but having low incidence

enzyme - an organic catalyst

enzyme-linked immunosorbent assay (ELISA) - a rapid test where an antibody or antigen is linked to an enzyme as a means of detecting a match between the antibody and antigen; a technique for detecting specific proteins by using antibodies linked to enzymes

EosFP - a fluorescent marker protein with UV-inducible green-to-red fluorescence conversion. EosFP was isolated from the stony coral Lobophyllia hemprichii

eosinophile - a type of white blood cell (leukocyte) that engulfs and digests bacteria. Eosinophils, along with other types of white blood cells, are important in an organism's internal defense system

eosinophylic - having an affinity for eosin dyes

ephemeral - short-lived, transitory

Image of ephyra larva

Ephyra larva of the jellyfish Aurelia. (Photo: Rick Gillis, Ph.D., Biology Dept., University of Wisconsin-La Crosse)


ephyra larva - a jellyfish (Scyphozoa) larval stage that develops into the adult medusa or jellyfish

epibenthic (epifaunal or epifloral) - refers to organisms living on the surface of the substrate

epibiont - an organism that lives on the outside of another organism

Epicratonic Sea - a large shallow sea covering a craton, which is an old and stable part of a continental lithosphere generally found in the interiors of tectonic plates

epidemic - the widespread outbreak of a disease, or a large number of cases of a disease in a single community or relatively small area

Image of spawning star coral

Star coral shedding gametes in an epidemic spawning event.


epidemic spawning - the simultaneous shedding of gametes by a large number of individuals

epidermis - the outer epithelial layer of the body

epifauna - animals that live upon or are associated with substratum features

epilithic - growing on rock or stone. Epilithic organisms live attached to rocks

epimeletic behavior - in animal behavior, the giving of care or attention, including parental care and also including non-parental care

epipelic - living on or in fine sediments, such as mud or sand

epiphyte - microalgal organism living on a surface (e.g., on a seaweed frond); a plant living on the surface of another plant

epipsammic - attached to, or moving through, sand particles

epistasis - the prevention or masking of the expression of an allele at one locus by an allele at another locus

epistome - the flap over the mouth of some lophophorates

epitheca - a layer of calcium carbonate that grows outside corallite walls

epithelium - a tissue layer of cells which lines body cavities and tubules, or covers surfaces. The cells may be ciliated or non-ciliated, and may be squamous (flat, scale-shaped), cuboidal (cube-shaped), or columnar (column-shaped) in shape. The cells may occur in a single layer, or may be multi-layered (stratified)

epithet - in taxonomy, part of the full name of a species. A complete species name consists of the name of the genus to which the species belongs, plus the specific epithet, plus the author of the species. For example, the foureye butterflyfish, Chaetodon capistratus Linnaeus, 1758

Image of polychaete

Palolo (Eunice viridis) are polychaete worms that are about 12 inches long and live in burrows dug into the coral pavement on the outer reef flat. The worm is composed of two distinct sections. The front section is the basic segmented polychaete with eyes, mouth, etc., followed by a string of segments, called the “epitoke,” that contain reproductive gametes. (Image: www.nps.gov)


epitoky - a mode of reproduction unique to polychaete worms in which the worm undergoes a partial or entire transition into a pelagic, sexually reproductive form, known as an epitoke. In many cases, epitoky involves loss or degeneration of digestive structures and enhancement of swimming, sensory, and reproductive structures. The epitoke is a considered a delicacy in some islands of the South Pacific.

epizoite - an animal that lives on the surface of another organism

epizootic - a temporal pattern of disease occurrence in an animal population in which the disease occurs with a frequency clearly in excess of the expected frequency in that population during a given time interval; an outbreak (epidemic) of disease in an animal population

epizootiology - the study of the factors which determine frequencies and distributions of infectious diseases among non-human animals

epoch - a division of geologic time next shorter than a period. Example: the Pleistocene epoch is in the Quaternary period

equilibrium - the state in which the action of multiple forces produces a steady balance, resulting in no change over time

equinox - either of the two times during a year when the sun crosses the celestial equator and the length of day and night are equal

era - a division of geologic time next smaller than the eon and larger than a period. Example: The Paleozoic era is in the Phanerozoic eon and includes, among others, the Devonian period

erectile - capable of being raised or erected, often referring to spines of bony fishes

erg - unit of work measuring force of one dyne applied over one centimeter

ergo - therefore; hence

erosion - the wearing away of the land surface naturally by wind or water, but is often intensified by human's land-clearing practices. The runoff is harmful to coral reefs

Image of polychaete

The polychaete Nereis sp. is an errant species of worm. (Photo: U.S. Geological Survey)


errant - motile or free swimming

erythrophore - a chromatophore which contains reddish pigments found in carotenoids and pteridines

esca - in some fishes, e.g. frogfishes, the first dorsal fin ray is modified to function as a fishing pole and lure. The part close to the body is the illicium or fishing pole. The esca is the lure on the end of the illecium

escape reaction - in animal behavior, a response to a predator or other dangerous situation, in which the threatened animal may flee, hide, or stay motionless in order to avoid contact

escarpment - a steep slope or long cliff that results from erosion or faulting and separates two relatively level areas of differing elevations

ESRI (Environmental Systems Research Institute, Inc.) - a world leader in GIS (geographic information system) software and technology

essential amino acid - an amino acid that cannot be synthesized by animals and therefore has to be ingested with food

Image of mangrove roots

This tangle of red mangrove roots in the Jobos Bay, Puerto Rico National Estuarine Research Reserve is an essential fish habitat, serving as a both a nursery area and protection for many marine animals. (Photo: NOAA)


essential fish habitat - under the Magnuson-Stevens Fishery Conservation and Management Act, those waters and substrates that fishes require to spawn, breed, feed, or grow to maturity

establishment - the subsequent growth and/or reproduction of a colonized species in a new territory

ester - a chemical compound formed by the reaction of an organic or inorganic acid with an alcohol, with the elimination of water

esterification - any chemical reaction between an acid and an alcohol that results in the production of an ester

esthetasc - a sensory seta covered by cuticle projecting from most antennules and antennae in crustaceans

esthete - a light sensitive organ in a minute vertical canal in the upper layer of the shell plate of a chiton

estimate - the best guess arrived at after considering all the information given in a problem

et-epimeletic behavior - in animal behavior, calling or signaling for care or attention, typically seen in young animals, but also occurring in adults

ethanoic acid - acetic acid

ethogram - an inventory or catalog of all of the behavioral patterns of an organism or a species

etiology - the science that is concerned with origins and causes of disease

euchromatic - describes cells or tissues that stain the color of the stain (dye) solution used to stain them

euchromatin - the genetically active portion of chromatin that is largely composed of genes

eukaryotic - descriptive of organisms with cells having a distinct nucleus with nDNA, and intracellular membranes. This includes all protists, fungi, plants and animals. The organisms are termed eucaryotes

eumetazoa - all multicellular animals excluding the sponges

euphotic depth - the depth to which significant phytoplankton photosynthesis can take place. It is typically taken to be the depth at which PAR falls to 1 percent of its value just below the surface

euphotic zone - the layer of the ocean that receives sufficient sunlight for photosynthesis. The depth to which 1% of incident light penetrates (1% is the minimum amount of light required for photosynthesis)

euryhaline - pertaining to an aquatic organism that can withstand a broad salinity range

euryokous - adapted to numerous ecological niches

euryphagous - describes an organism which gains its nourishment from a large variety of foods

eurythermal - pertaining to an aquatic organism that can withstand a broad temperature range

eurytopic - tolerating a wide variation in one or more environmental factors

Image of fish kill resulting from eutrophic conditions

Eutrophic conditions can result in large fish kills, as many fish die from reduced levels of oxygen in the water.


eutrophic - a situation in which the increased availability of nutrients such as nitrate and phosphate stimulates the growth of plants such that the oxygen content is depleted and carbon sequestered

evanescent - short-lived, temporary, soon disappearing

event - in probability, an event is an occurrence or the possibility of an occurrence that is being investigated

everted - turned inside out

evo-devo (evolutionary developmental biology) - a relatively new approach in biology that seeks to explain the causes biodiversity. It attempts to integrate every area of biology from molecular genetics through embryology, molecular and population genetics, comparative morphology, paleontology, molecular evolution, ecology and functional morphology. It offers both an account of developmental processes and also new integrative frameworks for analyzing interactions between development and evolution. A major challenge lies in integrating these approaches to understand the evolution of biodiversity at a mechanistic level. The "evo-devo" community needs access to genomic information on a wide range of organisms

evolutionarily significant unit (ESU) - a population or group of populations that is considered distinct (and hence a "species") for purposes of conservation under the Endangered Species Act. To qualify as an ESU, a population must 1) be reproductively isolated from other conspecific populations, and 2) represent an important component in the evolutionary legacy of the biological species

evolutionary significant unit (ESU) - a group of organisms that has undergone significant genetic divergence from other groups of the same species. Identification of ESUs is based on natural history information, range and distribution data, and results from analyses of morphometrics, cytogene

evolutionary tree - a lineage designed to show the evolutionary history of relationships among groups of organisms

ex situ conservation - the preservation of components of biological diversity outside their natural habitats, e.g., in zoos, aquaria, botanic gardens and gene banks

Exclusive Economic Zone (EEZ) - the sovereign waters of a nation, recognized internationally under the United Nations Convention on the Law of the SEA as extending out 200 nautical miles from shore. Within the U.S. the EEZ typically is between three and 200 miles from shore

excretion - a physiological process, originating in cells, that removes waste materials produced by the body

excurrent canal - in sponges, an excurrent canal discharges water received from the apopyle, into the spongocoel (atrium). The water then passes through the osculum or oscula to the outside

Executive Order 13089 - on June 11, 1998, President Clinton issued Executive Order 13089 directing all agencies to increase their efforts to protect our nation's coral reef resources. The executive order calls for the establishment of a U.S. Coral Reef Task Force, cochaired by the Secretaries of the Interior and of Commerce. The Task Force will develop and implement a comprehensive program of inventory, monitoring, and research to map and identify the major causes and consequences of degradation of coral reef ecosystems

Executive Order 13158 - on May 26, 2000, President Clinton signed Executive Order 13158 on Marine Protected Areas (MPAs) to strengthen the protection of U.S. ocean and coastal resources. This significant milestone in ocean conservation directs the Departments of Commerce and the Interior, and other federal agencies, to strengthen and expand a national system of MPAs by working closely with state, territorial, local, tribal, and other stakeholders

exhalant system - in sponges, part of the aquiferous system between the apopyle and the and osculum

exocoel - in coral and anemone polyps, the space between mesenteries belonging to different pairs

exocoelic tentacle - in anemones, a tentacle whose tenticular cavity communicates with the exocoel

exocrine gland - a gland that secretes its product through a duct

exogenous - having its origin external to the organism or ecosystem

exon - a nucleotide sequence (of DNA or RNA) in a gene that codes for part or all of the gene product or for some control function. In eukaryotes, exons are separated by non-coding sequences called introns; that part of the gene (a section of DNA) that is transcribed into messenger RNA and encodes a protein

exopinacoderm - the unicellular external surface of a sponge (ectosome composed of pinacocytes)

exoskeleton - an external skeleton or supportive covering of an animal formed from the ectoderm, as for example, the shell coverings of a crustacean, the calcium carbonate secretions of stony corals, or the bony plates of an armadillo

exothermic reaction - a reaction that gives off heat

exotic species - a non-native species that is introduced into an area; also referred to as alien or invasive species

exozoochory - the dispersal of plant seeds or spores accidentally attached to the outside of an animal's body; also called "epizoochory"

expected value - in statistics, the mean value calculated for a statistic over an infinite number of samples

explanate coral colony - a colony that spreads horizontally as the branches fuse into a solid or near solid plate

exponent - an expression of the number of times that a base is used as a factor

exponential growth - growth in the size of a population in which the rate of growth increases as the size of the population increases; change in a population growth rate that is proportional to the size of the population

exposure - contact with infectious agents such as bacteria or viruses in a manner that promotes transmission and increases the likelihood of disease

Image of diver in dry suit

Arctic diver with rebreather and heated dry suit prepares to descend into the ice. (Photo: NOAA/OAR/National Undersea Research Program)


exposure suit - a full or partial garment that is worn by scuba divers for protection against heat loss. Water conducts heat away from the body about 20 times faster than air. In addition to providing warmth, exposure suits also serve to protect from minor scrapes, stings and abrasions. There are three kinds of exposure suits: wet suits, which allows water to seep in between the insulated rubber covering and the skin. They come in a variety of thicknesses suitable for insulation in water as cold as 50 degrees F; body suits, made from Lycra or nylon.They provide full length abrasion protection, but only minimal insulation; and dry suits which are water-tight garments that keeps the divers body warm by providing insulation with a layer of gas, such as air. It is used for diving in waters that are too cold for comfortable wet suit protection, usually below 65 degrees F

expressed sequence tag (EST) - a small part of the active part of a gene, made from cDNA which can be used to fish the rest of the gene out of the chromosome by matching base pairs with part of the gene. The EST can be radioactively labeled in order to locate it in a larger segment of DNA

exsert corallite - a corallite that extends above the surface of the colony

extant - of a taxon: having living representatives; of a specimen: still in existence

extant species - species which are now living

Extended Reconstruction of SST (ERSST) data set - a globally complete reconstruction of of sea surface temperatures (SST) based on in situ measurements and satellite data produced at a monthly, 2 degree resolution

extension rate (linear extension rate) - annual normalized growth rate (measured as accretion or lengthening) of a branching coral caused by calcification

exteroreceptor - a neurological receptor that receives information from the environment external to the organism

extinct - of a taxon: having no living representatives.

extinct species - species for which there are no living representatives

Image of manatee and pup

Manatees are threatened with extinction from both habitat destruction and direct effects from human encroachment, such as serious wounds from boat propellors.


extinction - the evolutionary termination of a species caused by the failure to reproduce and the death of all remaining members of the species; the natural failure to adapt to environmental change

extinction coefficient - a coefficient measuring the rate of extinction, or diminution, with distance of transmitted light in sea water

extirpated species - a species that has been destroyed or removed completely from a particular area, region, or habitat

extra- - outside, beyond

extra-embryonic membrane - membranes possessed by amniote (reptiles, birds, and mammals) embryos that allow these classes of vertebrates to be free of aquatic habitats for reproduction, and in their evolution, occupy terrestrial habitats. The membranes are the amnion, yolk sac, allantois and chorion

extracellular digestion - a form of digestion that takes place within the lumen of the digestive system. The resulting nutrient molecules are transferred into the blood or body fluids through the process of absorption

extratentacular budding - an asexual form of reproduction where daughter corallites grow from the outside wall of the parent corallites

Image of tube worms with crabs

Crowded life in an extreme environment—Riftia tubeworms, mussels, and scavenging crabs found at a hydrothermal vent site in the Pacific Ocean. Most hydrothermal vents are found at an average depth of about 2,100 meters (7,000 ft) in areas of seafloor spreading along the Mid-Ocean Ridge system. (Photo courtesy of C. Van Dover.)


extreme environment - an environment characterized by extremes in growth conditions, including temperature, salinity, pH, among others

extremophile - a microorganism that lives at extreme levels of pH, temperature, pressure or salinity

extrinsic - not forming an essential part of a thing; arising or originating from the outside

extrinsic factor - a biotic or abiotic factor acting on an organism or population from outside the organism or population.These are the physical and chemical features of the environment, as well as other organisms

Image of small Pacific jellyfish

The rounded upper surface of this jellyfish is the exumbrella.


exumbrella - the upper surface of the bell of a medusa or jellyfish

exuvia - the cast off exoskeleton and related body parts left behind after moulting

Image of flatworm

Eyespots of a free-living flatworm, Planaria dugesia.


eyespot - any light-sensing structure in some primitive organisms that consists of a pigmented area and light sensitive cells; also called an ocellus

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