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  Marine Biology Glossary

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Abiotic Factor The physical, chemical and other non-living components of the environment that an organism lives in. These factors include all aspects of climate, geology, and atmosphere that affect ecological systems. Compare biotic factor.

Aboral Opposite the end/side on which the mouth is located (Kozloff, 1996).

Abundance Scale A modified leaf at the base of a flower stalk (OED, 2008).

Abyssal plain The deep ocean floor, an expanse of low relief at depths of 4,000 to 6,000 m. Seafloor Zones

Abyssopelagic Zone 'Abysso' meaning 'no bottom', this zone of the ocean begins 4000 m below the surface of the ocean and extends down to the sea floor. This zone is home to a variety of unique critters that are specially adapted to the inhospitable conditions that these depths create. Visit the Abyssopelagic Zone on OceanLink to learn about life in the abyss.

Acclimation Given a change of a single parameter, a readjustment of the physiology of an organism, reaching a new steady state.

Accretion Build up or accumulation of sediment.

Acontia Plural of 'acontium'.

Acontium: Thread-like nematocyst bearing organ attached to lower end of a mesentery or septal filament in the gastrovascular cavity in some Actinaria (Anemones). The acontia my be protruded through perforations (cinclides) of the body wall (adapted from Manuel, 1988 and Stachowitsch, 1992).

Adambulacral Plates: Of Echinodermata (echinoderms); a series of calcareous plates on either side of the ambulacral furrow (Southward & Campbell, 2006).

Adambulacral Spines: Of Echinodermata (echinoderms); spines on adambulacral plates (Southward & Campbell, 2006).

Adaptation: Any change in the structure or function of an organism which makes it better suited to its environment. For example, the torpedo body shape of a tuna allows for fast swimming in the open ocean.

Aggregation: Organisms (usually referring to of the same species) living closely together, but not physically connected (cf. 'colony').

Aggregated spatial distribution: A case where individuals in a space occur in clusters too dense to be explained by chance.

Ahermatypic: Non-reef-building (referring to scleractinian corals).

Algal Mat: A dense mass of green or other algae (e.g. Enteromorpha spp., Ulva spp.) which blankets the substratum in a littoral or shallow-water environment, often in areas of freshwater influence or where eutrophication occurs.

Alien Species: A non-established introduced species (q.v.), which is incapable of establishing self-sustaining or self-propagating populations in the new area without human interference (cf. 'introduced species'; 'non-native').

Allele: One of the alternative forms of a particular gene. Each gene is comprised of two alleles, one inherited from the father and one from the mother. However, within a population, many alleles may exist for one gene. Hair colour in humans is a great example! See also genes.

Allochthonous: Exogenous, originating outside and transported into a given system or area (Lincoln et al., 1998).

Allopatric speciation: The differentiation of geographically isolated populations into distinct species.

Allozyme: A variant of an enzyme type. These may be variants of a specific enzyme (e.g., cytochrome c) that are the products of a single genetic locus.

Ambulacra: Plural of 'ambulacrum'.

Ambulacral: Adjective of 'ambulacrum'.

Ambulacral Furrow: Of Echinodermata (echinoderms); groove or furrow in the oral side of the arm holding the tube feet (Southward & Campbell, 2006).

Ambulacral Furrows: Plural of 'ambulacral furrow'.

Ambulacrum: Of Echinodermata (echinoderms); radially arranged regions of the body of Echinoderms that bear the tube feet (Southward & Campbell, 2006).

Amensal: Negatively affecting one or several species. 

Amino acids: Basic structural unit of proteins. 

Amphidromic Point: The central point of a cyclonic tidal system, at which the vertical astronomical tidal range is nil, or very small, increasing progressively with increasing distance from this central point (from Ministry of Defence, 1987.)

Amphipod: A crustacean belonging the Order Amphipoda (cf. Amphipoda).

Amphipoda: An group of crustaceans recognized by their laterally compressed bodies, lack of a carapace, and numerous, differently modified legs (Hayward et al., 1996).

Anadromous fish: Fish that spends most of its life feeding in the open ocean but that migrates to spawn in fresh water.

Anaerobic: An environment in which the partial pressure of oxygen is significantly below normal atmospheric levels; deoxygenated (Lincolnet al., 1998).

Anal: Relating to or near the anus, or the posterior opening of the alimentary canal (Abercrombie et al., 1973).

Androdieocious: Reproductive strategy in which individuals are either strictly males or hermaphrodites.

Anisogamous: Having flagellate gametes of different size, shape or behaviour (from Bold, 1977 and Lincoln et al., 1998).

Annulated: Where the external surface is divided into a chain of rings or 'annuli' by furrows giving the appearance of segments (Barnes et al., 1993).

Anoxic: Lacking oxygen.

Aquaculture: Fish and other organisms farmed or raised in freshwater environments. 

Arrow worms: Members of the phylum Chaetognatha, a group of planktonic carnivores.

Asexual reproduction: Reproduction of the individual without the production of gametes and zygotes.

Assimilation efficiency: The fraction of ingested food that is absorbed and used in metabolism. 

Assortative mating: The mating of a given genotype mates with another genotype at a frequency disproportionate to that expected from random encounter.

Atoll: A horseshoe or circular array of islands, capping a coral reef system perched around an oceanic volcanic seamount.

Attenuation (of light): Diminution of light intensity; explained, in the ocean, in terms of absorption and scattering.

Autotroph: An organism that synthesizes organic molecules from inorganic starting materials through photosynthesis or chemosynthesis. Autotrophs are ecologically important as primary producers as they ultimately provide energy for all heterotrophic organisms. See also chemosynthesis, photosynthesis; compare heteroptroph.

Autotrophic algae: Algae capable of photosynthesis and growth using only dissolved inorganic nutrients. 

Auxotrophic algae: Algae requiring a few organically derived substances, such as vitamins, along with dissolved inorganic nutrients for photosynthesis.

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Bathypelagic Zone: The zone of the ocean that extends from 1000m to 4000m below the surface of the ocean. Visit the Bathypelagic Zone on OceanLink for pictures and cool facts about life at this depth.

Benthic: Refers to organisms that live on or in the ocean bed. Benthic epifauna are organisms that live on the ocean floor or upon bottom objects sch as sea anemones and barnacles, whereas benthic infauna are organisms that live within the surface sediments such as clams and worms. Compare pelagic.

Benthic-pelagic coupling: The cycling of nutrients between the bottom sediments and overyling water column.

Benthos: Organisms that live associated with the sea bottom. Examples include burrowing clams, sea grasses, sea urchins, acorn barnacles.

Berm: A broad area of low relief in the upper part of a beach.

Between-habitat comparison:. A contrast of diversity in two localities of differing habitat type (e.g., sand versus mud bottoms).

Bioaccumulation:> The process whereby pollutants are taken up, retained and concentrated in the cells of plants and animals.

Biodiversity: The variation in life on Earth reflected at all levels, from various ecosystems and species, to the genetic variation within a species. See also ecosystem diversity, species diversity, genetic diversity.

Biogenically reworked zone: The depth zone, within a sediment, that is actively burrowed by benthic organisms.

Biogenic graded bedding: A regular change of sediment median grain size with depth below the sediment-water interface caused by the activities of burrowing organisms.

Bioluminescence: Meaning living (bio) light (luminescence) is the light produced by living organisms and the emission of such biologically produced light. Also commonly referred to as 'phosphorescence'. For more info, visit the Salty Science Seaweeds Page.

Biomass: The amount of living material per unit area or volume; may be expressed as grams of carbon, total dry weight, and so on.

Biotic Factor: A living component of the environment which arises from and affects living organisms (distinct from physical factors). For example, the interaction between predators and prey is a biotic interaction. Compare abiotic factor.

Blood pigment. A molecule used by an organism to transport oxygen efficiently, usually in a circulatory system (e.g., hemoglobin). 

Bloom: (phytoplankton) A population burst of phytoplankton that remains within a defined part of the water column. 

Bohr effect: When blood pH decreases, the ability of hemoglobin to bind to oxygen decreases. An adaptation to release oxygen in the oxygen starved tissues in capillaries where respiratory carbon dioxide lowers blood pH.

Boreal: Pertaining to the Northern Hemisphere, north temperate zone. 

Boring: Capable of penetrating a solid substratum by scraping or chemical dissolution.

Bottom-up control: Refers to food webs. A control of a population that comes from change lower in a food web (e.g., control of a population of mussels by abundance of phytoplankton food).

Boundary layer: A layer of fluid near a surface, where flow is affected by viscous properties of the fluid. At the surface, fluid velocity must be zero, and the boundary layer is a thin film that depends on surface texture, fluid velocity in the "mainstream of flow," and fluid mass properties such as salinity.

Brackish sea: Semienclosed water body of large extent in which tidal stirring and seaward flow of freshwater do not exert enough of a mixing effect to prevent the body of water from having its own internal circulation pattern. 

Browsers: Organisms that feed by scraping thin layers of living organisms from the surface of the substratum (eg., periwinkles feeding on rock-surface diatom films; urchins scraping a thin, filmy sponge colony from a rock).

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Calcareous: Made of calcium carbonate

Calcification: the process by which corals extract calcium from seawater and create calcium carbonate.

Carrying capacity: The total number of individuals of a population that a given environment can sustain.

Carnivore: An organism that captures and consumes animals.

Catadromous fish: Fish that spawns in seawater but feed and spends most of its life in estuarine or fresh water.

Chaetognaths: See Arrowworms.

Character displacement: A pattern in which two species with overlapping ecological requirements differ more when they co-occur than when they do not. The difference is usually in a morphological feature related to resource exploitation, as in the case of head size, which may be related to prey size.

Chemosynthesis: The process whereby chemical energy is used to make organic compounds from inorganic compounds. One example is the oxidation of ammonia to nitrite by nitrifying bacteria. Compare photosynthesis.

Chlorinity: Grams of chloride ions per 1000 grams of seawater.

Chloroplast: In eukaryotic organisms, the cellular organelle in which photosynthesis takes place,

Chromosome: A linear sequence of genes wound up with proteins into a single unit that is found in the nucleus of cells. See also DNAgenes.

Cladogram: A tree-like diagram showing evolutionary relationships. Any two branch tips sharing the same immediate node are most closely related. All taxa that can be traced directly to one node (that is they are "upstream of a node") are said to be members of a monophyletic group.

Cnidaria: an animal phylum characterized by stinging cells called nematocysts, that contains the stony (hard) corals, anemones, sea fans, sea pens, hydras, and jellyfish.

Coastal reef: A coral reef occurring near and parallel to a coastline.

Comb jellies: Members of the phylum Ctenophora, a group of gelatinous forms feeding on smaller zooplankton.

Commensal: Having benefit for one member of a two-species association but neither positive nor negative effect on the other.

Community: A naturally occurring group of plants and animals that live within a certain environment and interact with each other. Communities are often defined by a dominant species (e.g. kelp forest community) or the major physical characteristics of the area (e.g. mud flats).

Compensation depth: The depth of the compensation light intensity.

Compensation light intensity: That light intensity at which oxygen evolved from a photosynthesizing organism equals that consumed in its respiration.

Competition: An interaction between or among two or more individuals or species in which exploitation of resources by one affects any others negatively. 

Complex life cycle: A life cycle that consists of several distinct stages (e.g., larva and adult).

Conformer: An organism whose physiological state (e.g., body temperature) is identical to, and varies identically with, that of the external environment.

Continental drift: Horizontal movement of continents located in plates moving via sea-floor spreading.

Continental shelf: A broad expanse of ocean bottom sloping gently and seaward from the shoreline to the shelf-slope break at a depth of 100 to 200 m.

Continental slope: See Slope.

Conservation Biology: A field of science that deals with threats to biodiversity. The goals of conservation biology are to investigate human impacts of biodiversity and to develop approaches to prevent extinction through stewardship of entire biological communities.

Convergence: The contact at the sea surface between two water masses converging, one plunging below the other.

Convergent Evolution: The development of similar structures in organisms that do not share recent common ancestor (e.g. eyes of squid and humans). Comparedivergent evolution; see also evolution.

Copepod: Order of crustaceans found often in the plankton. Copepods

Coprophagy: Feeding on fecal material. 

Coral: a group of marine animals belonging to the phylum cnidaria, that exist as small sea anemone-like polyps, typically in colonies of many identical individuals; or the skeletal remains of coral polyps. Coral

Coral bleaching: the loss of color in corals due to stress-induced expulsion of symbiotic zooxanthellae algae.

Coral polyp: a small individual coral animal with a tube-shaped body and a mouth surrounded by tentacles.

Coral reef: A wave-resistant structure resulting from cementation processes and the skeletal construction of hermatypic corals, calcareous algae, and other calcium carbonate-secreting organisms. Coral.

Corer: Tubular benthic sampling device that is plunged into the bottom in order to obtain a vertically oriented cylindrical sample. 

Coriolis effect: The deflection of air or water bodies, relative to the solid earth beneath, as a result of the earth's eastward rotation.

Countercurrent exchange mechanism: Mechanism by which two vessels are set side by side, with fluid flowing in opposite directions, allowing efficient uptake and retention of heat, oxygen, or gas, depending upon the type of exchanger.

Countershading. Condition of organisms in the water column that are dark-colored on top but light-colored on the bottom.

Counter-illumination: Having bioluminescent organs that are concentrated on the ventral surface so as to increase the effect of countershading (see also countershading).

Critical depth: That depth above which total integrated photosynthetic rate equals total integrated respiration of photosynthesizers.

Critical salinity: A salinity of approximately 5 to 8% that marks a minimum of species richness in an estuarine system.

 Cryptic Colouration: >To protect themselves against predators, many animals acquire colouring and markings to match and conceal them in their usual surroundings. For example the nudibranch (sea slug), Rostanga pulchra, is practically indiscernible from the red sponges on which it lays its eggs.

Ctenophora: See Comb jellies. 


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Daily estuary: An estuary in which tidal movements cause substantial changes in salinity at any one location on a daily basis. 

Decomposer: An organism which gains energy by breaking down the final remains of living things. Predominantly bacteria and fungi, decomposers are important in freeing the last of minerals and nutrients from organics and recycling them back into the food web. See also decomposition; compare detrivore.

Decomposition: The biochemical process where biological materials are broken down into smaller particles and eventually into basic chemical compounds and elements. See also decomposer.

Deep layer: The layer extending from the lowest part of the thermocline to the bottom. 

Deep-scattering layer: Well-defined horizon in the ocean that reflects sonar; indicates a layer usually consisting of fishes, squid, or other larger zooplankton.

Demographic: Referring to numerical characteristics of a population (e.g., population size, age structure). 

Density: (seawater) Grams of sea water per milliliter of fluid. 

Density-dependent factors: Factors, such as resource availability, that vary with population density. 

Deposit feeder: An organism that derives its nutrition by consuming some fraction of a soft sediment. 

Detritus Particulate material that enters into a marine or aquatic system. If derived from decaying organic matter it is organic detritus. See also detrivore.

Detrivore: An organism that feeds on large bits of dead and decaying organic matter. What detrivores leave behind is used by decomposers. Crabs and seabirds are examples of detrivores. Compare decomposer; see also detritus.

Diatoms: Microscopic algae with plate-like structures composed of silica.

Diffusion: The net movement of units of a substance from areas of higher concentration to areas of lower concentration of that substance. 

Digestion efficiency: The fraction of living food that does not survive passage through a predator's gut. 

Dinoflagellate: Dominant planktonic algal form, occurring as a single cell, often biflagellate. 

Directional selection: Preferential change in a population, favoring the increase in frequency of one allele over another. 

Dissolved organic matter: Dissolved molecules derived from degradation of dead organisms or excretion of molecules synthesized by organisms. 

Disturbance: A rapid change in an environment that greatly alters a previously persistent biological community.

Divergent Evolution: The evolution of one species to a number of different forms. Compare convergent evolution; see also evolution.

Diversity: A parameter describing, in combination, the species richness and evenness of a collection of species. Diversity is often used as a synonym for species richness. 

Diversity gradient: A regular change in diversity correlated with a geographic space or gradient of some environmental factor.

DNA: Deoxyribonucleic acid. The primary genetic material of a cell that makes up genes and chromosomes. See also geneschromosome.

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Ecology: The study of the physical and biological interactions between an organism and its natural environment.

Ecological Niche: The role a plant or animal plays in its community. The niche of an organism is defined by what it eats, its predators, salt tolerances, light requirements etc. Two species cannot live stably in the same habitat if they occupy identical niches.

Ecosystem: A community of plants, animals and other organisms that are linked by energy and nutrient flows and that interact with each other and with the physical environment. Rain forests, deserts, coral reefs, and grasslands are examples of ecosystems.

Ecosystem Diversity: The diversity of biological communities and their physical environment. Diversity is determined by the species composition, physical structure and processes within an ecosystem. This is the highest level of biodiversity. See alsobiodiversity; compare species diversity, genetic diversity.

Effluent: Industrial or urban waste discharged into the environment.

Endangered: A species or ecosystem that is so reduced or delicate that it is threatened with or on the verge of extinction. Compare extinct, extirpated, threatened, vulnerable.

Endemic: An animal or plant species that naturally occurs in only one area.

Environment: All of the physical, chemical, and biological factors in the area where a plant or animal lives.

Epilithic: A term for organisms that live attached to rocks.
Latin translation: epi = upon, lith = rock. This term is general to terrestrial and marine habitats, ie. some lichens are epilithic.

Epipelagic Zone: see Photic Zone.

Estuarine flow: Seaward flow of low-salinity surface water over a deeper and higher -alinity layer. 

Estuarine realms: Large coastal water regions that have geographic continuity,are bounded landward by a stretch of coastline with fresh-water input, and are bounded seaward by a salinity front. 

Estuary: A semienclosed body of water that has a free connection with the open sea and within which seawater is diluted measurably with freshwater that is derived from land drainage. 

Euphausiid: Member of an order of holoplanktonic crustacea. 

Eutrophic: Water bodies or habitats having high concentrations of nutrients. 

Eutrophication: Enrichment of a water body with nutrients, resulting in excessive growth of phytoplankton, seaweeds, or vascular plants, and often depletion of oxygen.

Evenness: The component of diversity accounting for the degree to which all species are equal in abundance, as opposed to strong dominance by one or a few species.

Evolution: The process by which a species' structural and behavioural characteristics change over many generations, sometimes in response to changes in environmental conditions. "New" species develop in this way. For example, scientists think that whales gradually evolved from land animals. See also convergent evolutiondivergent evolution.

Extinct: A species which no longer exists. The Stellar sea cow is an example of a species which once lived on the Pacific's East Coast and is now extinct. Compare extirpated.

Extirpated: A species no longer existing it's natural location, but occurring elsewhere on Earth. Compare extinct.

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Fecal coliform bacteria: Technically, all the facultative anaerobic gram negative non-spore forming rod shaped bacteria that fermet lactose in EC medium with gas production within 24h at 44.5 degrees C. A measure of bacteria mostly originating from guts that enter waters. Believed to be correlated with disease-causing (pathogenic) bacteria. 

Fecal pellets: See Pellets 

Fecundity: The number of eggs produced per female per unit time (often: per spawning season). 

Fertilization: The joining or fusion of the male gamete (sperm) and the female gamete (egg) to form a zygote during sexual reproduction. See also gamete, zygote.

Foliose coral: A coral whose skeletal form approximates that of a broad, flattened plate. 

Food Chain: A linear sequence of organisms that exist on successive trophic levels within a natural community, through which energy is transferred by feeding. Primary producers capture energy from the environment (through photo- or chemo-synthesis) and form the base of the food chain. Energy is then passed to primary consumers (herbivores) and on to secondary and tertiary consumers (carnivores and top carnivores) (e.g. phytoplankton -> zooplankton -> herring -> salmon -> killer whales). Once they die, these organisms are in turn consumed and their energy transferred to detrivores and decomposers. Compare food web

Food chain efficiency:Amount of energy of some other quantity extracted from a trophic level, divided by the amount of energy produced by the next-lower trophic level.

Food Web: A non-linear network of feeding between organisms that includes many food chains, and hence multiple organisms on each trophic level. For example, both sharks and tuna eat herring, and sharks also eat tuna. Visit and tour around the Pacific Northwest Food Web on the OceanLink website. 

Foraminifera: Protozoan group, individuals of which usually secrete a calcareous test; both planktonic and benthic representatives. 

Founder principle: A small colonizing population is genetically unrepresentative of the source of population. 

Freshet: An increase of water flow into an estuary during the late winter or spring, owing to increased precipitation and snow melt in the watershed. 

Front: A major discontinuity separating ocean currents and water masses in any combination. 

Fugitive species: A species adapted to colonize newly disturbed habitats.

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Gamete: A mature reproductive cell that is capable of fusing with another gamete of the opposite sex to form a zygote. Male gametes are typically known as sperm and female gametes a typically known as eggs. See also fertilizationzygote.

Gametophyte: Haploid stage in the life cycle of a plant. 

Generation time: The time period from birth to average age of reproduction. 

Generalist: A species that can live in many different habitats and can feed on a number of different organisms. For example, shore crabs on the Pacific coast live in a wide variety of habitats, such as mud, sand and rock, and feed on everything from the algae growing on rocks to invertebrates to detritus. Compare specialist.

Genes: The hereditary material coded in cells that determine how an organism will look and behave. A gene is a single unit located on a chromosome and is thereby passed from one generation to the next. Genes are what make each species and individual unique. For example, genes are responsible for hair colour and texture in humans. See also chromosomeDNA.

Genetic Diversity: The genetic variation that occurs within a population or species. For example, there are several different colour dog whelk shells and ochre sea stars. See also biodiversity; compare ecosystem diversity, species diversity.

Genetic drift: Changes in allele frequencies that can be ascribed to random effects. 

Genetic locus: A location on a chromosome (possibly of a diploid organism with variants that segregate according to the rules of Mendelian heredity). 

Genetic polymorphism: Presence of several genetically controlled variants in a population. 

Genotype: The genetic makeup of an organism. The actual appearance of an individual (the phenotype) depends on the interaction between different forms (or alleles) of genes and between the genotype and the environment. Compare phenotype.

Genus: (plural: genera) The level of the taxonomic hierarchy above the species but below the family level. 

Geostrophic flow: Movement of water in the oceans as a combined response to the Coriolis effect and gravitational forces created by an uneven sea surface. 

Geotactic: Moving in response to the earth's gravitational field. 

Gillnet: A net set upright in the water to catch fish by entangling their gills in its mesh.

GIS. Geographic Information System: A system that allows automatic location of information suitable for mapping. Usually involves a software system that takes geographic position data and other data (e.g., type of bottom sediment) in order to create a map. Data on processes (e.g., current speed) can be incorporated to make a geographic model of flow. 

Global warming: Predicted increase in the earth's oceanic and atmospheric temperature, owing to additions of carbon dioxide to the atmosphere, as a result of human activities. 

GPS. Global Positioning System: An electronic device that uses positioning signals from satellites in order to locate precisely latitutude and longitude. Now used nearly exclusively for locating ship sampling stations at sea, but also useful for locations near and on shore. 

Grab: Benthic sampling device with two or more curved metal plates designed to converge when the sampler hits bottom and grab a specified volume of bottom sediment. 

Grazer: A predator that consumes organisms far smaller than itself (e.g., copepods graze on diatoms). 

Greenhouse effect: Carbon dioxide traps solar-derived heat in the atmosphere near the earth.

Gregarious settling: Settlement of larvae that have been attracted to members of their own species. 

Gross primary productivity: The total primary production, not counting the loss in respiration. 

Guild: A group of species, possibly unrelated taxonomically, that exploit overlapping resources. 

Gyre: Major cyclonic surface current systems in the oceans.G

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Habitat:The immediate space where an animal or plant lives and has food, water and protection. Habitat loss, which includes the destruction, degradation, or fragmentation of habitats, is the primary cause of decreasing biodiversity.

Halocline: Depth zone within which salinity changes maximally.

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

Harmful algal bloom: A bloom of (usually) planktonic microalgae belonging to a strain of a species that has a toxic harmful to marine organisms or humans consuming marine organisms. 

Herbivore: A plant-eating animal. Sea urchins re a good example of a marine herbivore as they feed on kelp. See also heterotrophprimary consumer.

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

Hermaphrodite: An animal that has both male and female reproductive organs. Nudibranchs (sea slugs) are a good example o a hermaphrodite. 

Hermatypic: Reef-building. 

Heterotroph: An organism that is unable to synthesize organic compounds (and thus its energy) from the environment and therefore fulfils its energy requirements by feeding on other organisms or organic matter. Compare autotroph.

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

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

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

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

Holoplankton: Organisms spending all their life in the water column and not on or in the sea bed. 

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

Homogeneous: Similar or uniform structure or composition throughout.

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

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

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

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

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Infaunal: Living within a soft sediment and being large enough to displace sedimentary grains. 

Inorganic: Part of or derived from non-biological material. A chemical compound that does not include a carbon chain. Compare organic.

Interspecific competition: Condition in which one species' exploitation of a limiting resource negatively affects another species.

Interstitial: Living in the pore spaces among sedimentary grains in a soft sediment.

Introduced Species:> An organism that has been brought into an area, usually by humans, where it does not normally occur. Introduced species often compete with and cause problems for native species. Introduced species are also called exotic, nonnative, and alien species. See also invasive speciesnative species.

Invasive Species: An introduced species that out-competes native species for space and resources. Scotch Broom is an invasive species that out-competes local vegetation and results in a monoculture, and hence a decrease in local diversity. See alsointroduced speciesnative species.

Isotonic: Having the same overall concentration of dissolved substances as a given reference solution.

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Keystone Predator: The dominant predator or the top predator that has a major influence on community structure. For example, sea otters are a keystone predator in kelp beds. Sea otters eat urchins that feed on kelp which house a huge diversity of other organisms. If sea otter populations are lowered in an area the kelp beds are generally reduced and urchin barrens appear.

Keystone Species: A species that has a major influence on community structure.

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Laminar flow: The movement of a fluid where movement of the entire fluid is regular and with parallel streamlines. 

Larva: A discrete stage in many species, beginning with zygote formation and ending with metamorphosis. 

Larvacea: A group of planktonic tunicates that secrete a gelatinous house, used to strain unsuitable particles (large particles are rejected). An inner filter apparatus of the house, the so-called food trap or particle-collecting apparatus, is used to retain food particles. 

LD50: The value of a given experimental variable required to cause 50% mortality. 

Leaching: The loss of soluble material from decaying organisms. 

Lecithotrophic larva: A planktonic-dispersing larva that lives off yolk supplied via the egg. 

Leeward: The side of an island opposite from the one facing a persistent wind. 

Life table: A table summarizing statistics of a population, such as survival and reproduction, all broken down according to age classes. 

Litter: Accumulations of dead leaves in various states of fragmentation and decomposition. 

Locus: See Genetic locus. 

Logistic population growth: Population growth that is modulated by the population size relative to carrying capacity. Population growth declines as population approaches carrying capacity, and is negative when population size is greater than carrying capacity. 

Longshore current: A current moving parallel to a shoreline.



Macrobenthos: (macrofauna or macroflora) Benthic organisms (animals or plants) whose shortest dimension is greater than or equal to 0.5 mm. 

Macrofauna: Animals whose shortest dimension is greater than or equal to 0.5 mm. 

Macrophyte: An individual alga large enough to be seen easily with the unaided eye. 

Macroplankton: Planktonic organisms that are 200-2,000 micrometers in size. 

Mainstream flow: The flow in a part of the fluid (e.g., in a tidal creek) that is well above the bottom or well away from a surface and essentially not under the influence of the boundary layer (see boundary layer). 

Mangel: See Mangrove forest. 

Mangrove forest: A shoreline ecosystem dominated by mangrove trees, with associated mud flats. 

Marine snow: Fragile organic aggregates, resulting from the collision of dissolved organic molecules or from the degradation of gelatinous substances such as larvacean houses. Usually enriched with microorganisms. 

Marine protected area: A conservation geographic unit designed to protect crucial communities and to provide reproductive reserves for fisheries that hopefully will disperse over wider areas. 

Maximum sustainable yield: In fisheries biology, the maximum catch obtainable per unit time under the appropriate fishing rate. 

Meiobenthos (meiofauna or meioflora): Benthic organisms (animals or plants) whose shortest dimension is less than 0.5 mm but greater than or equal to 0.1 mm. 

Meiofauna: Animals whose shortest dimension is less than 0.5 mm but greater than or equal to 0.1 mm. 

Megaplankton: Planktonic organisms that are greater than or equal to 2000 micrometers in size. 

Meroplankton: Organisms that spend part of their time in the plankton but also spend time in the benthos (e.g., planktonic larvae of benthic invertebrates). 

Mesopelagic Zone: Also called the "twilight zone" of the ocean, this area from 200m to 1000m in depth, is bordered by the photic zone above and darkness below. It's in this zone where you start to see bioluminescence on all sorts of animals. Visit theMesopelagic Zone on OceanLink to learn about the animals unique to these depths.

Metabolic rate: The overall rate of biochemical reactions in an organism. Often estimated by rate of oxygen consumption in aerobes. 

Metamorphosis: Major developmental change as the larva develops into an immature adult. 

Metapopulation: A group of interconnected subpopulations, usually of subequal size. The features of individuals now founnd in one subpopulation might have been determined by conditions affecting them when they were located in another subpopulation. 

Microbenthos: (microfauna or microflora) Benthic organisms (animals or plants) whose shortest dimension is less than 0.1 mm. 

Microfauna: Animals whose shortest dimension is less than 0.1 mm. 

Mixing depth: The water depth to which wind energy evenly mixes the water column. 

Mixoplankton: Planktonic organisms that can be classified at several trophic levels. For example, some ciliates can be photosynthetic but also can ingest other plankton and are heterotrophic.

Moderately stratified estuary: An estuary in which seaward flow of surface low-salinity water and moderate vertical mixing result in a modest vertical salinity gradient. 

Molecular diffusion: the process by which carbon dioxide moves freely between air and sea. The exchange occurs in a film of water at the surface. Carbon dioxide travels wherever concentrations are lowest. If levels in the atmosphere are high, the gas goes into the ocean. If they are higher in the sea, as they have been for much of the past, the gas leaves the water and enters the air. 

Monophyletic: Refers to a group of species that all have a single common ancestral species. 

Morphology: The form and structure of an organism, in particular its outside features.

Mucous-bag suspension feeder: Suspension feeder employing a sheet or bag of mucus to trap particles nonselectively.

Mutualism: An interaction between two species in which both derive some benefit.

Mutualistic: Conferring reciprocal benefit to individuals of two different associated species.


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Nanoplankton: Planktonic organisms that are 2-20 micometers in size. 

Native Species: A species that occurs naturally in an area (i.e. is not introduced). Compare introduced speciesinvasive species.

Natural Selection: The main mechanism of evolutionary change. In a given population of organisms, there are heritable traits that enable some members to contribute a larger number of offspring than others. If these offspring also have a greater reproductive success, then the genetic composition of the population is altered, thus evolution. See also selection pressure.

Neap tides: Tides occurring when the vertical range is minimal. 

Nekton: Pelagic organisms that are free-swimming and so whose movements are independent of the tides, currents and waves. Such animals include fish, whales, squid, crabs and shrimps. The distribution of nekton is limited by temperature and nurtient supply and decreases with decreasing depth. Compare benthicplankton.

Neritic: Seawater environments landward of the shelf-slope break. 

Net primary productivity; Total primary production, minus the amount consumed in respiration. 

Neuston: Planktonic organisms associated with the air-water interface. 

Niche: A general term referring to the range of environmental space occupied by a species. 

Niche overlap: An overlap in resource requirements by two species. 

Nitrogen fixation: The conversion of gaseous nitrogen to nitrate by specialized bacteria. 

Nonrenewable Resource: A resource that is in limited supply and can't be replenished by natural processes, at least not for thousands of years or more. Fossil fuels are nonrenewable resources. Compare renewable resource.

No-take Reserves: Geographic areas where by law no one is allowed to fish or collect biological specimens. Rules could apply to one or all species. 

Nuisance bloom: A rapid increase of one or only a few species of phytoplankton, resulting in densities high enough to cause discoloration of the surface water, possible increase of toxins, and degradation of water quality aspects such as dissolved oxygen. 

Nutrient cycling: The pattern of transfer of nutrients between the components of a food web. 

Nutrients: Those constituents required by organisms for maintenance and growth (we use this term in this book in application to plants).

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Organic: Part of or derived from living organisms. Or a chemical compound that contains carbon as an essential component. Compare inorganic.

Over-consumption: The use of resources at a rate that exceeds the ability of natural processes to replace them.

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Pelagic: Refers to the plants and animals that live in the water column or in the open waters of the ocean rather than the ocean floor (see benthic). Life is found throughout the pelagic zone, however is more concentrated at shallower depths. Pelagic organisms can be further divided into the plankton and nekton. Compare benthic.

Pesticides: Chemical products used to reduce or eliminate unwanted organisms, regarded as "pests". Pesticides are often used on gardens, agricultural land, roadsides, and golf courses to eliminate species considered undesirable or damaging.

Phenotype: The physical appearance of an individual that is the result of that individual's genotype and the interaction of the genotype with the environment during development. Hence, individuals with the same genotype may have different phenotypes in different environments. Compare genotype.

Phosphoresence: see Bioluminescence.

Photic Zone: The surface layer of the ocean that is penetrated by sunlight. The photic zone is the layer of the ocean that has been explored the most as it is relatively easy to access with conventional diving equipment. In the photic zone phytoplankton flourish and it is where the fish, marine mammals, and marine invertebrates that most people are familiar with are found. Light can penetrate down to approximately 200m which marks the end of the photic zone. Also referred to as the Sunlight Zone or the Epipelagic Zone.

Photosynthesis: A chemical process whereby plants and algae use a sun's energy to make sugars (organic compounds) from carbon dioxide and water (inorganic compounds). See also autotroph, chemosynthesis.

Phylum: The broadest classification for animals. Compare species.

Phytoplankton: The plant and algae component of the plankton; the primary producers of almost all marine food webs. Compare zooplankton.

Plankton: Pelagic organisms that float through the water column, not attached to any substrate and unable to move against the currents and tides. Plankton can be further divided into phytoplankton and zooplankton, meroplankton and holoplankton. Compare nekton.

Population: The number of individuals of a particular species that live within a defined area.

Predator: An animal that hunts and kills other animals for food.

Prey: An animal that is used by other animals for food.

Primary Consumer: A heterotrophic, herbivorous organism that feeds directly on a primary producer. Urchins are a primary consumer as they feed on kelp. See alsoheterotrophfood chain; compare secondary consumer.

Primary Producer: An autotrophic organism that makes complex organic compounds from inorganic compounds through the process of photosynthesis or chemosynthesis. For example, phytoplankton synthesize sugars (organic compounds) from carbon dioxide and water(inorganic compounds) using energy from the sun. Phytoplankton is one example of a marine primary producer. See also autotrophfood chain.

Primary Production: Synthesis of organic matter by plants, which is the main source of energy and nutrition for other consumers in the ecosystem (e.g. herbivores). See also chemosynthesisphotosynthesis.

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Quadrat: An ecological sampling unit that consists of a square frame of known area. The quadrat is used for quantifying the number or percent cover of a given species within a given area. See also transect.

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Renewable Resource: A resource that can be replenished through natural processes within a human life span, if it is soundly managed. Compare nonrenewable resource.

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Salinity: the concentration of salts dissolved in water.
Secondary Consumer: A heterotrophic, carnivorous organism that feeds on a primary consumer. Herring feeding on zooplankton are an example of a secondary consumer. See also food chainheterotrophprimary consumer.
Selection Pressure: A measure of the effectiveness of natural selection in altering the genetic composition of a population. See also natural selection.

Specialist: A species with a very narrow range in habitat or food requirements. For example, the Marbled Murrelet nests in old-growth forests on thick branches high up in the forest canopy. Compare generalist.

Speciation: The evolution of one or more species from an existing species.

Species: A group of organisms that differ from all other groups of organisms and that are capable of breeding and producing fertile offspring. This is the smallest unit of classification for plants and animals. Compare phylum.

Species Diversity: A measure of both species abundance and species richness. An area that has a large number of species and many representative individuals from each species is more diverse than an area that has only a single species. See alsobiodiversity; compare ecosystem diversitygenetic diversity.

Species Abundance: The total number of individual of a species within a given area or community. Compare species richness.

Species Richness: The number of different species that exist within a given area or community. Compare species abundance.

Substrate: The material upon or within which a plant or animal live or grows (e.g. rocky or sandy substrate). See also benthic.

Sunlight Zone: see Photic Zone.

Sustainable: A sustainable way of life is one in which human needs are met without diminishing the ability of other people, wild species, or future generations to survive.

Symbiosis:< An interaction between two different species where either both, one or neither of the organisms benefit from the relationship. Many symbiotic relationships are obligatory (e.g. tropical reef building corals and their algal symbiont).

Systematics: The area of biology that deals with the diversity of living organisms, their relationships to each other through evolution, and their classification. Can also be referred to as taxonomy.

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Taxonomy: The theory and practice of describing, naming and classifying plants and animals. See also systematics.

Threatened: A species likely to become endangered if limiting factors are not reversed. Compare endangered, extinct, extirpated, vulnerable.

Transect: A straight line placed on the ground along which ecological measurements are taken. If an ecologist wanted to sample the diversity of intertidal organisms in the intertidal, he/she would place a number of transects perpendicular to the shore and take samples at predetermined interval lengths. See also quadrat.

Trophic levels: The energy levels or steps in a food chain or food web: primary producer, primary consumer, secondary consumer, tertiary consumer etc.

Twilight Zone: see Mesopelagic Zone.

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Vulnerable: A species of special concern because of characteristics that make it particularly sensitive to human activities or natural events. Compare endangered,extinctextirpatedthreatened.

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Water Cycle: The continuous circulation of water from the atmosphere to the earth, including the ocean, and back to the atmosphere through condensation, precipitation, evaporation, and transpiration.

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Zooplankton: Animal component of the plankton that feed on phytoplankton and other zooplankton (primary consumers). Compare phytoplankton.

Zooxanthellae: any of various yellow-green algae that live symbiotically within the cells of other organisms, such as reef-building coral polyps.

Zygote: A fertilized egg. Or the diploid cell that results from the joining of two haploid gametes (sperm and egg) during sexual reproduction, that will cleave to form an embryo. See also fertilizationgamete.

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