Nannoplankton: Plankton smaller than 10 microns, which pass through an ordinary plankton net but can be removed from the water by centrifuging samples
Natal: Pertaining to birth or hatching.
Native Species: A species that occurs naturally in an area (i.e. is not introduced). Compare introduced species, invasive species.
Natural killer (NK) cell: Bone marrow-derived, mononuclear white blood cells (large granular lymphocytes) that are able to kill invading microorganisms without activation by cells of the immune system. They are, therefore, part of the innate immune system. They are specialized in killing virus-infected cells and cells transformed to develop cancer.
Natural Mortality (M): Deaths in a fish stock caused by predation, pollution, senility, etc., but not fishing.
Natural selection (Darwin's definition, 1859): "As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequent recurrent struggle for existence, it follows that any being, if it varies however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected." Link to a simulation on natural selection. Natural selection tends to reduce heritability because strong (directional or stabilizing) selection leads to reduced variation.
Nature-nurture debate: The debate on the relative contributions of genetics (nature) and environment (nurture) to the characteristics of an organism. An example is the debate on whether gene(s) and/or environmental factors determine the sexual orientation of an individual. Finding a gene playing a role in the development of a condition does not necessarily mean it is a purely genetic trait.
Negative assortative mating: A type of nonrandom mating in which individuals of unlike phenotype mate more often than predicted under random mating conditions.
nekton pelagic animals that are active swimmers; for example, adult squid, fish, and marine mammals
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 benthic, plankton.
Nematocyst: Older name for a cnidocyst.
Nemertea: A phylum of unsegmented, elongate marine worms having a protrusible proboscis and no body cavity, and live mostly in coastal mud or sand; nemertean.
Neoteny: Retention of juvenile features in sexually mature adult animals. Neoteny frequently correlates with recent evolution of the species (like Homo sapiens).
Neritic: The region of shallow water adjoining the seacoast; e.g. bays, lagoons, mangroves, salt marshes, etc.
Nerve: A bundle of neurons, or nerve cells. More properly, it is a bundle of axons.
Nerve cord: Primary bundle of nerves in chordates, which connects the brain to the major muscles and organs of the body.
Net Increase (or Decrease): New body substance elaborated in a stock, less the loss from all forms of mortality.
Neuron: A specialized cell that can react to stimuli and transmit impulses. A neuron consists of a body which contains the nucleus; dendrites, which are short branches off the body that receive incoming impulses; and a long axon which carries impulses away from the body and to the next neuron.
Neurospora crassa: Haploid, heterothallic, filamentous Ascomycete fungus (bread mold). It has two mating types (A and a) operating as sexual compatibility system, and 11 het loci operating as heterokaryon compatibility system in vegetative phase. Link to Neurospora website.
Neurotoxic Shellfish Poisoning (NSP) - A human neurological disease caused by the ingestion of toxic marine shellfish (filter-feeding bivalves). Symptoms are similar to those of ciguatera poisoning and include temperature reversal sensations, as well as headache, chills, and muscle and joint pain (Hallegraeff 1995, Steidinger 1993). Cases have been reported from the southeast US and eastern Mexico (Steidinger 1993).
Neuston: Organisms that live on or just under the water surface, often dependent on surface tension of support.
Neutral theory: Link to a lecture on neutral theory of evolutionary change.
niche the full range of biological and physical conditions under which an organism can live and reproduce. The realized niche is largely determined by interactions with other species
Niche: The fundamental niche is the full range of abiotic and biotic factors under which a species can live and reproduce. The realized niche is the set of actual conditions under which a species or a population of a species exists, and is largely determined by interactions with other species.
Nocturnal: Refers to night, or animals that are active during night.
Nominal Catch: The sum of the catches that are landed (expressed as live weight or equivalents). Nominal catches do not include unreported discards.
Non-coding region: Parts of a gene that include sequences which are not translated. Both 5' and 3' untranslated regions (UTRs), upstream promoter region and introns are classified as non-coding regions.
Non-disjunction: Due to failure in pairing of homologous chromosomes in meiosis, the two members of one pair migrate to the same pole, giving rise to unbalanced gametes, one of which contain both homologous chromosomes, and the other none (most frequent in sex chromosomes). The non-disjunction event is much more frequent in maternal meiosis I. This may be due to the fact that in a mature woman, oocytes have been held in the ovary for a very long time at prophase I of meiosis from before her birth to shortly before ovulation of the oocyte in question.
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. .
Nonsense mutation: A mutation that changes an amino acid specifying codon to one of the three termination (stop) codons.
Notochord: A rod that forms in the embryonic mesoderm and which establishes the front-to-back orientation of vertebrate embryos. It also initiates the formation of the nervous system, the skeleton and most muscles.
Nuclease: An enzyme that breaks bonds in nucleic acids. Deoxyribonuclease (DNAase) and ribonuclease (RNAase).
Nucleoid: The loosely tangled clump of DNA within the cytoplasm of a prokaryotic cell.
Nucleolar organizer: A region on a chromosome that is associated with formation of a new nucleolus following cell division. It contains the genes for several species of ribosomal RNA (rRNA), i.e., 18S, 5.8S, 5S and 28S in eukaryotes.
Nucleolus: The site of synthesis of rRNA within the nucleus of a eukaryotic cell.
Nucleoside: A small molecule composed of a purine or pyrimidine base linked to a five-carbon sugar (pentose: ribose or deoxyribose). With the addition of a phosphate group, it becomes a nucleotide. Nucleosides in RNA are adenosine, guanosine, cytidine and uridine; in DNA, they are (d)adenosine, (d)guanosine, (d)cytidine and (d)thymidine.
Nucleosome: A beadlike structure of eukaryotic chromosomes. It consists of a core of eight histone molecules and a DNA segment of about 150 base pairs. Each nucleosome is separated from another by a linker DNA sequence of about 50 base pairs. Nucleosome structure helps to fold DNA into a compact form in the interphase nucleus. Otherwise the length of a chromosome, when linear, is many orders of magnitude greater than the diameter of the nucleus.
Nucleotide: The monomeric unit that makes up the DNA or RNA, formed by a phosphate group, a pentose and one of the nitrogenous bases (A, T/U, C, G). Nucleotides in RNA are adenylate, guanylate, cytidylate and uridylate; in DNA, they are (d)adenylate, (d)guanylate, (d)cytidylate and thymidylate.
Nucleus: A membrane-bound organelle in eucaryotic cells which contains a large percentage of the genetic material in the cell. In dinoflagellates, it is most often referred to as a mesokaryon or a dinokaryon due to its unique feature: chromosomes are permanently condensed.
nudibranchs soft-bodies, gastropod mollusks; sea slugs
Nutrients: In the ocean, any one of a number of inorganic or organic compounds or ions used primarily in the nutrition of primary producers; nitrogen and phosphorus compounds are examples