O

obligate symbiont Organism that is physiologically dependent on establishing a symbiotic relationship with another.

observability Not all of the members of a group are always seen the same proportion of time. Animals of different ages, sexes, or of different dominance status may be seen more or less than other animals.

observational learning The tendency to perform an appropriate action or response as the result of having observed another animal's performance of similar actions in the same situation.

observational method In an historical context, the notion, developed by C. Lloyd Morgan, that only data gathered by direct experiment and watching the animals under study could provide the basis for generalizations. This was in contrast to Romanes's notion that much about animal behavior could be gained by inference from what was seen. In the modern sense, this refers to any of a number of techniques used to watch and record the actions and interactions of animals.

observations Objectively measuring or recording the results of testing a theory; watching and evaluating.

obtect pupa Pupa with wings and legs tightly appressed to its body and covered by an external cuticle.

Obturator L. A disk covering an opening.

ocean desert The concept of a region in the ocean in which lack of nutrients is reflected in a lack of plants and animals; a region of low biological productivity.

ocean shorelines Rocky coasts and sandy beaches along the oceans; support rich, stratified communities.

ocean thermal electric conversion (OTEC) Energy derived from temperature differentials between warm ocean surface waters and cold deep waters. This differential can be used to drive turbines attached to electric generators.

ocean thermal energy conversion (OTEC) A process for obtaining energy by exploiting depth differences in temperature.

oceanic ecosystems The marine ecosystem that extends from the continental shelves into the unproductive open ocean.

oceanic islands Islands in the ocean; formed by breaking away from a continental landmass, volcanic action, coral formation, or a combination of sources; support distinctive communities.

oceanic zone The pelagic environment beyond the shelf break.

oceanic Pertaining to the portion of the marine environment that overlies the deep ocean basins.

ocellus A simple eye or eyespot in many types of invertebrates.

octomerous Eight parts, specifically, symmetry based on eight.

odontophore Tooth-bearing organ in molluscs, including the radula, radular sac, muscles, and cartilages.

off-center, on-surround cell A ganglion cell that generates an action potential when light shines on the periphery of its receptive field but is inhibited when light shines on the center of its receptive field.

off-surround The photoreceptor cells in the outer portion of a receptive field that have an inhibitory effect on ganglion cells if stimulated by light.

offset allowances A controversial component of air quality regulations that allows a polluter to avoid installation of control equipment on one source with an "offsetting" pollution reduction at another source.

oil A triglyceride that is liquid at room temperature.

Olecranon olekranon = elbow.

olfaction The act of smelling; the sense of smell.

olfactory sacs Structures on both sides of the head of fishes that are sensitive to chemical stimuli.

olfactory Pertaining to the sense of smell.

Oligochaeta The class of annelids whose members are characterized by having few setae and no parapodia. Monoecious with direct development. The earthworm (Lumbricus) and Tubifex.

oligopod larva Usual larva in Coleoptera and Neuroptera, with a well-developed head and thoracic legs.

oligotrophic Condition of rivers and lakes that have clear water and low biological productivity (oligo 5 little; trophic 5 nutrition); are usually clear, cold, infertile headwater lakes and streams.

omasum The third compartment of the stomach of a ruminant mammal.

Omentum omentum = fat skin. A folded peritoneal membrane from the stomach to the dorsal body wall.

ommatidia The functional units of the invertebrate compound eye, each one of which has a lens, focusing cone, photoreceptive cells, and forms an image.

ommatidium One of the optical units of the compound eye of arthropods.

omnivore An animal that uses a variety of animal and plant material in its diet.

on-center, off-surround cell A ganglion cell that generates an action potential when light shines on the center of its receptive field and is inhibited when light shines on the periphery of its receptive field.

on-center The inner circle of cells of a receptive field that stimulate ganglion cells if they are stimulated by light.

onchocercoma Subcutaneous nodule containing masses of the nematode Onchocerca volvulus.

onchosphere The larva of the tapeworm contained within the external embryonic envelope and armed with six hooks and cilia. Typically referred to as a coracidium when released into the water.

oncogene Any of a number of genes that are associated with neoplastic growth (cancer). The gene in its benign state, either inactivated or carrying on its normal role, is a proto-oncogene.

oncomiracidium A ciliated larva of a monogenetic trematode.

oncosphere Rounded larva common to all cestodes, bears hooks.

one-gene--one-polypeptide theory The concept that one gene in DNA codes for a sequence of amino acids in a specific polypeptide.

ontogenetic niche The multitude of inherited ecological and social traits that are, in effect, passed on from generation to generation and that play integral roles, in concert with genetic inheritance, influencing behavior development.

ontogeny The course of development of an individual from egg to senescence.

Onychophora A phylum of terrestrial animals with 14 to 43 pairs of unjointed legs, oral papillae, and two large antennae. Onycophorans live in humid tropical areas of the world. Their ancestors may have been an evolutionary transition between annelids and arthropods. Velvet worms or walking worms.

oocyst Cystic form in the Apicomplexa, resulting from sporogony; the oocyst may be covered by a hard, resistant membrane (as in Eimeria), or it may not (as in Plasmodium).

oocyst residuum Cytoplasmic material not incorporated into the sporocyst within an oocyst; seen as an amorphous mass within an oocyst.

oocyte Stage in formation of ovum, just preceding first meiotic division (primary oocyte) or just following first meiotic division (secondary oocyte).

ooecium Brood pouch; compartment for developing embryos in ectoprocts.

oogamy The union of a large nonmotile egg with a small motile or nonmotile male sperm.

oogenesis The process by which an egg cell forms from an oocyte.

oogenotop Female genital complex of a flatworm, including oviduct, ootype, Mehlis' glands, common vitelline duct, and upper uterus.

oogonium In algae and fungi, a specialized egg-containing cell; a single-celled female gametangium, containing one or more eggs.

ookinete The motile zygote of malarial parasites.

Oomycetes An assemblage of organisms, previously grouped with the fungi, but currently assigned to the kingdom Protista based in part on the presence of cellulose cell walls, rather than chitinous walls as found in the fungi.

oostegites Modified thoracic epipods in females of the crustacean superorder Peracarida. They form a pouch for brooding embryos.

ootheca Egg packet secreted by some insects; may be covered with sclerotin.

ootid Stage of formation of ovum after second meiotic division following expulsion of second polar body.

ootype Part of oviduct in flatworms that receives ducts from vitelline glands and Mehlis' gland.

Opalinata The protozoan subphylum where members are cylindrical; covered with cilia. Examples: Opalina, Zelleriella.

open access The concept of international law that permits free access by any nation to marine resources existing outside national jurisdictions.

open access system A commonly-held resource for which there are no management rules.

open canopy A forest where tree crowns cover less than 20 percent of the ground; also called woodland.

open circulatory system A circulatory system in which blood is not confined to vessels in a part of its circuit within an animal; blood bathes tissues in blood sinuses.

open range Unfenced, natural grazing lands; includes woodland as well as grassland.

open system A system that exchanges energy and matter with its environment.

operant (instrumental) conditioning A type of learning that involves the animal's forming a variety of associations during which it learns to associate its behavior with the consequences of that behavior. That is, the sequence of events is dependent upon the behavior of the animal.

operational sex ratio The number of sexually mature males and females in a population.

operator A gene responsible for the activation and deactivation of the structural genes.

operculum A cover. 1. The cover of a gill chamber of a bony fish (Chordata). 2. The cover of the genital pores of a horseshoe crab (Meristomata, Arthropoda). 3. The cover of the aperature of a snail shell (Gastropoda, Mollusca).

operon A genetic unit consisting of a cluster of genes under the control of other genes, found in prokaryotes.

Ophiuroidea The class of echinoderms whose members have arms sharply set off from the central disk. Tube feet without suction disks. Brittle stars.

ophthalmic Pertaining to the eye.

opisthaptor Posterior attachment organ of a monogenetic trematode.

opisthomastigote A form of Trypanosomatidae with the kinetoplast at the posterior end. The flagellum runs through a long reservoir to emerge at the anterior. There is no undulating membrane. An example is Herpetomonas.

opisthosoma The portion of the body of a chelicerate arthropod that contains digestive, reproductive, excretory, and respiratory organs.

opposite arrangement Two leaves borne per node and arranged across the stem from each other.

opposite Leaf arrangement in which they are attached at a node directly across from one another on the stem.

opsonization Modification of the surface characteristics of an invading particle or organism by binding with antibody or a nonspecific molecule in such a manner as to facilitate phagocytosis by host cells.

Optic The eye.

optimality model A model that attempts to predict the particular combination of costs and benefits of a behavior that will maximize an individual's inclusive fitness.

oral Having to do with the mouth. The end of an animal containing the mouth.

oral cavity The cavity within the mouth.

oral sucker The sucker on the anterior end of a tapeworm, fluke, or leech.

orbital Discrete pathways or bands surrounding an atom through which electrons move.

Orbito- Circle, track.

orchitis Inflammation of the testis.

order Taxonomic rank consisting of a group of related families.

Ordovician A geological period beginning around 500 million years ago, believed to be the time when plants may have begun to migrate onto land.

organ A structure composed of different tissues, such as root, stem, leaf, or reproductive (e.g., flower) parts.

organ of Ruffini Sensory receptor in the skin believed to be a sensor for touch-pressure, position sense of a body part, and movement. Also known as corpuscle of Ruffini.

organ system A set of interconnected or interdependent parts that function together in a common purpose or produce results that cannot be achieved by one of them acting alone.

organelle A body within the cytoplasm of eukaryotic cells; several different types of organelles occur, each with a specialized function such as the chloroplast, which functions in photosynthesis.

organic Living or once living material; compounds containing carbon formed by living organisms.

organic compound A molecule that contains carbon, hydrogen, and usually oxygen.

organic evolution The change in an organism over time; a change in the sum of all genes in a population.

organic molecule A molecule that contains one or more carbon atoms.

organization, level of The extent of specialization and organization of the cells of an organism. Organization may be at the cellular, tissue, or organ level.

organizational effects Refers to the effects of exposure to certain hormones during critical periods in development that affect the central nervous system and other structures, producing morphological and behavioral changes in adulthood. Contrast with activational effects.

organizational effects of hormones Changes resulting from the presence of hormones at critical time periods such that specific developmental pathways for specific brain regions and developing gonadal tissues are influenced to become either female or malelike.

organizer Area of an embryo that directs subsequent development of other parts.

oriental sore Disease caused by Leishmania tropica. Also called Jericho boil, Delhi boil, Aleppo boil, or cutaneous leishmaniasis.

orientation The way an organism positions itself in relation to external cues.

Oroya fever Clinical form of Carrion's disease, caused by the bacterium Bartonella bacilliformis and transmitted by sand flies.

orthogenesis A unidirectional trend in the evolutionary history of a lineage as revealed by the fossil record; also, a now discredited, anti-Darwinian evolutionary theory, popular around 1900, postulating that genetic momentum forced lineages to evolve in a predestined linear direction that was independent of external factors and often led to decline and extinction.

oscillator The internal mechanism that is the clock in a biological rhythm.

osculum Excurrent opening in a sponge.

osmoconformer An organism whose body fluids have the same or very similar osmotic pressure as that of its aquatic environment; a marine organism that does not utilize energy in osmoregulation.

osmole. Molecular weight of a solute, in grams, divided by the number of ions or particles into which it dissociates in solution. Adj., osmolar.

osmoregulation The maintenance of osmolarity by an organism or body cell with respect to the surrounding medium.

osmoregulator Any substance, either organic or inorganic, that functions to change the solute potential of a solution, and thereby controls the water relations of that solution by osmosis.

osmosis The movement of water through a selectively permeable membrane due to the existence of a concentration gradient.

osmotic conformersO rganisms that tolerate large variations of internal ionic concentrations without serious damage by remaining isosmotic with the water around them.

osmotic potential. Osmotic pressure.

osmotic pressure In hypoosmotic conditions, the internal fluid pressure that develops from the osmotic inflow of water.

osmotic shock Alternating seawater and dilute freshwater in intertidal zones causes erratic osmotic balance in plants growing in these areas.

osmotroph A heterotrophic organism that absorbs dissolved nutrients.

osphradium A sense organ in aquatic snails and bivalves that tests incoming water.

ossicles Small separate pieces of echinoderm endoskeleton. Also, tiny bones of the middle ear of vertebrates.

Osteichthyes The class of fishes whose members are characterized by the presence of a bony skeleton, a swim bladder, and an operculum. Bony fishes.

osteoblast A bone-forming cell.

osteoclast A large, multinucleate cell that functions in bone dissolution.

osteocyte A bone cell that is characteristic of adult bone, has developed from an osteoblast, and is isolated in a lacuna of the bone substance.

osteon Unit of bone structure; Haversian system.

osteostracans A group of Paleozoic (Upper Silurian to Upper Devonian) agnathans belonging to the order Cephalaspidiformes.

ostium Opening.

otoacariasis Infestation of the external ear canal by ticks or mites.

otolith Calcareous concretions in the membranous labyrinth of the inner ear of lower vertebrates, or in the auditory organ of certain invertebrates.

outbreeding depression A reduction in the fitness of offspring resulting from matings among individuals from different populations or demes, possibly caused by the breaking up of coadapted gene complexes.

outcrosser A flower that must be cross-pollinated to successfully complete the reproductive process.

outgroup. In phylogenetic systematic studies, a species or group of species closely related to but not included within a taxon whose phylogeny is being studied, and used to polarize variation of characters and to root the phylogenetic tree.

outwelling The export of detritus and other organic matter from estuaries to other ecosystems.

ovarian cycle The cycle in the ovary during which the oocyte matures and ovulation occurs.

ovary The primary reproductive organ of a female; where eggs (ova) are produced.

overburden Overlying layers of noncommercial sediments that must be removed to reach a mineral or coal deposit.

overdispersion Nonrandom dispersion of individuals in a habitat, such as when a minority of host individuals bears a majority of parasites.

overnutrition Receiving too many calories.

overpopulation Too many organisms (especially humans) for the available resources to support in a sustainable way.

overshoot The extent to which a population exceeds the carrying capacity of its environment.

overturn The sinking of surface water that has become more dense than the water below .

ovicapt Sphincter on the oviduct of a flatworm.

oviger Leg that carries eggs in pycnogonids.

oviparity Reproduction in which eggs are released by the female; development of offspring occurs outside the maternal body. Adj., oviparous

oviparity a condition that describes the habit of releasing eggs that later hatch

oviparous An animal that releases eggs.

ovipositor A modification of the abdominal appendages of some female insects that is used for depositing eggs in or on some substrate (Arthropoda, Hexapoda).

ovisac External sac attached to the somite that bears openings of gonoducts in females of many Copepoda. Fertilized eggs pass into the ovisacs for embryonation.

ovovitellarium Mixed mass of ova and vitelline cells; found in the monogenean genus Gyrodactylus and in a few tapeworms.

ovoviviparity Reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent, or immediately after laying. Adj., ovoviviparous.

ovoviviparous Describes reproduction in which embryos develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after emerging.

ovulation The release of an egg from the ovary.

ovule Structure that will become a seed following fertilization; literally an integumented megasporangium that contains the embryo sac before fertilization.

ovuliferous scale In conifers, an axillary, scalelike shoot that bears one or more ovules. Each strobilus may contain one or many ovuliferous scales, depending on the species.

ovum Mature female germ cell (egg).

oxidation The loss of an electron by an atom or molecule; sometimes addition of oxygen chemically to a substance. Opposite of reduction, in which an electron is accepted by an atom or molecule.

oxidative phosphorylation The electron transport system associated with aerobic respiration and mitochondria. In the release of energy through a series of cytochromes, three molecules of ATP are made.

oxidization The loss of electrons or hydrogen from an atom or molecule.

oxygen An element that is one of the constituents of water, organic matter, and many other chemicals. Oxygen gas (O₂), composed of two oxygen atoms, is needed for respiration and is produced by photosynthesis.

oxygen cycle The circulation and reutilization of oxygen in the biosphere.

oxygen debt The amount of oxygen that must be supplied following physical exercise to convert accumulated lactic acid (lactate) to glucose.

oxygen minimum layer A layer of water at a depth of approximately 500 m (1,600 ft) where oxygen is depleted.

oxygen minimum zone the ocean layer below the photic zone where dissolved oxygen concentration is lowest

oxygen sag Oxygen decline downstream from a pollution source that introduces materials with high biological oxygen demands.

oxyhemoglobin Compound formed when oxygen combines with hemoglobin.

oyster reef A dense oyster bed present in some estuaries and other marine environments.

ozone A highly reactive molecule containing three oxygen atoms; a dangerous pollutant in ambient air. In the stratosphere, however, ozone forms an ultraviolet absorbing shield that protects us from mutagenic radiation.

ozone layer Ozone (O₃) in the atmosphere that deflects ultraviolet radiation, which is harmful to life.