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655 The glossary that follows defines over  specialized terms that appear in the text of this encyclopedia. Included are a number of terms that may be familiar to the lay reader in their common sense but that have a distinctive meaning within this field of study. Definitions have been provided by the encyclopedia authors so that all of these terms can be understood in the context of the articles in which they occur. abalone A large edible marine gastropod of the genus Haliotis, having an ear-shaped shell with a row of holes along the outer edge and a large muscular foot used to hold the animal tightly onto a rock. abiotic Due to physical processes; not due to biological processes. An ABIOTIC FACTOR is any nonbiological environmental condition, such as temperature, salinity, pH, or desiccation, as opposed to biotic (biological) factors such as competition or predation. aboral surface The top (dorsal) side of a sea star; the side opposite the sea star’s mouth. absorptance For algae, the fraction of solar irradiance absorbed. acceleration The rate at which velocity changes through time, typically expressed as meters per seconds squared (m/s ). The change can be in either the direction or the magnitude of velocity. acceleration reaction A force, proportional to volume, exerted on an object along the axis of flow, caused by the relative acceleration of the body and fluid. Also, ACCELERATIVE FORCE. acclimation The ability of an organism to modify its environmental (e.g., thermal) optimum or tolerance limits when exposed to a change in its environment; as measured in controlled studies in the laboratory, where only a single factor such as temperature is varied. When observed in the field (e.g., in response to seasonal changes in the environment), this ability is termed ACCLIMATIZATION. acidity Atermdescribingthehydrogenionconcentrationof a solution when the pH is below  (the pH of neutrality). acoelomate Lacking a fluid-filled body cavity between the outer body wall and the gut. acoustic Doppler current profiler (ADCP) Aninstrument that uses the Doppler shift in sound waves to measure current velocities throughout the water column. acrosome reaction Release from the sperm cell of a secretory granule containing sperm proteins that consequently form long, narrow extensions, which can bind to sperm receptors on an egg. actinula The specialized planktonic larva of some Hydrozoa. advection The horizontal movement of an object (e.g., seawater or larvae). aeolid A nudibranch belonging to the suborder Aeolidoidea, having cerata on the back and lacking a rhinophoral sheath. algae Any of various chiefly aquatic, eukaryotic, photosynthetic organisms, ranging in size from single-celled forms to the giant kelp. aliasing The erroneous appearance of a low-frequency signal, caused by too low a sampling rate. alkalinity A term describing the hydrogen ion concentration of a solution when the pH is above  (the pH of neutrality). Oceanographers define seawater alkalinity as the number of milliequivalents of H+ ions required to titrate one kilogram of seawater to the bicarbonate equivalence point (~pH .). allele A version of a gene that codes for a particular trait; a specific DNA sequence occupying a given locus (position) on a chromosome. GLOSSARY 656 G L O S S A R Y allelopathy The use of chemical compounds to defend space against interspecific competitors (e.g., by inhibiting their growth or larval settlement). alternative states The ability of more than one type of community to develop and be maintained in the same place at different times or in adjacent, physically similar habitats at the same time. Also, MULTIPLE COMMUNITY OUTCOMES. altricial Of young animals, born or hatched in a state that is not highly developed. In contrast to marine mammals, most terrestrial carnivores are born in an altricial state. ambient light The light existing in a subject area, prior to any added by a photographer. ambulacral groove A groove on the underside of each limb of an echinoderm, through which the tube feet of the water vascular system extend. amino acids Organic compounds containing amino (NH2) and carboxyl (COOH) groups. Amino acids are linked together by peptide bonds to form proteins. amoebocyte A morphologically and functionally diverse group of blood cells that typically exhibit amoeboid locomotion. In tunicates, some amoebocytes carry vanadium granules, while others may be used for storage, phagocytosis, and defense against pathogens. amplitude Of a sinusoidal wave, half the vertical distance between the crest and trough. Wave height is twice the wave amplitude. ampullae (singular ampulla) Internal, muscular bulbs that contract to elongate...

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