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Macro-Algae: Complex celled algae found in marine aquaria; e.g., Calurpa.

Macrobenthos: Benthic organisms (e.g., 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.

Macroinvertebrate: An animal lacking a backbone and visible without the aid of magnification.

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.

Magnesium (Mg): No specific recommendation, but high calcium is associated with hardness, total dissolved solids problems and can cause aesthetic problems.

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).

Malnutrition: The lack of proper nutrition resulting from a poorly balanced diet.

Mandible: Jaw; one of a pair of mouthparts located behind the upper lip (labrum).

Manganese (Mn): The MCL is 0.05 mg/L. Manganese is primarily regulated because of the aesthetic problems associated with elevated levels of manganese, i.e., a secondary drinking water standard. Elevated manganese levels can disrupt the nervous system and regeneration of hemoglobin. For freshwater the concentration should be less than 1.5 mg/L.

Mangel: See Mangrove Forest.

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

Mangroves Ecosystem: The environment of saline-tolerant trees that live in the tidal zone of the ocean, and all the other non-living and living things that have relationships there.

Mantle: Large, pigmented fleshy portion of tridacnid clams that is exposed to the light by gaping of the shell valves. Also called siphonal tissue. Also, the coral tissue in fleshy polyps (e.g., Catalaphyllia).

Marine Biologists: Scientists who study the life and processes of the ocean.

Marmelade Cat: See Orange-Blotched.

Mimicry: When an animal mimics, or copies the appearance of another animal in order to gain an advantage, like camouflage or a better defense. A good example is the marine Mimic Tang.

Margin: Border or edge of shell.

Mariculture: Cultivation of marine organisms under controlled conditions.

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.

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.

Marinelife: Animals that live in the ocean including coral polyps, sea urchins, clams, shells, worms, crabs, octopuses, squid, etc.

Mask: A covering of the eyes and nose that allows the clear viewing of underwater environments.

Maximum Contaminant Levels (MCLs): Legally enforceable drinking water standards required by the Safe Drinking Water Act. Standards set by the Environmental Protection Agency establish the maximum permissible concentration of selected contaminants in public water supplies. Contaminants are included on the list if they pose a public health risk. For example, 10 ppm is the MCL for nitrate-nitrogen (NO³-N).

Maximum Contaminant Level Goals (MCLGs): Public drinking water standards that serve as non-enforceable goals for selected contaminants contained in drinking water that pose no health risk to people over a lifetime of exposure. A MCLG is a suggested level set by EPA as a guideline for water utilities.

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

Maximum Sustained Yield: Maximum number or amount of a species that can be harvested each year without steady depletion of the stock; the remaining stock is able to replace the harvested members by natural reproduction.

Mbunas: Common name for rock-dwelling African Cichlids from Lake Malawi.

Mechanical Filtration: Filters that mechanically remove particles from the water by filtering water through a substrate such as polyester, which can remove the impurities as it passes through the media. Examples are under gravel filters, canister filters, and wet/dry pre-filters.

Mecury (Hg): The MCL is 0.002 mg/L for organic mercury. Mercury has been associated with kidney disease. For freshwater the concentration should be less than 0.00005 mg/L.

Medial: In the middle; at or toward the midline of the body.

Median: At or on the midline of the body.

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

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

Meiobenthos: Very small animals living buried in the sediments of the seafloor.

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

Membrane: A thin film of tissue, usually transparent.

Membranous: Like a membrane; thin and more or less transparent.

Meroplankton: Floating developmental stages (eggs and larvae) of organisms that as adults belong to the nekton and benthos.

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: The 150-2000 m depth zone, seaward of the shelf-slope break.

Mesothorax: The second or middle thoracic segment.

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

Metal Halide: A very intense type of lighting used in marine and freshwater plant aquaria.

Metal Halide Lighting: Metal halide lights are a type of light bulb that emits very white and very bright light. They require a special fixture and ballast. Metal halide lights deliver a very wide spectrum of light, making them a great substitute for real sunlight. They are considered by most hobbyists to be the best method for lighting planted and reef tanks. They are very efficient in terms of lumens/watt, although they burn much hotter than incandescent lights (HO and VHO). Do not confuse them with halogen bulbs, which have a very yellow light not appropriate for aquarium use.

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

Metamorphosis: See Incomplete or Complete Metamorphosis.

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.

Metathorax: The third or most posterior thoracic segment.

Methemoglobinemia: A condition that limits the oxygen-carrying capacity of red blood cells. The condition occurs when bacteria in the digestive tract convert nitrate to nitrite. Nitrite reacts with hemoglobin in the blood, producing methemoglobin, which cannot carry oxygen. The resulting oxygen starvation causes a bluish discoloration of the body. The condition is largely confined to infants less than 9 months old. Excessive amounts of nitrates may be ingested with water or food. Often foods, such as fresh vegetables, are a major source of nitrates.

Micro-Algae: Single celled algae often growing in strands called hair algae.

Microbenthos: Benthic organisms (e.g., animals or plants) whose shortest dimension is less than 0.1 mm

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

Microplankton: Net plankton, composed of individuals below 1mm in size but large enough to be retained by a small mesh net.

Micro-Siemens/cm: A unit of electrical conductance: how easily electricity flows through a material.

Milt: Sperm.

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 Adsorption Pads: Polyester pads that have been chemically treated to absorb certain substances from the water. This form of chemical filtration is placed in a canister or power filter. Note that they sometimes will remove good trace elements as well as pollutants.

Mollusks: An animal phylum of marine animals that includes bivalves (mussels), snails, slugs, oysters, and nudibranches.

Molt: The process of shedding the exoskeleton.

Monoculture: Cultivation of only one species of organism in an aquaculture system.

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

Mooching: A method of fishing in which you leave your bait on the bottom and jerk it upwards periodically to attract fish.

Mottling: To mark with blotches of different colors or shades.

Mouth: The opening through which animals, including coral polyps, take in food, nutrients and liquids.

Mouth Brooder: Fish that protect un-hatched eggs in their mouth. Sometimes called tooth-carps. Some species are maternal mouthbrooders, such as most African Cichlids, which means that it is the mother that guards the eggs in her mouth. There are also some species that are paternal mouthbrooders, and still others that switch mid-term, like Arowanas.

Mucous-Bag Suspension Feeder: Suspension feeder employing a sheet or bag of mucus to trap particles non-selectively.

Mussel Scar: A mark on the underside of a shell where a mussel was attached.

Mutualism: An intimate association between different organisms in which both organisms benefit.

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