- macrochromosome
- macula
- male
- mammal
- mandible
- mandibular symphysis
- mane
- mangrove
- marbled
- marine
- maritime
- marker species
- marsh
- mass stranding
- mature
- maxilla
- meadow
- medial
- meiofauna
- melanism
- melon
- membrane
- mesad
- mesal
- mesial
- mesic
- mesoderm
- mesopelagic
- Mesozoic
- metabolism
- metacentric
- metamorphosis
- metazoa
- microchromosomes
- micropaleontology
- mimicry
- Miocene
- mitosis
- molar
- mollusk
- monogamy
- monograph
- monophyletic
- monotypic
- montane
- moorland
- morpha
- morphology
- morphometric
- mottled
- moult
- mud
- mudflats
- multifurcate
- muzzle
- MY
- mysid
- mystacial area