Rotifer in the context of Microplankton

Marine microorganisms are defined by their habitat as microorganisms living in a marine environment, that is, in the saltwater of a sea or ocean or the brackish water of a coastal estuary. A microorganism (or microbe) is any microscopic living organism or virus, which is invisibly small to the unaided human eye without magnification. Microorganisms are very diverse. They can be single-celled or multicellular and include bacteria, archaea, viruses, and most protozoa, as well as some fungi, algae, and animals, such as rotifers and copepods. Many macroscopic animals and plants have microscopic juvenile stages. Some microbiologists also classify viruses as microorganisms, but others consider these as non-living.

Marine microorganisms have been variously estimated to make up between 70 and 90 percent of the biomass in the ocean. Taken together they form the marine microbiome. Over billions of years this microbiome has evolved many life styles and adaptations and come to participate in the global cycling of almost all chemical elements. Microorganisms are crucial to nutrient recycling in ecosystems as they act as decomposers. They are also responsible for nearly all photosynthesis that occurs in the ocean, as well as the cycling of carbon, nitrogen, phosphorus and other nutrients and trace elements. Marine microorganisms sequester large amounts of carbon and produce much of the world's oxygen.

Microfauna (from Ancient Greek mikros 'small' and Latin fauna 'animal') are microscopic animals and organisms that exhibit animal-like qualities and have body sizes that are usually <0.1 mm. Microfauna are represented in the animal kingdom (e.g. nematodes, small arthropods) and some other heterotrophic, microscopic eukaryotes. A large amount of microfauna are soil microfauna which includes eukaryotic microbes, rotifers, and nematodes. These types of animal-like eukaryotic microbes and true animals are heterotrophic, largely feeding on bacteria. However, some microfauna can consume other things, making them detritivores, fungivores, or even predators.

Haplodiploidy is a sex-determination system in which males develop from unfertilized eggs and are haploid, and females develop from fertilized eggs and are diploid. Haplodiploidy is sometimes called arrhenotoky.

Haplodiploidy determines the sex in all members of the insect orders Hymenoptera (bees, ants, and wasps) and Thysanoptera ('thrips'). The system also occurs sporadically in some spider mites, Hemiptera, Coleoptera (bark beetles), and rotifers.

Philip Henry Gosse FRS (/ɡɒs/; 6 April 1810 – 23 August 1888), known to his friends as Henry, was an English naturalist and populariser of natural science, prolific author, "Father of the Aquarium", scientific illustrator, lecturer, entrepreneur, and pioneer in the study of marine biology and ornithology. Gosse created and stocked the world's first public marine aquarium at London Zoo in 1853, and coined the term "aquarium". His 1854 work The Aquarium: An Unveiling of the Wonders of the Deep Sea was the catalyst for the aquarium craze in mid-Victorian England. Over thirty years later, Gosse co-authored a three-volume work on Rotifera (microscopic aquatic animals) considered at the time "the most complete and exhaustive history of the Rotifera in any language", with drawings of "extreme minuteness, accuracy, and beauty".

In addition, Gosse was one of the chief figures among Brethren (British evangelical Christians frequently referred to by the misnomer "Plymouth Brethren"). For over half his life, he advanced his religious outlook by lecturing, evangelising, teaching, preaching, and watching for the Second Advent, as well as helping to spread the movement across the world.

Bdelloidea /ˈdɛlɔɪdiə/ (from Greek βδέλλα, bdella 'leech') is a class of rotifers found in freshwater habitats all over the world. There are over 450 described species of bdelloid rotifers (or 'bdelloids'), distinguished from each other mainly on the basis of morphology. The main characteristics that distinguish bdelloids from related groups of rotifers are exclusively parthenogenetic reproduction and the ability to survive in dry, harsh environments by entering a state of desiccation-induced dormancy (anhydrobiosis) at any life stage. They are often referred to as "ancient asexuals" due to their unique asexual history that spans back to over 25 million years ago through fossil evidence. Bdelloid rotifers are microscopic organisms, typically between 150 and 700 μm in length. Most are slightly too small to be seen with the naked eye, but appear as tiny white dots through even a weak hand lens, especially in bright light. In June 2021, biologists reported the restoration of bdelloid rotifers after being frozen for 24,000 years in the Siberian permafrost.

Meiobenthos, also called meiofauna, are small benthic invertebrates that live in marine or freshwater environments, or both. The term meiofauna loosely defines a group of organisms by their size—larger than microfauna but smaller than macrofauna—rather than by their taxonomy. This fauna includes both animals that turn into macrofauna later in life, and those small enough to belong to the meiobenthos their entire life. In marine environments there can be thousands of individuals in 10 cubic centimeters of sediment, and counts animals like nematodes, copepods, rotifers, tardigrades and ostracods, but protists like ciliates and foraminifers within the size range of the meiobenthos are also often included. In practice, the term usually includes organisms that can pass through a 1 mm mesh but are retained by a 45 μm mesh, though exact dimensions may vary. Whether an organism will pass through a 1 mm mesh also depends upon whether it is alive or dead at the time of sorting.

The terms meiobenthos and meiofauna were first coined and defined in 1942 by marine biologist Molly Mare, with both terms being derived from the Greek “μείων” (méio - "less"). However, organisms that fit into the modern meiofauna category have been studied since the 18th century.