Strictly speaking, the abyss is a particular zone extending between 3000 and 6000 m depth. By extension, the term is also used to designate the deep oceans overall.

Pertaining to benthos (from the Greek, meaning “depth”), which designates the oceanic substratum. Refers to the fauna living on the bottom, by contrast to the pelagic fauna, which lives in the open ocean.

The total quantity or mass of living material within a specified area at a given time. The concept of biomass allows one to express the idea of the abundance of animal presence in volume without having to use headcounts as is the case when speaking of density. Useful in that living organisms vary too broadly in size for density to be a meaningful measure.

A living organism, often microscopic, surrounded by a calcareous or siliceous envelope. Foraminifers are abundant in sedimentary layers and provide information on the origin and evolution of the deep substratum.

A reproductive gland.

Pertaining to the shore areas that alternate between submergence and nonsubmergence due to tidal oscillation.

Elements such as nitrogen and phosphorus, which are indispensable for plant growth and planktonic production. The oceans’ fertilizers.

Pertaining to a nutrient-poor body of water, such as the central zones of the oceans which have very little plankton. From the Greek oligo, meaning “few, and trophê, meaning “nourishment).

Pertaining to fish and animals that live in the open sea, away from the sea bottom (benthos)(from the Greek pelagos, meaning “open sea”).

From the Greek photos, meaning “light.” The photic zone is the space within lakes or oceans that is penetrated by sunlight. The lower limit of this zone depends on the particles in suspension in the water. In the open sea, it typically extends down to 200 m.

One of the largest divisions of the animal or plant kingdoms. Within the hierarchy of taxonomic classification, a phylum is situated between kingdom and class.

Planktonic organisms belonging to the plant kingdom. Many of them are microscopic algae and diatoms (unicellular organisms) that photosynthesize, producing the first level in the oceans’ food chain.

The very long geological period extending from the Earth’s formation (some 4.5 billion years ago) to the Cambrian Period (about 540 million years ago), marked by the appearance in abundance of life, now recorded in fossils.

Upwelling occurs when polar surface waters sink to the bottom, slowly migrate across the deep oceanic basins, and rise to the surface. These waters contain nutrients from the sunken carcasses of small organisms. This surfacing thus brings nutrients to the surface, nourishing plankton, and is an important aspect of the marine food web.

Planktonic organisms belonging to the animal kingdom. The majority are small crustaceans (copepods, krill), arrowworms, and gelatinous creatures that feed primarily on phytoplankton.