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Marine Biology Terminologies

The list of words and scientific terms used in marine science is extensive. Furthermore, some of the marine biology definitions can be more confusing than they need to be.

This help guide contains simple explanations about the vocabulary used by scientists to describe sea life plants and marine animals.

Basic Glossary of Marine Biology Definitions

The content in this website references most of the biological terms and scientific vocabulary used in ocean life.

This section lists the common terminology with simple definitions, from 'abdomen' to 'zooxanthellae'.

But, before we start, let's answer a few basic misunderstandings about the subject of marine biology.

In a nutshell, marine biology refers to the study of marine organisms, their behavioural patterns, and their interaction with environments.

There are four (4) disciplines of marine science. It is a multidisciplinary approach to the study of nature in the oceans. Thus, marine scientists will need to be experts in biology, geology, chemistry, and physics.

What are the Marine Ecosystems?

Depending upon the extent of your fact-finding research, a general agreement is that the recognised and accepted marine ecosystems are a combination of:

What are the 3 Different Groups of Marine Life?

  1. Benthos (plants and animals that live on the ocean floor)
  2. Nekton (creatures that swim in water)
  3. Plankton (organisms that float in the water)


The abdomen is the region of a vertebrate's body that is furthest from its mouth. It contains the digestive and reproductive organs.


In simple terms, abiotic means devoid of life (e.g. it is not biological). Some abiotic factors can shape the environment, such as light, temperature, water, or salinity and ocean currents in marine ecosystems.


The anus is found at the end of the digestive tract or gut through which waste products of digestion are excreted.

Benthic Zone

The benthic zone is the ecological region at the lowest level of a body of water. This could be an ocean or a lake, or it may also include the sediment surface layers.

Bilateral Symmetry

It's often called 'plane symmetry' in animals. The term means that the animal has the 'sameness' in both right and left sides (e.g. displaying a mirror image of the other).


The removal of calcium carbonate substrate is known as bioerosion. Corals increase in size due to calcium carbonate or limestone formation. This is how they form a skeleton (or a framework) to support coral reef organisms.

But, the crunching action of bioeroders breaks down the substrate into sand. Fortunately nature has a way of controlling the delicate balance between excessive erosion and insufficient accretion.

Bioluminescent Bacteria

Some bacteria produce bioluminescence light, such as those that are present in sea water and in marine sediments.

You can also find luminous bacteria living in the gut of marine animals (e.g. squid) and on the surface of decomposing fish.

Bacterial species reported to possess bioluminescent qualities belong within families Enterobacteriaceae, Vibrionaceae, or Shewanellaceae (all assigned to the class Gammaproteobacteria).


The marine biome is the largest biome in the world. It is a geographical area notable for the species living within it.

The planet's features exist in two principle biome categories which are the terrestrial biome (land) and the aquatic biome (water).

Caudal Fin

The caudal fin is the tail fin of a fish used for steering, balancing, or efficient propulsion (e.g. like a motor on a small boat).


Nudibranchs use finger-like, branched or thread-like tentacles for breathing and to help with digestion. In some cases, they can also use the appendages for defensive protection.

Ciliary Tract

Ciliary tracts are lines of tiny hair-like extensions. Some creatures use them in water currents to move food and mucus.


A cnidocyst is the stinging cell or nematocyst of a cnidarian (coelenterate). The three most common types of cnidocytes are nematocysts, ptychocysts, and spirocysts.


Many open-ocean fish species use countershading as camouflage. Thus, one side of the animal will be dark (e.g. dorsal) and the other light (e.g. ventral) to blend in with the natural surroundings.

Note: A good example of a marine animal that uses 'counter shadowing' is the blue glaucus dragon fish - a soft-bodied, gastropod mollusk.


A diverse group of arthropods (animals with jointed limbs and hard exoskeleton) such as crabs, lobsters, shrimps, and barnacles.


Dentition refers to the arrangement or physical condition of the teeth in a particular species, usually on the upper and lower jaws. Even so, some fish have them on their lips, in their mouth, tongue, and even inside their throat.


Detritus means non-living particulate organic material. Thus, fragments of dead organisms, including faecal and decomposed sediment, are defined as detritus debris.


Sexual dimorphism is the strikingly obvious observable characteristics that differ between male and female genders of the same species.

Dorsal Fin

The dorsal is one of the 'unpaired' fins found on the back of a fish's or whale's body. In general, fishes use dorsal fins to increase the lateral surface of their body when swimming


Ecosystems (biomes) are complex subjects. Nonetheless, one simple way to define an ecosystem is the combination of (all):

  • Abiotic factors and the interdependent biological communities in a single area.
  • Diverse species and how they interact with the geographic area.
  • The physical and chemical processes that occur within the environment.


The epidermis is the outermost layer of skin. There are two basic layers in all fish species, the epidermis (outer) and the dermis (inner).


In marine biology terminology, the word extant means it is still in existence or surviving (e.g. not extinct).


Gametes are mature haploid male or female germ cells that are able to unite with others of the opposite sex to form a zygote as a result of sexual reproduction.


Gonochorism (or gonochoristic in marine biology terms) indicates that an organism has unisexualism. It describes the state of having only one of two or more distinct sexes.


Hermaphrodites are organisms that have male and female sex organs. Besides being able to produce eggs and sperm during its adult life, they may also display other sexual characteristics in both genders.


A heterotroph describes a plant or animal that is unable to produce its own food source. So, heterotrophic organisms require organic compounds of carbon and nitrogen for nourishment.


Simply put, hypoxia means there is a low amount of oxygen reaching the tissues. Aquatic hypoxia creates stress for marine organisms because they are receiving low concentrations of oxygen in the water.


This type of marine algae is big enough to see with the naked eye. Typical examples of macroalgae include gulfweed, kelp, rockweed, and seaweed.

Mammalian Dive Reflex

It has several aliases, such as diving bradycardia and the diving response. But, what does the mammalian diving reflex do and why is it important for deep-diving freedivers?

In simple terms, it is a physiological reaction that mammals and humans have in response to submersion in water - especially cold water. This automatic, protective action results in certain body parts shutting down to conserve energy (e.g. for survival).


There are several notable characteristics that help to define the phylum of protostome mollusks (based on their embryological development).

We use the calcium carbonate shell, mantle, and unsegmented body to characterise this group of marine invertebrates (e.g. snails, slugs, shellfish, octopuses, and squid).


A monospecific species (or genus) is one that contains only a single species. So, for example, chirodectes maculatus is an extremely rare monospecific genus of the box jellyfish (Chironex fleckeri) in the taxonomic family Chirodropidae.


Snail slime, called mucopolysaccharide, is an external mucus secretion produced by almost all invertebrate marine snails. Most gastropods, sea slugs, and land snails produce and discharge some kind of mucus as they move around their habitat.


Mucus is usually a sticky, slimy, substance secreted by certain organisms. They often use it for locomotion, lubrication, or for protection against foreign particles.

As a result, mucous bag suspension feeders employ a sheet or a bag of mucus to 'non selectively' trap food particles suspended in the water column.


Some of the smallest planktonic organisms (e.g. flagellates) are unicellular and measure between two (2) and twenty (20) microns (μm) in size.


In relation to marine biology, the simplest definition of nocturnal is an organism that sleeps during the day and is most active at night time.


A simple definition of a notochord is a stiff, yet flexible, rod of tissue cells running along the back of a body. Thus, notochords are the primitive backbones in animals that fall under the phylum chordata.

Ocean Acidification

Over a period of time, additions of carbon dioxide (CO2) to the atmosphere, and the ensuing dissolution of it in seawater, will decline the pH in the oceans.


Most common in gastropods, the operculum is a hard, organic, lid-like plate that covers (or it completely closes) an aperture, such as gill covers.


Some individuals suffer an overwhelming fear of snakes - to the point where it often results in an anxiety disorder. Typical symptoms of ophidiophobia include dizziness, nausea, profuse sweating, and an increased heart rate.


Found most often in living sea sponges, the osculum is an opening used for the filtration and discharge of wastewater.


Found in most segmented marine worm species, parapodia structures are a form of locomotary organs. For example, they are lateral, fleshy profusions that the sea butterfly marine snail uses to travel through the water.

Pectoral Fins

The pectorals are a pair of fins found on each side of the body, behind the head and gills. So, fish use their pectoral fins for movement control, balance, and for abrupt stops.


The tendency of an animal or organism to stay in a particular area (or return to it frequently) is known as philopatric behaviour.

There are many reasons why this unique conduct takes place. But, animals that return to their birthplace to breed, such as banded sea kraits and turtles, may be among the most common.

Planula Larva

Planula larva (planulae for plural) is usually a free-swimming larval form that is common in most species of the phylum Cnidaria. Typical examples include coral, jellyfish, and sea anemones).

In most cases, the planula body is cylindrical in shape, although some are egg-shaped. They use cilia (tiny hairlike projections) for movement or locomotion.

In fact, it is the polyp form in sea anemones and some anthozoans (including the medusa form in most of the cnidarians (coelenterates) that produce planulae.

Radula Structures

The vast majority of marine mullusks use a radula as the primary feeding organ. It is a rasping tongue-like ribbon that helps them scrape or cut its food before swallowing it.

Scuttling Ships

Despite being a rare occurrence, the deliberate sinking of a ship is called 'scuttling'. In general, abandoned, old, or captured vessels may be scuttled to prevent them from becoming navigation hazards.

Note: A typical example of a scuttled ship is the Um El Faroud Libyan oil tanker sunk off the coast of Wied iz-Zurrieq, in Malta.


Sedentary describes the way in which particular animals feed. Sedentary filter feeders strain particles or small organisms out of the water column by circulating them through their digestive system.


A symbiotic interaction is a close and often long-term relationship between two or more different biological species. An example would be the symbiosis between the goby fish and the burrowing shrimp.

Teleplanic Larvae

The term refers to the kind of larva that is capable of dispersal over long-ranging distances (e.g. across oceans).


In simple terms, a tentacle is an elongated organ that is flexible and movable. In general, most marine invertebrates (e.g. cnidarians, mollusks) use their tentacles to help them feed, move, and grasp things to gather sensory information.

Hence, a tentacle-tube-foot suspension feeder is an organism that traps food particles on the tentacles (or tube feet in echinoderms).


The World Register of Marine Species (WoRMS) aims to provide a comprehensive list of names of marine organisms and information on synonymy.

Valid names have the highest priority. Even so, including other 'common names' means the register also serves as a guide for interpreting taxonomic literature.


Zooid means that animals or new individuals arise from the budding or division of other organisms within the colony.


Floating or weak swimming plankton consisting of small animals and the immature stages of larger animals make up part of the planktonic food supply.

Note: Check out our zooplankton facts and information section for more fun and interesting anecdotes.


Zooxanthellae are minute cells that live within most types of coral reef polyps. They provide a food source through photosynthesis, which in turn, helps the coral thrive and survive.

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