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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.


A simple definition of an anadromous fish species is one that spawns in fresh water, then migrates to the ocean (for foraging and maturing), and then returns to a river to repeat the process.

Typical examples of anadromous fish are bull trout, eels, lamprey vampire fish, and the chinook salmon.

Animal Migration

In ecology, migration refers to the movement of long distances by animals, often a seasonal occurrence. All major animal groups contain some migratory creatures, especially birds, fish (e.g. whale sharks), reptiles, and marine mammals.


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


Someone who owns, or manages, one or more aquariums is called an aquarist. For the most part, the term refers to fish tanks or vivariums.

Pro Tip: Another section explains the basics of fish keeping for beginners, with advice for troubleshooting problems with glass aquariums and community fish tanks.

Asexual Reproduction

In simple terminology, asexual reproduction in animals refers to a process where a single parent is able to produce an offspring by themselves (e.g. a clone of the parent) and without any need for male fertilisation.


Most batoids are cartilaginous fish (e.g. boneless skeleton) with a flat body and large pectoral fins fused to the sides of the head.

Thus, they tend to be bottom feeders, such as sharks, skates, and rays, that bury themselves in the loose substrate to wait for prey.

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), such as in the octopus phylum.


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).

Buccal Pumping

Some sharks and ray species use a mechanical procedure to 'pump' seawater over their gills. These buccal pumpers, such as manta ray pups, can "manipulate" buccal cavity volume even as they remain motionless inside the female.

Cannibalistic Animals

Cannibalism refers to the act of consuming (eating as a source of food) another individual of the same or of a similar species.

In the animal kingdom (Animalia), records show that this common ecological interaction involves more than 1,500 species.

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).

Caudal Peduncle

The swimming muscles, called the caudal peduncle, of a fish are found at the base of the caudal fin, and vary in size. Thus, a long caudal peduncle will act as a 'propeller' to assist fast movement in water.

Cephalic Fins

Often referred to as 'devil horns', manta rays (genus Mobula) use their cephalic lobes to guide tiny marine animals (zooplankton) into their mouth (e.g. while filter feeding) as well as for social communication.


The strange flattened and extended shape of caphalofoils, especially the T-shaped version in hammerhead sharks, enables the animal to turn or raise its head in a swift movement when catching its prey.


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.


The chelicerae in spiders, such as the subphylum Chelicerata (claw-bearers), form part of the mouth. Often referred to as 'jaws', chelicerae look more like pincers or articulated fangs.

Ciguatera Fish Poisoning

Eating certain species of fish that contain marine microalgae toxins, such as some grouper fish species, can cause severe illness in humans.

The typical symptoms of ciguatera include nausea (with vomiting), and neurologic tingling in the fingers or toes.

Ciliary Tract

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


The cloaca in fishes refers to an opening that functions as a digestive, reproductive, and urinary tract. So, the base of the tail is the usual location for cloacae. But in some, such as the pirate perch (Aphredoderus sayanus), the cloaca is under the head.


There are thought to be more than ten thousand cnidarian species, such as corals and jellyfish. All cnidaria are aquatic invertebrates and the majority of them live in the ocean.


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


Some aquatic species share some kind of long-term interaction with another plant or animal, also known as symbiosis. In other words, commensalism occurs when one member gains a benefit - but the other species does not.

The species benefiting from the association (the commensal) often gets nutrients and a safe shelter, such as in the squat anemone shrimp.

Hence, this kind of commensal relation often occurs between a larger host (sea anemone) and the smaller commensal (the crustacean).


A corallivore is one of the marine biology terms relating to animals that feed on coral polyps (e.g. annelids, butterflyfishes, marine mollusks, and echinoderms).

As a consequence of that, corallivorous creatures can influence the abundance and distribution of coral reef formations.

Cosmopolitan Distribution

In simple terms, if the range of habitats for any plant or animal species extends across the vast majority of the world's oceans, it has geographical distribution.

Typical examples of sea creatures with "cosmopolitan distribution" are the blue whale, ray fish species, and the great white shark.


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 a hard exoskeleton) such as crabs, lobsters, shrimps, and barnacles.

Ctenoid Scales

Typically, ctenoid scales in some species of fish, such as Mediterranean cardinalfish (Apogon Imberbis), are comb-like bony structures that help to protect the body against hazardous environmental impacts and their predators.


Osmotic lysis, also called cytolysis, can occur if a cell bursts because of the osmotic imbalance that causes excess water to diffuse into a skin cell.

A typical example of this process may occur during the first aid treatment for Portuguese man-of-war sting injury.


This terminology refers to an organism with a colonial body (e.g. Hydrozoa) but devoid of a mouth. Thus, the main function is to capture food for nutritive members of the colony.


In fish families, the term "demersal" refers to creatures that live and feed near to (or on) the bottom of a water body. This contrasts with pelagic fish species that tend to live close to the surface of oceans or lakes, often drifting with the currents.


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.

Dermal Denticles

In simple terminology, a denticle is the toothlike scale (sometimes platelike) that most cartilaginous fishes have. Hence, a large mass of denticles creates a tough outer skin in some sea creatures, especially sharks and marine rays.


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.


An ectoparasite refers to an organism that lives on the external skin of a host species. In fact, like most of the underwater vampires in the sea, this is how they get their sustenance for life.


Marine biologists use the term 'emarginate' to describe the posterior border of a caudal fin. In most cases, it will curve inwardly (e.g. slightly concave in appearance).


For the most part, all warm-blooded animals are endotherms, especially mammals and birds. In essence, it means they can maintain a constant body temperature independent of their surroundings or 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).


Exoskeletons are rigid coverings that form outside of an animal's body. The outer shell (skeleton) provides support for its internal organs as well as protection for some invertebrate animals (e.g. arthropods, crabs, crustaceans, coral reef formations).


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


Objects with a spindle-like shape, meaning wider in the middle and tapering at both ends, are said to be fusiform. Other examples include lemons and carrot roots (e.g. with a conical tip).

In fish, especially the shark phylum, the fusiform body shape is somewhat rounded and tapered. In other words, the enhanced aerodynamic body shape helps some fish reduce drag and improve efficiency when swimming.


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.


Ichthyophile (plural ichthyophiles) refers to someone, a hobbyist, who loves keeping fish or a group of fish, known as a school or shoal.

Lek Mating

The fish mating system known as lekking involves aggregations of males that form 'leks' or mating arenas (e.g. wrasse fish).

Hence, the courtship display that follows attracts females into visiting the lek. So, the main purpose of lek mating is for a female to choose a 'suitable' male for breeding purposes.


A leucistic animal (e.g. mostly birds, mammals, and reptiles) is one with an abnormal condition of reduced pigmentation of hair, feathers, or skin.

Sometimes, a genetic mutation causes paleness or patches of reduced colouring due to a lack of melanin and other pigments, especially with certain species of marine rays.


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

Macrobenthos Feeding

In general, macrobenthos refers to organisms that live on or near hard structures at the bottom of a water column, such as green algae. Typical macrobenthos feeders that eat algae as a primary food source include the giant goby (Gobius cobitis) and most marine crustaceans.

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).

Meristics in Fish

Meristics, in ichthyology and herpetology, refers to quantitative features used to describe a species of fish or reptile (e.g. the number of scales that it has).

Metamerism in Zoology

The repetition of homological body segments (e.g. similarity in physiology and structure) is commonplace in marine annelids, crustaceans, and other segmented worms.


A marine biology dictionary describes metamorphosis as being a change in physical form or structure. In other words, it refers to the alteration in appearance and character of a juvenile into adulthood.


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.

Multiple Paternity

High levels of multiple paternity, having more than one father, is the process considered as being beneficial for increasing reproductive success, especially in lizards, marine turtles, nurse sharks, and snakes.


In biology, the term 'mutualistic relationship' refers to two (or more) organisms of a different species that benefit from 'working together'. For the most part, they will have evolved together and each is part of the other's environment.

A typical example of mutualism takes place when boxer crabs (Lybia tessellata) carry living sea anemones for extra protection against predators.


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.


An omnivorous fish species, such as the red fire goby, is one that can eat and survive on different kinds of plant and animal matter.

Even though omnivores are able to digest carbohydrates, protein, and fat, some species have the ability to incorporate algae, bacteria, and fungi into their diet.


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.


Such as in marine arthropods, ovigers are special appendages that can function as a cleaning device for the long legs and trunk. But, they can also carry eggs.


A simple definition of an oviparous creature is one that incubates eggs inside the maternal body (e.g. birds, fishes, reptiles). In general, the eggs will hatch soon after being released by the parent.


The process refers to the production of young by means of eggs which actually hatch within the body of the parent. Typical examples include some marine rays and sea snakes.


Found in most segmented marine worm species, parapodia structures are a form of locomotory 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.


A piebald fish is one that is suffering from a condition where the absence of melanocytes causes changes in skin and hair colour.

When cells fail to produce the pigment melanin, it can cause piebaldism in fish species, especially the nurse shark (Ginglymostoma cirratum) and some freshwater catfish.

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.


The 'pleuston' refers to marine organisms that are held at the surface of the water by some kind of floating mechanism. A typical example would be the gas-filled bladder (pneumatophore) of the Portuguese man-of-war species.

Preopercle Bones

In most fish, excluding butterfly fish species, preoperculars are flattish boomerang-shaped bony structures that help to form the posterior and lower segments of the cheek.


Typically, any elongated appendage that protrudes from the head of an animal, including vertebrates and invertebrates, is called a proboscis.

But, in invertebrates, this marine life terminology relates to a tubular mouthpart that an organism uses to suck food, such as sea spiders.

Radula Structures

The vast majority of marine mollusks 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.

Ram Ventilation

In marine biology terms, ram ventilation refers to the extraction of oxygen from water as it passes over the gills of a fish. In other words, there is very little, if any, active muscle pumping used during the breathing process.

Pro Tip: Not all fish species use this system. For example, the whitetip reef shark has developed an inner pumping system that passes water over the gills. Thus, it is one of the few sharks that can lay dormant on the seabed.

Retia Mirabilia

The rete mirabile of some warm-blooded vertebrates (especially cetaceans and manta rays) uses a process of countercurrent blood flow (e.g. flowing in opposite directions) as a crosscurrent exchanger (e.g. for heat exchange) between vessel walls.


In coral, the term sclerites refers to small aggregates of calcium carbonate that embed in the soft tissue of Alcyonaria (Octocorallia). These small, pointed structures help some soft corals keep their shape, such as dead man's fingers (Alcyonium digitatum).

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 small spiracle in some shark species provides the eye (and brain) with oxygenated blood. However, it is much larger in ray fish (batoids) and used to pump water over the gills. This mechanism means they can breathe motionless while buried in the sand.


The term refers to the region directly above the eye sockets (e.g. eyebrows in humans). In some species, such as the horn shark (Heterodontus francisci), it can signify strong anatomical features.


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.

Tapetum Lucidum

In some species, especially the blue shark (Prionace glauca), the tapetum lucidum is a layer of reflective cells that sit behind the retina. The feature means they can see much better in darkness or cloudy water, especially in deep oceans.


The taxonomic reference of a creature, an organism, or a plant is the science of naming it, describing it, and classifying it for the benefit of others.

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).

Terminal Mouth

A simple description of a terminal mouth in fish species is one that's located in the middle and front of the head and points forward (e.g. the slippery dick fish).

In general, because both jaws will be a similar length, these fish are mid-water feeders that prey on other fishes in the water column.


In simple terminology, a thermocline forms a distinct boundary between less dense warm water and denser colder water. In some cases, the abrupt change in water temperature can be several degrees - especially as you dive deeper.


In fishes, tubercles are solid, raised, keratin skin nodules. Keratin is the same material that produces hair and fingernails in humans.

In some species, such as the lumpsucker fish (Cyclopterus lumpus), tubercles are most often found on the males (e.g. during the breeding season) and then shed soon after.


In marine biology, a vestigial structure is one that has already lost a major ancestral function, often smaller as a result.

For example, whale sharks do not attack or eat humans even though they have 300 rows of vestigial teeth.


A viviparous animal is one that fertilises and develops the embryo inside the body of the mother and then gives birth to a live offspring.

So, a typical example of viviparity (live-bearing) is the one that occurs during the bull shark reproduction process.


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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