The Classification of Organisms in the Ocean

Taxonomy is the science of classifying all living organisms by assigning each species a degree of relatedness to other species. In general, the more alike one species is to another, the closer together those two species will be classified. The same system is used to classify ocean organisms as land organisms.

Although some species have been reclassified and new strata developed, the taxonomic system still used today is still mostly the same as when it was originally developed by Carolus Linnaeus in the 18th century. It is based on 7 basic tiers: kingdom, phylum, class, order, family, genus, species. Several subtiers, such as subphylums, are also included in modern taxonomy. Some taxonomists also include domain, just above kingdom.

Domains and kingdoms

The same basic 3 domains are found in the ocean as on land. These are:

* Archaea – single-celled protozoans which lack a cell nucleus and are not bacteria
* Bacteria – single-celled bacteria and phytoplankton
* Eukaryotes – multicellular organisms, as well as single-celled protozoans which have a cell nucleus

Plankton are defined by their ecological niche at the bottom of the food chain rather than by taxonomy, so different types of plankton can be found in each of these domains. Diatoms, the simplest form of single-celled colony organism, are eukaryotes, as are the much larger pyrosomes. Cyanobacteria, also known as blue-green algae, are the basis of photosynthesis in the ocean. They belong to the bacterial domain.

The green sulfur bacteria (phylum Clorobi) and purple sulfur bacteria (Proteobacteria) have unique forms of photosynthesis or chemosynthesis which are based on hydrogen sulfide. The GBS1 species of green sulfur bacteria is capable of photosynthesis using only the glow from the nearby thermal vent. It is the only species known to use photosynthesis completely without the sun in its natural environment.

Along with chemo-autotrophic and chemosynthetic bacteria, marine archaea are at the bottom of the food chain in most extreme environments, including hydrothermal vents. They are the base of all ocean biological communities which are not driven by solar energy. These organisms are extremophiles which thrive in conditions of extreme heat, cold, salt, acid, or alkalinity which kill most other organisms. Many of them are anoxic and still live in oxygen-deprived or completely anoxic environments. These organisms may have evolved before the Earth’s atmosphere changed from hydrogen and methane to an oxygen-nitrogen mix.

The most common archaea in the oceans are the chrenarcheota. Where kingdoms are used instead of domains, chrenarcheota are sometimes given their own kingdom. Otherwise, they are included under the kingdom Archaea.

Where kingdoms are used instead of domains, these organisms are divided into either 5 or 6 kingdoms. The eukaryotes are always divided into the same 4 kingdoms:

* Protista – single-celled protozoans which have a cell nucleus
* Fungi – multicellular organisms which are neither plant nor animal and cannot photosynthesize
* Plantae – plants
* Animalia – animals

If 5 kingdoms are used, the fifth is Monera, which are all the single-celled protozoans which lack a cell nucleus. If 6 kingdoms are used, Monera is divided into Bacteria and Archaea, which is sometimes called Archaebacteria.

Phylums, subphylums, and divisions (Non-Animalia Eukaryotes)

The greatest diversity in ocean eukaryotes is found among the Protista. Brown, red, and green algae are classified as Ochrophyta, Rhodophyta, and Chlorophyta respectively. Tubulinea covers all the different kinds of amoebas, which move by cellular extensions. Haptophyta and Dinophyta cover the single- and multicellular organisms which move by using flagella.

The classification of marine fungi also includes many different molds and yeasts, including fish diseases. However, the exact taxonomy of oceanic fungi at every level is under constant debate.

Tidal fungi can be found in symbiotic relationships with algae or cyanobacteria in intertidal ecologies. This symbiotic relationship creates a composite organism known as a lichen. The alga provides the photosynthetic energy, while the fungus keeps the alga from drying out during low tide. Rocks in intertidal regions are often covered with lichen.

Among plants, only Anthophyta is found in a marine environment. Algae, which used to be classified as plants, are now usually included under Protista. Most seaweeds are chromist algae.

The only other true plants which are found in an ocean environment are the seagrasses. These are monocot angiosperms, Order Alismatales, which includes flowering plants in aquatic environments. Most Alismatales are found in freshwater environments. Sedentary animals, such as sea cucumbers, corals, and anemones, fill most of the other ecological niches usually filled by plants on dry land.

Animalia (Non-Chordates)

From smallest to largest, the major ocean animal phylums are Porifera, Cnidaria, Bryozoa, Echinodermata, Mollusca, and Chordata. Most non-microscopic ocean animal life falls into the first 5 phylums.

The Porifera, or sponges, are the simplest of all marine animals. They have no nervous, digestive, respiratory, or circulatory system. All biological processes are accomplished by filtering water through intake and output holes.

The Cnidaria are the jellyfish colony organisms, with specialized stinging cells along the tentacles. This ocean phylum also includes the reef corals (Order Zoantharia), sea anemones (Class Anthozoa), and free-standing corals (Subclass Octocorallia). They have no excretory or circulatory system, but some classes of Cnidaria do have a rudimentary digestive cavity. They may also show simple radial symmetry. Most Cnidaria, even the sessile species, are free-floating before they become adults.

Like the Porifera, the Bryozoa are filter feeders. Many of them have been found to cause skin diseases on fish and other  marine animals. They have simple digestive tracts and mineralized exoskeletons.

Echinodermata are the starfish, sea cucumbers, and sea urchins. They are all flat and radially symmetrical, with 5 or more muscular limbs radiating out from the mouth area. Most echinoderms are able to regenerate limbs, and some can even regenerate internal organs. They reproduce both sexually and asexually.

As their name suggests, Mollusca are the familiar shellfish, although they also include squids, snails, and the octopus. They are characterized by a central soft body with a muscular foot or limbs. They have flow-through digestive systems, a respiratory system based on gills, and well-developed nervous systems. Most molluscs also have separate sexes. Marine molluscs are divided into the following classes:

* Polyplacophoria – articulated shell with 8 plates
* Gastropodia – foot slides over the surface and cannot attach to soft surfaces, most have coiled shells
* Bivalvia – filter feeder, paired gills, double shell
* Cephalopoda – muscular tentacles reaching outwards from the head, sharp beaks, can move by jet propulsion

The cephalopods are considered the most intelligent invertebrates. Some, particularly the octopus, show evidence of very high levels of intelligence. Octopi have been observed using tools to build their nests. Some evidence indicates that some cephalopods may be capable of observational learning and even communication.


The chordates include all vertebrates and closely related invertebrates which have a dorsal nerve cord. These are the most structurally complex organisms in the ocean. The invertebrates are divided into the following subphylums:

* Tunicata (or Urochordata) – chordate features only in larval form, including rudimentary brains
* Cephalochordata – no brains or skulls, filter feeders
* Vertebrata (or Craniata) – skulls, spinal columns, digestive tracts

Most fish species fall under Craniata. These range all the way from jawless fish such as hagfish (Class Myxini) and lampreys (Class Petromyzontida), through sharks and rays (Class Chondrichithyes), to lobe-finned bony fishes such as coelacanths and lungfishes (Class Sarcopterygii).

Other major vertebrate classes include:

* Amphibia – cold blooded, unshelled eggs, often with juvenile marine stages
* Reptilia – cold blooded, shelled egg, includes crocodilians, turtles, and sea snakes
* Aves – birds
* Mammalia – mammals

These are the most familiar ocean organisms because they are the largest and they dwell nearest the surface of the ocean. Nearly all ocean birds and mammals also spend at least some of their time on land. Modern amphibians cannot tolerate ocean waters, but a few frogs and toads, such as Bufo marinus, can live in mangrove habitats and other estuarine environments.

The largest ocean animals of all are the whales (Order Cetacea). They are divided into the baleen whales (Suborder Mysticeti) and the toothed whales (Suborder Odontoceti). Baleen whales, including the largest animal on earth, the blue whale (Balanoptera musculus), feed almost exclusively on krill, and can be considered a specialized type of filter feeder. Toothed whales, such as sperm whales, orca, and dolphins, are carnivores.

However, the vertebrates are not usually the apex predators of the ocean. That distinction belongs to the sharks and giant squids. Only the elephant seals and toothed whales can compete with them for an apex position, and then only in the topmost ocean layer. The ocean deeps belong to other creatures.