What is Classification?

Classification is a way of categorizing organisms that enables an easy process of identification of such organisms. All living things are classified by grouping them under specific categories. The grouping is done based on the close relativeness of the organisms. The classification tool requires knowledge of characteristics, differences, and similarities between different organisms.

What are Marine Organisms?

Marine organisms are living entities that inhabit vast areas of seas and oceans. These organisms live in saline water bodies that support a wide range of lives. Marine organisms can be found floating on the surface as well as inhabiting the deep sea. Some marine plants carry out photosynthesis, while some creatures derive food from chemical substances where sunlight does not penetrate.

Marine Organisms classification

Like all organisms, marine organisms are classified based on a hierarchical system. It includes the highest position kingdom, followed by phylum, class, order, family, genus, and species. Marine organisms can be classified by considering various factors, including habitat and mobility, metabolism, size, and developmental stages. These organisms are also scientifically named using genus and species. Genus is ranked above species in the hierarchy.

Types of Marine Organisms

Marine organisms are divided into several groups. These comprise several genera and species of bacterium, protozoans, planktons, marine invertebrates, and vertebrates. Based on habitat and mobility, there are three main groups of marine animals: plankton, nekton, and benthos. Based on the five-kingdom classification, there are numerous bacteria, protozoans, planktons, a very few fungi, plants, and diverse groups of animals. Based on the presence and absence of backbone, there are marine invertebrates and vertebrates. Different groups of marine organisms are discussed below.

Plankton

Planktons are considered floaters. They can be phytoplankton, zooplanktons, or bacterioplankton (plankton containing a bacterium's component). Phytoplankton is also called drifting algae, while zooplanktons are drifting protists and other minute animals. There are marine prokaryotes in a floating or attached state. These are mainly the archaea.

Two types of zooplanktons exist, such as mesozooplankton and macrozooplankton. Copepods are zooplanktons that filter out the phytoplankton present in the marine habitat. Macrozooplanktons are larger than copepods. They mostly serve as nutrition for whales.

Bacterioplankton includes all the bacteria that drift in the water. A bacterium called blue-green algae, which is a cyanobacterium, is the most common bacterioplankton. This bacterium is found in abundant quantities in the ocean. Examples of marine phytoplankton also include diatoms and dinoflagellates. Dinoflagellates are found in abundance, and most species are associated with producing bioluminescence.

Nekton

All swimmers of the marine habitat belong to the nekton community. These swimmers are found at all depths of the seas and oceans. Most of the marine vertebrates are swimmers. Fish are the most common creatures belonging to the nekton community. They are found in open water bodies and are also bottom dwellers. Based on their habitat, they are called pelagic fish and demersal fish.

The nekton community also includes mammals that spend their lives completely in the sea as well as animals that spend their lives both on the sea and on lands. Some examples include whales and seals, respectively.

Benthos

The benthos community comprises marine life, such as marine animals and plants that live in the benthic region of the marine habitat. The benthic region is the sea bottom that extends from the littoral zone to the open sea. Marine life is diverse in this region, and hence the benthic region has a huge variety of species. Out of all the known marine animals, 16% of them belong to the benthos community.

The benthos community is again subdivided into epi-flora and epifauna and in-flora and infauna. Epi-flora and epifauna inhabit the surfaces of sea bottoms. On the other hand, in-flora and infauna live partially or completely buried into the sea bottom. The benthos community feeds on organic matter and nekton animals. Examples of benthos animals include sea stars, several species of mollusks, and scavengers like hagfish.

Plants and Algae

Some plants, such as salt marsh grasses, are found in the saline habitat. Algae such as seaweed also inhabit the seas and oceans. True and false plants, such as salt marsh grasses and seaweeds, respectively, are primary producers. They are capable of photosynthesis owing to the region they are found. It is the neritic region where they can be abundantly observed, and sunlight penetrates this region.

Marine invertebrates

Diverse groups of invertebrates exist in the seas and oceans. Examples include jellyfish, starfish, sponges, and sea worms. Some of the marine invertebrates include Brachiopods, Phoronida, Nemertea, Cephalochordate, and Hemichordate. 

• Brachiopoda are phyla of marine organisms with hard upper and lower shells. 
• Phoronida is a phylum of filter feeders with soft bodies. They have upright chitin tubes for protection. 
• Bryozoan is a phylum consisting of sedentary marine invertebrates. Such organisms live in colonies and are known as sea mats.
• Nemertea is another phylum of marine invertebrates. They comprise worms such as ribbon worms or proboscis worms.
• Porifera is a phylum that includes marine sponges. They are multicellular organisms with several pores and channels. They have cellular-level organization and lack body symmetry. The mode of nutrition is heterotrophic. They also possess spongin and spicules and lead a sedentary marine life.
• The cnidaria phylum of marine organisms has a tissue-level organization. The organisms under this phylum exhibit radial symmetry. The most common examples are sea anemones and jellyfish.
• Ctenophora are swimmers that possess cilia. The organisms under this phylum exhibit rotational symmetry. This phylum comprises organisms like comb jellies.
• Annelida consists of several worms and makes up a large phylum. Examples include burrowing polychaetes and sea leeches.
• Arthropoda phylum has more than 11 lakh species. These species have jointed appendages with strong moving and sensing characteristics. Crustaceans are marine arthropods, and examples include lobsters, crabs, and shrimps.
• Platyhelminthes is another phylum constituting worms. These marine tapeworms are found in the gut of large marine animals. An example includes a marine tapeworm called Polygonoporus giganticus.
• The Mollusca phylum consists of classes of organisms such as Gastropoda, Bivalvia, and Cephalopoda. It is the largest marine phylum. Examples include snails, octopuses, oysters, squid, and cuttlefish.
• Echinoderm phylum consists of organisms that belong to only marine habitats. They have a hard and spiny outer covering or skin. Most of the organisms are intertidal, and a few live on the ocean floor. Examples include sea cucumbers and sea urchins.

Other phyla belonging to marine invertebrates include Tardigrada and Lobopodia. The non-craniate phylum belonging to the marine invertebrates that are closely related to the marine vertebrates are as follows:

• A tunicate is a group of invertebrates found in the seas and oceans. They are found attached to the rocks or other surfaces of the ocean floor. Examples of tunicate include sea squirts and salps.
• Cephalochordata belongs to the phylum chordata but lacks a backbone. Amphioxus is an example of this group of invertebrates found in marine habitats.
• The phylum hemichordate includes deuterostome animals, such as acorn worms. Acorn worms are named as such because of their acorn-shaped front end.

Marine vertebrates

The marine vertebrates have a backbone and comprise the fish, reptiles, and mammals of the seas and oceans. These marine organisms are discussed below.

• Fish are ectothermic vertebrates that are also found in saltwater and lead marine life. They have a streamlined body apt for swimming. The appendages are fins that allow them to cut across the water. The body of a fish is covered with scales for protection. The respiration process occurs through the gills. The gases are exchanged through the gills, and oxygen is taken up from the water. Examples include sardines, sharks, and eels. Some of these marine fish also have swim bladders that help in buoyancy. Some fish also exhibit bioluminescence.
• Reptiles are also found in marine habitats. However, only a few types are found. Examples include sea turtles, snakes, and crocodiles. Apart from sea snakes, all these reptilian species lay eggs on land.
• Mammals are also found in the seas and oceans. They belong to several groups, such as cetaceans, sirenians, mustelids, and pinnipeds. Examples include whales, dolphins, sea lions, sea otters, seals, and walruses. All of these marine lives belonging to the class mammal are endothermic. They give direct birth to the young, are adapted to marine life such as swimming, and their kidneys excrete salt.

Common Mistakes

• Classification and taxonomy are often confused. Classification is assigning and grouping organisms based on two or more attributes. On the other hand, taxonomy is the science of classification, providing hierarchical relationships.
• Cephalochordata and hemichordate are two such marine invertebrates that are classified based on the absence of backbone in each and the region of the presence of notochord. Cephalochordata has a notochord from head to tail, whereas hemichordate has a notochord in the proboscis region.

Context and Applications

The topic is significant for undergraduate, graduate, and doctoral courses, especially for:

• Bachelor of Science in Marine Biology
• Bachelor of Science and Master of Science in Oceanography
• Bachelor of Science and Master of Science in Aquatic Ecology
• Doctor of Philosophy in Oceanography
• Taxonomical Studies
• Curator in museum

Related Concepts

• Marine ecosystem
• Marine organisms
• Classification of freshwater organisms
• Biodiversity of marine life
• Taxonomy

Practice Problems

Q1: Which of the following is not a classification of marine organisms?

(a) Plankton

(b) Neuston

(c) Benthos

(d) Nekton

Correct Choice: (b)

Q2: Benthos organisms live on which of the following?

(a) Ocean floor

(b) Open sea

(c) Ocean basin

(d) All of the above

Correct Choice: (a)

Q3: Identify the example of a benthic organism.

(a) Dolphin

(b) Sardines

(c) Cuttlefish

(d) Sea stars

Correct Choice: (d)

Q4: Which of the following is NOT radially symmetric?

(a) Sea urchin

(b) Jellyfish

(c) Sponges

(d) Coral

Correct Choice: (c)

Q5: Marine sponges are ______.

(a) sedentary

(b) swimmers, or nekton

(c) pelagic

(d) None of the above

Correct Choice: (a)