Phylum Definition

The Definition of Phylum, with List of Marine Phyla and Examples

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The word phylum (plural: phyla) is a category used to classify marine organisms.

How Are Marine Organisms Classified?

There are millions of species on Earth, and only a small percentage of them have been discovered and described. Some organisms have evolved along similar paths, although their relationship to each other isn't always obvious. This evolutionary relationship between organisms is known as the phylogenetic relationship and can be used to categorize organisms.

Carolus Linnaeus developed a system of classification in the 18th century, which involves giving each organism a scientific name, then placing it in broader and broader categories according to its relationship to other organisms. In order of broad to specific, these seven categories are Kingdom, Phylum, Class, Order, Family, Genus, and Species. 

Definition of Phylum

As you can see, Phylum is one of the broadest of these seven categories. While animals in the same phylum can be very different, they all share similar characteristics. For example, we are in the phylum Chordata. This phylum includes all animals with a notochord (vertebrates). The rest of the animals are divided up into a very diverse array of invertebrate phyla. Other examples of chordates include marine mammals and fish. Even though we are very different from fish, we share similar characteristics, such as having a spine and being bilaterally symmetrical.

List of Marine Phyla

The classification of marine organisms is often under debate, especially as scientific techniques get more sophisticated and we learn more about the genetic makeup, range, and populations of different organisms. The major marine phyla currently known are listed below.

Animal Phyla

The major marine phyla listed below are drawn from the list on the World Register of Marine Species.

  • Acanthocephala - These are parasitic worms that live in the guts of vertebrates and invertebrates. They have a thorny proboscis and may also have spines on their bodies.
  • Annelida - This phylum contains segmented worms. Earthworms are a familiar type of annelid to us. In the ocean, segmented worm species include such beautiful animals as Christmas tree worms.
  • Arthropoda - Many familiar types of seafood, such as lobsters and crabs, are arthropods. Arthropods have a hard exoskeleton, a segmented body, and jointed legs.
  • Brachiopoda - This phylum includes lamp shells.
  • Bryozoa - Bryozoans are invertebrates that are also known as moss animals. They are colonial organisms that primarily live in colonies of individuals, and may encrust seagrassesmangrove roots, shells, pilings, docks, and other underwater structures.
  • Cephalorhyncha - A group of worms that includes spiny-crown worms, loriciferans, horsehair worms, and priapulid worms.
  • Chaetognatha - This is another group of worms called arrow worms.
  • Chordata - This phylum is probably one of the most familiar to us. We are included in the Phylum Chordata, which includes all animals with a nerve cord (called a notochord) at some stage of their development. Marine life in this phylum includes marine mammals (cetaceans, pinnipeds, sirenians, sea otterspolar bears), fishtunicates, seabirds, and reptiles.
  • Cnidaria - This phylum includes such colorful sea creatures as corals, sea anemones, sea jellies (jellyfish), sea pens, and hydras.
  • Ctenophora - Ctenophores (pronounced "teen-o-fors") are jelly-like animals. This phylum includes comb jellies or sea gooseberries. These are clear, often bioluminescent animals that don't have stinging cells like the cnidarians.
  • Cycliophora - The World Register of Marine Species recognizes two species of this organism, also known as a wheel wearer.
  • Dicyemida - Dicyemids are parasitic organisms that live in cephalopods.
  • Echinodermata - This phylum includes sea stars, brittle stars, basket stars, sea lilies, feather stars, sand dollars, sea urchins, and sea cucumbers.
  • Echiura - Echiurans are also called spoon worms. They have a proboscis and small hooks on their posterior (back) end.
  • Entoprocta - This phylum contains entoprocts, or goblet worms. These are small, transparent worms that are fixed to a substrate and may live individually or in colonies.
  • Gastrotricha - This phylum includes several hundred species of small animals that live on plants, in between grains of sand and on detritus.
  • Gnathostomulida - This is another phylum containing worms, called jaw worms. They are named because of their forceps-like jaw.
  • Hemichordata - This phylum contains worm-like animals that share some characteristics with chordates, including having nerve cords.
  • Mollusca - This diverse phylum includes an estimated 50,000 to 200,000 species of snails, sea slugs, octopuses, squids, and bivalves such as clams, mussels, and oysters.
  • Nematoda - ​Nematodes, or roundworms, are worm-like organisms that are very abundant in nature, and can be decomposers or parasitic. An example of roundworms in the marine environment is animals in the genus ​Robbea, which live in the sediment around seagrass beds.
  • Nemertea - The Phylum nemertea contains ribbon worms, slender worms of which there are more than 1,000 species. Some ribbon worms can grow more than 100 feet in length.
  • Phoronida - This is another phylum that contains worm-like organisms. These are called horseshoe worms, and they are thin organisms that live in chitinous tubes they secrete.
  • Placozoa - Placozoans are simple animals that were discovered in the 1800s in an aquarium in Europe. All that is known of these animals have been learned from animals observed in aquaria.
  • Platyhelminthes - Animals in the platyhelminthes phylum are flatworms. Flatworms are unsegmented worms that may be free-living or parasitic.
  • Porifera - The phylum porifera includes sponges. The word porifera comes from the holes in the sponges—it comes from the Latin words porus (pore) and ferre (bear), meaning "pore-bearer". The holes are pores through which the sponge draws in water for feeding, and expels wastes.
  • Rotifera - This phylum contains rotifers, also known as "wheel animals" from the wheel-like motion of the cilia on their head.
  • Sipuncula - The Phylum Spipuncula contains animals called peanut worms, because some are shaped like peanuts. This phylum contains several hundred species, most of which live in shallow waters. Species may burrow in sand, mud, or even rock. They may also live in crevices or shells.
  • Tardigrada - Animals in the Phylum Tardigrada are called "water bears." These tiny animals look and move surprisingly like a bear. Some tardigrades live in the ​Arctic Ocean.

Plant Phyla

According to the World Register of Marine Species (WoRMS), there are nine phyla of marine plants. Two of them are the Chlorophyta, or green algae, and the Rhodophyta, or red algae. The brown algae are classified in the WoRMS system as their own Kingdom—Chromista.

References and Further Information:

  • Morrissey, J.F. and J.L. Sumich. 2012. Introduction to the Biology of Marine Life. Jones & Bartlett Learning. 467pp.
  • WoRMS Editorial Board. 2015. World Register of Marine Species
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Kennedy, Jennifer. "Phylum Definition." ThoughtCo, Aug. 26, 2020, Kennedy, Jennifer. (2020, August 26). Phylum Definition. Retrieved from Kennedy, Jennifer. "Phylum Definition." ThoughtCo. (accessed April 1, 2023).