Marine Animals: Characteristics, Types, and Examples

Written and verified by the biologist Georgelin Espinoza Medina

Last update: 02 January, 2024

Planet Earth is mostly made up of water, both fresh and salt water. The latter covers more than 70% of the Earth’s surface, representing 90% of the habitable space. This environment is home to a great diversity of marine animals, which inhabit all types of ecosystems, from coastal to oceanic.

These animals are adapted to these habitats with all the physical and chemical characteristics of their waters. In addition, they cover almost all the existing groups in the kingdom, and they’re numerous and variable in shape, size, coloration, and lifestyle. Therefore, we invite you to get to know the existing types, their distinctive features, and some examples. You can’t miss it!

The characteristics of marine animals

Although it’s difficult to delimit the characteristics of marine animals, thanks to the great diversity that exists, let’s look at some aspects that stand out in them:

1. Size

First of all, it’s important to mention that they don’t have a uniform size, as they range from very small–even microscopic–up to much larger proportions. In the heavyweight category is the blue whale (Balaenoptera musculus), the largest marine animal in the world, with almost 30 meters in length.

2. Shape and structure

Just as their size can’t be unified, neither can their morphology. Thanks to the variety of taxa existing in these ecosystems, they have different aspects and characteristics, such as the following:

• Elongated
• Flattened
• Irregular
• Circular shapes
• With or without a shell

Some have fins, tentacles, thick skins, fangs, and teeth, among many other important structures and adaptations.

These animals also possess the ability to regulate the amount of salt in the environment. This is achieved through various mechanisms that allow them to maintain their metabolism in complete equilibrium.

3. Movement

There are sessile species–those that don’t move, that is, they’re fixed to a substrate- or mobile species. Some can move, albeit slowly, while others are agile and fast swimmers. Similarly, certain types of animals have little movement and move at the mercy of ocean currents.

Other animals don’t even spend their entire lives in the water. But they do spend most of the time there. In this case, we have as an example the seabirds that feed in coastal environments and live in the surroundings.

In addition, there are marine organisms that carry out large migrations per year, which are subject to the following variables:

• Environmental conditions
• Reproductive or breeding season
• Resources available for feeding

4. Feeding

The diet of marine animals varies according to the type of organism. Intake may include bacteria, algae, plants, or animals. The basis of the aquatic food chain is phytoplankton.

5. Respiration

Regarding the way in which they obtain oxygen from the environment, some marine animals take it from the water, while others take it from the air. Therefore, they present different structures based on each species. Some of these include the following:

• Through the skin
• With lungs
• With gills

Thus, some of these animals can’t spend all their time submerged underwater and need to come to the surface to breathe.

6. Reproduction

The reproductive classification of these beings is diverse, according to the species. In this regard, there are organisms with separate sexes or hermaphrodites. In addition, they can be distinguished based on the type of fertilization (internal or external), and even by the gestation process, that is, if they are oviparous, ovoviviparous, or viviparous.

There are also animals that multiply asexually, and others have the capacity to regenerate.

What are the marine animals?

As we’ve already mentioned, there are many types of animals that inhabit marine waters. In fact, almost all taxa are represented, both invertebrates and vertebrates, occupying everything from the coasts to the depths of the sea. Let’s take a look at some types and examples!

Among all the existing diversity, we have marine organisms in the following phyla of the animal kingdom:

1. Porifera (Porifera)

Sponges or porifera are aquatic animals that are often colonial. They inhabit marine areas, although there are also freshwater sponges. Their bodies are made up of numerous pores and canals, with a simple organization that doesn’t go beyond the tissue level. This means that the cells of the organisms are grouped in a layer or even tissue.

Adults are sessile and live attached to the substrate. They lack nerve cells and organized muscle fibers. In addition, they reproduce both sexually and asexually.

There are about 9,655 species of sponges. Most of them occupy marine environments in tropical, temperate, and polar waters.

Examples of marine porifera include the following:

• Ciliated sponge or Sicom ciliatum, creamy yellow in color.
• Fire sponge or Tedania ignis, bright orange in color.
• Giant barrel sponge or Xetospongia muta, variable in color from salmon pink to violet.

2. Cnidarians (Cnidaria)

A taxon that includes diverse animals such as polyps, hydras, jellyfish, sea anemones, and corals. It owes its name to the presence of unique stinging cells: Cnidocytes. They have two body forms, the polyp and the jellyfish. The former is sessile and fixed to a substrate, while the latter is floating and swimming. Some live in association with other species, in commensalism or mutualism. Examples of cnidarians include the following:

• The Portuguese man o’war or Physalia physalis.
• The golden anemone or Condylactis aurantiaca.
• The staghorn coral or Acropora cervicornis.

3. Flatworms (Platyhelminthes)

Flatworms have soft, unsegmented bodies and are noted for their regenerative capacity. Although most are parasitic, there are free-living species included in the turbellarians group. These have a variety of sizes and can range from microscopic to half a meter in length, but most measure less than 5 millimeters.

Some forms of flatworms are parasites of animals such as mollusks, arthropods, and annelids, among others.

As an example of marine flatworms, we can mention the species Thysanozoon nigropapillosum, which has the common name of yellow-spotted flatworm. This is due to its appearance, as it has papillae with yellow tips that stand out on its brown body.

4. Mollusks (Mollusca)

This is the second most successful group of animals, due to the large number of living species (after arthropods). Just as there are numerous specimens, we find a diversity of forms in this taxon.

Mollusks are soft-bodied animals, which may be enclosed in a calcium carbonate shell. They also have diverse sizes, from a few millimeters or inches – as is the case of some gastropods – to various feet in length. Therefore, we have some examples:

• Giant clams (Tridacna) that measure up to 4.5 feet
• Sea hares (Aplysia) measuring between 1.3 and 3.3 feet centimeters, such as A. dactylomela, which can reach up to 1 foot in length.
• Giant squids (Architeuthis), which can reach impressive body lengths of 26.25  feet, and up to 72 feet with their tentacles extended, according to Encyclopedia Britannica.

5. Annelids (Annelida)

These animals have a vermiform or worm-like body that consists of several segments. These usually extend to the outer plane, with circular grooves or rings. Most have hair-like structures, which may be located on appendages called parapodia.

Annelids have well-developed internal organs.

They also occupy a variety of habitats. Some are free-living (marine, freshwater, or terrestrial), and some are parasitic. Examples of annelids living in marine environments include:

• The Bobbit worm, or Eunice aphroditois, which can reach up to 10 feet in length.
• The sand castle worm, or Phragmatopoma californica, which is about 2.75 inches long.
• The Korean earthworm, or Perinereis aibuhitensis, which is a sediment-dwelling species that can measure up to 7.8 inches long.

6. Arthropods (Arthropoda)

They’re characterized by a chitinous body covering that forms a protective exoskeleton and molts from time to time. In addition, they usually have a segmented body with jointed appendages, a feature to which they owe their name.

It’s a large group. In fact, it’s the most species-rich in the kingdom Animalia. They’re found in all habitats of the planet, with a diversity of adaptations. They can be found in both aquatic and marine environments. Here are some species names:

• The sea spider Pycnogonum litorale (Pycnogonida), which has a great regenerative capacity
• The Antarctic krill (Euphausia superba), which is a small crustacean that’s very important for the feeding of animals such as whales
• The giant isopod Bathynomus raksasa, a crustacean with an average length of 13 inches nicknamed the “sea roach.”

7. Echinordemos (Echinodermata)

This group is characterized by a hard and spiny body shell. They also have an internal skeleton of calcareous nature and a remarkable aquifer system. There’s a diversity of shapes and sizes, from a few inches to several feet, as is the case of certain sea cucumbers.

There are more than 10,000 species, all of them marine. Here are a few examples:

• The red sea urchin Strongylocentratus franciscanus
• The brittle sea star Ophioderma occultum.
• The sunflower sea star Pycnopodia helianthoides, with a greater number of arms (between 15 and 24) and a great weight (11 pounds) and size (around 2.6 feet in diameter).

8. Chordates (Chordata)

Here we have a variable group, with three subphyla, which have the following marine representatives:

• Tunicates
• Vertebrates
• Cephalochordates

They’re characterized by a rod or support structure called notochord, a post-anal tail, a doral nerve cord, and a segmented body, among other aspects.

They exist in different sizes, from a few millimeters to several feet. They can occupy the water column (pelagic) or the bottom (benthic). They’re also solitary or colonial. Here, we find a diverse group of vertebrates, such as fish, reptiles, birds, and mammals. Here are some examples:

• The rough sea squirt (Styela canpus), which is benthic, filter-feeding, and barely 1 inch long.
• The leatherback sea turtle (Dermochelys coriacea), a pelagic, swimming, carnivorous specimen, measuring up to 7 feet in length.
• The killer whale (Orcinus orca), a pelagic, swimming, carnivorous cetacean, which can measure around 26 feet in length.

Other phyla that include marine animals

The phyla named so far are the most well-known. However, they’re not the only existing ones, and there are other less popular ones that include marine species. Among them, we can mention the following:

1. Placozoa (Placozoa)

Also called plate animals, they’re very small, about 2 or 3 millimeters long, and a few microns thick, although there’s one species that measures more than that. They’re similar in appearance to amoebae, ciliated, and without specialized organs. They inhabit tropical and subtropical marine waters.

Placozoans represent one of the oldest phyla of the animal kingdom. Despite this, many aspects of their biology are still unknown, and there are only four species.

2. Ctenophora (Ctenophora)

Popularly called comb jellies, they’re cnidarian-like creatures. The phylum comprises about 214 species, all marine. They have a variety of shapes and sizes, some less than 1 centimeter and others up to 6.5 feet in length. Most of them are colorless, and they can also be bioluminescent.

3. Nemerteans (Nemertea)

These are other types of unsegmented worms, mostly marine, that can swim freely. They occupy diverse environments, from the intertidal region to deeper waters. They have different sizes (some are only a few millimeters, while others measure several feet long) and colors. There are also commensal or parasitic species.

4. Rotifera (Rotifera)

These are aquatic invertebrates that have representatives from marine or brackish environments, although most of them inhabit fresh waters. They’re small, from just a few microns to 3 millimeters in length, with a diversity of shapes.

Their name derives from a ciliated crown they use for locomotion and feeding, with which they make rotating movements. However, some species are sessile or sedentary.

5. Cyclophores (Cycliophora)

In this case, we refer to marine invertebrates that are symbionts of lobsters, as they inhabit their mouthparts. They can be found from the intertidal zone to depths of 2,362 feet. Only two species have been discovered.

6. Gastrotrichs (Gastrotricha)

These worms are called hairybellies or hairybacks. They’re small in size (up to 3 millimeters in length). There are marine, freshwater, brackish, or even semi-terrestrial representatives. Marine species are generally found in interstitial spaces in looser sediments. Their bodies are transparent and have a head and a trunk.

7. Entoproctos (Entoprocta)

These are usually members of marine environments and are distributed among the world’s oceans, from intertidal zones to depths of 1,640 feet, although most of them are coastal. They’re sessile, solitary, or colony-forming.

They’re notable for their hydroid cnidarian-like appearance, with cup-shaped bodies possessing a ring of ciliated tentacles.

8. Bryozoa

Here, we have aquatic beings (marine or freshwater) of microscopic sizes. Nevertheless, they form large colonies that cover algae, stones, shells, and even ship hulls and water pipes. They’re diverse and abundant. Therefore, there are about 20,585 living species.

9. Chetognathans (Chaetognatha)

They’re popularly known as arrow worms due to the presence of a pair of lateral fins and a caudal fin that resemble these objects. They occupy marine environments and some estuaries. They’re small, from 1 millimeter to 4.7 inches in length. They can be transparent, orange, or opaque in color.

10. Kinorrhynchus (Kinorhyncha)

These are little-known marine animals that can also be found in brackish waters. They’re found from intertidal to abyssal zones at depths of 26,247 feet. They’re benthic (bottom-dwelling in aquatic ecosystems), mobile, and have a yellowish-brown body covered with spines, which can barely reach 1 millimeter in length.

11. Priapulids (Priapulida)

This is a phylum with a small number of 22 species. They’re marine animals and live buried in mud or sandy bottoms. They’re only a few centimeters long and have a cylindrical body divided into two regions: A proboscis (or prosoma) and the trunk.

12. Nematodes (Nematoda)

These marine animals look like worms, are elongated, and vary in size. Although most nematodes are small, some reach up to more than three feet in length. They can be free-living (found in the sea, soil, and freshwater) or parasitic, both on plants and animals, including humans.

13. Nematomorphs (Nematomorpha)

Also known as horsehair worms, there are only four planktonic marine species. They can measure up to 3.3 feet long and 3 millimeters wide and have dark shades. In addition, they have the peculiarity of being parasites of crustaceans during their larval stage.

14. Brachiopods (Brachiopoda)

Brachiopods are similar to bivalve mollusks due to the presence of a shell. They’re marine, benthic, solitary, and filter-feeding. Likewise, they have small dimensions, measuring from 1 millimeter to 3.5 inches.

15. Phoronids (Phoronida)

Phoronids are a small taxon of marine animals with about three species. They’re marine animals and sessiles and are found in all parts of the world. They have a cylindrical trunk from which a crown of ciliated tentacles emerges at one end.

Endangered marine animals

Among such a variety of creatures, it should also be mentioned that many marine animals are in danger of extinction. The threats are diverse and include fishing, pollution, and climate change, to name a few. Examples of species on the International Union for Conservation of Nature (IUCN) red list include the following:

• The staghorn coral is critically endangered by extreme temperatures and ocean acidification.
• The great white shark (Carcharodon carcharias) is classified as vulnerable as a result of being captured worldwide.

Diverse and surprising marine life

As we’ve seen, the diversity of specimens is astonishing. We find everything from microscopic beings to others of unimaginable proportions, as among them, we have the largest mammal, measuring almost 100 feet in length. Did you know about all these types of marine animals? We invite you to continue discovering the curiosities hidden in the animal world!