Classifying marine organisms

Classification is an important tool used by scientists to show how organisms are related to each other and to group them by their characteristics, but this can be difficult for some marine organisms!

What does classification involve?

Classification involves grouping organisms into a series of hierarchical categories: kingdom, phylum, class, order, family, genus and species. These categories were first developed by Carl Linnaeus in the 18th century and have remained in common use ever since.

However, there is significant debate amongst scientists about the groupings of organisms within these categories. For example, when Linnaeus first described his system, he named only 2 kingdoms – animals and plants. Today, most scientists recognise at least 5 kingdoms, some argue that there are at least 10 and others debate the value of a further category, called a domain, that would sit above kingdom.

Six kingdoms of marine organisms

All kingdoms are represented in the marine environment, and most scientists classify marine organisms into one of the following 6 kingdoms.

• Bacteria are single-celled organisms that reproduce by splitting in two. Bacteria live throughout the marine environment. They play a crucial role in ecosystems, breaking down organic material and making nutrients available for the phytoplankton.
• Protozoans are single-celled organisms that are generally much larger than bacteria. They may be autotrophic or heterotrophic. In the marine environment, this kingdom is well represented and includes amoebae.
• Chromists range from very small organisms such as diatoms (a type of phytoplankton) to seaweeds. Most chromists photosynthesise but there are some significant differences that have led scientists to classify them separately to plants, for example, they use a different kind of chlorophyll.

• Fungi rely on breaking down organic material as they are not able to make their own food. There are very few fungi in the marine environment.
• Plants are multi-cellular and autotrophic – they use photosynthesis to produce food using sunlight. Plants are much more widespread on land and in freshwater, and there are only a few types that thrive in the marine environment, for example, eel grass and mangroves. Seaweeds were previously classed as plants before they were reclassified as chromists.
• Animals are typically large and multi-cellular. They are heterotrophic and rely on other organisms for food. Animals in the marine environment include jellyfish, sponges, sea spiders, bryozoans, mussels, sea stars, fish and whales.

Find out more about how scientists are using a new technique called DNA barcoding to help classify species.

Animal characteristics

Animals can be broadly divided into invertebrates (such as crabs and sea stars) and vertebrates (such as fish and dolphins). Key characteristics of animals include the following:

• They need to obtain energy by consuming other organisms (heterotrophic): Marine animals have evolved a huge variety of adaptations to help them find food. For example, a sea star can extend its stomach outside its body to digest its prey, such as mussels. Cockles have siphon like structures that they use to filter organic material and phytoplankton suspended in the water. Bryozoans band together into colonies to make it easier to capture food as it floats past.
• They are able to move: This applies to almost all animals although there are a couple of examples in the marine environment where this might be more difficult to spot! For example, a number of marine animals only move about in their larval stage. As adults, they may be permanently attached to a surface, for example, barnacles.
• Their bodies are made up of multiple cells In almost all animals, their cells are arranged into tissues that perform different functions. This sets them apart from other organisms such as bacteria that are made up of a single cell.
• Sexual reproduction Almost all animals reproduce sexually, when a sperm and an egg combine to produce a juvenile animal. Some animals, such as sea stars, are also able to reproduce asexually and can reproduce by splitting themselves in half.

Plant or animal?

With some marine organisms, it can be difficult to tell what kingdom they belong to. For example, a colony of bryozoans can often look a lot like a plant. However, look very closely, and you will see that they are actually made of hundreds of tiny individual animals called zooids. These zooids look like tiny worms with a nose that has tentacles around the outside.