Marine Herbivores: Species and Characteristics

An herbivore is an organism that feeds on plants. These organisms are referred to with the adjective herbivorous. The word herbivore comes from the Latin word herba (a plant) and vorare (devour, swallow), meaning "plant-eating." An example of a marine herbivore is the manatee.

The opposite of an herbivore is a carnivore or "meat-eater." Organisms that eat herbivores, carnivores, and plants are referred to as omnivorous.

Size Matters

Many marine herbivores are small because only a few organisms are adapted to eat phytoplankton, which provides the bulk of the "plants" in the ocean. Terrestrial herbivores tend to be larger since most terrestrial plants are large and can sustain a large herbivore.

Two exceptions are manatees and dugongs, large marine mammals who survive primarily on aquatic plants. These animals live in relatively shallow areas, where light is not limited, and plants can grow larger. 

Advantages and Disadvantages of Being an Herbivore

Plants such as phytoplankton are relatively abundant in ocean areas with access to sunlight, such as in shallow waters, at the surface of the open ocean, and along the coast. An advantage of being an herbivore is that food is pretty easy to find and eat. Once it is found, it can't escape like a live animal might.

One of the disadvantages of being an herbivore is that plants are often more difficult to digest than animals. More plants may be needed to provide adequate energy for the herbivore. 

Examples of Marine Herbivores

Many marine animals are omnivores or carnivores. But there are some marine herbivores that are well-known. Examples of marine herbivores in various animal groups are listed below.

Herbivorous Marine Reptiles:

• Green sea turtle (who are named for their green fat, which is green because of their plant-based diet)
• Marine iguanas

Herbivorous Marine Mammals:

• Manatees
• Dugongs

Herbivorous Fish

Many tropical reef fish are herbivores. Examples include: 

• Parrotfish
• Angelfish
• Tangs
• Blennies

These coral reef herbivores are important to maintaining a healthy balance in a reef ecosystem. Algae can dominate and smother a reef if herbivorous fish aren't present to help balance things out by grazing on the algae. Fish can break down the algae using a gizzard-like stomach, chemicals in their stomach, and intestinal microbes.

Herbivorous Invertebrates

• Some gastropods, including as limpets, periwinkles (e.g., the common periwinkle), and queen conchs.

Herbivorous Plankton

• Some zooplankton species

Herbivores and Trophic Levels

Trophic levels are the levels at which animals feed. Within these levels, there are producers (autotrophs) and consumers (heterotrophs). Autotrophs make their own food, while heterotrophs eat autotrophs or other heterotrophs. In a food chain or food pyramid, the first trophic level belongs to the autotrophs. Examples of autotrophs in the marine environment are marine algae and seagrasses. These organisms make their own food during photosynthesis, which uses energy from the sunlight.

Herbivores are found at the second level. These are heterotrophs because they eat the producers. After herbivores, carnivores and omnivores are at the next trophic level, since carnivores eat herbivores, and omnivores eat both herbivores and producers.

Sources

• “Herbivory in Fish.” Herbivory in Fish | Department of Microbiology, https://micro.cornell.edu/research/epulopiscium/herbivory-fish/.
• Map of Life - Convergent Evolution Online, http://www.mapoflife.org/topics/topic_206_Gut-fermentation-in-herbivorous-animals/.
• Morrissey, J.F. and J.L. Sumich. Introduction to the Biology of Marine Life. Jones & Bartlett Learning, 2012.