Rostrum

Definition of Rostrum, As Used in Marine Life

Image Showing a Lobster Rostrum
Cooked lobster, with the rostrum visible as the projection that extends between the animal's eyes. Foodcollection/Getty Images

The term rostrum is defined as an organism’s beak or a beak-like part. The term is used in reference to cetaceans, crustaceans and some fish. 

The plural form of this word is rostra.

Cetacean Rostrum

In cetaceans, the rostrum is the upper jaw or “snout” of the whale.

According to the Encyclopedia of Marine Mammals, the term rostrum also refers to the skull bones in the whale that provide support for the rostrum.

Those are  the forward (anterior) parts of the maxillary, premaxillary and vomerine bones. Essentially, it is made up of the bones we have between the bottom of our nose and our upper jaw, but the bones are much longer in cetaceans, especially baleen whales. 

Rostrums look different in toothed whales (odontocetes) versus baleen whales (mysticetes). The toothed whales have a rostrum that is usually dorsally concave, while baleen whales have a rostrum that is ventrally concave.  More simply put, the top part of a toothed whale's rostrum is shaped more like a crescent moon, while a baleen whale's rostrum is shaped more like an arch. The differences in rostrum structure become pretty evident when viewing images of cetacean skulls, as is shown in the FAO identification guide here.

The rostrum in a cetacean is a strong, relatively hard part of the anatomy. Dolphins can even use their rostra to 

Crustacean Rostrum

In a crustacean, the rostrum is the projection of the animal's carapace that extends forward of the eyes. It projects from the cephalothorax, which is present in some crustaceans and is the head and thorax together, covered by a carapace.

The rostrum is a hard, beak-like structure.  In a lobster, for example, the rostrum projects between the eyes.

It looks like a nose, but it is not (lobster smell with their annentules, but that's another topic). Its function is thought to be simply to protect the lobster's eyes, especially when two lobsters have a conflict.

The Lobster Rostrum's Contribution to History

In the 1630's, European warriors wore a "lobster tail" helmet that had overlapping plates hanging from the back to protect the neck and a nasal bar in the front, modeled after a lobster's rostrum.  Oddly enough, lobster rostrums have also been used as a cure for kidney stones and urinary diseases. 

In shrimp, the rostrum is also known as the head spine, which is a hard projection between the animal's eyes. 

In barnacles (which are crustaceans but don't have visible eyes like lobsters do, the rostrum is one of the six shell plates that make up the animal's exoskeleton. It is the plate located on the anterior end of the barnacle. 

Fish Rostrum

Some fish have body parts that are referred to as a rostrum. These include billfish such as sailfish (the long bill) and sawfish (the saw).

Rostrum, As Used in a Sentence

  • When the minke whale surfaces to breathe, its rostrum usually appears first, followed by the top of its head and its back.
  • I needed to pass a kidney stone, so I roasted a lobster's rostrum and then mashed it up and dissolved it in wine (yes, this was purportedly a cure for kidney stones in the Middle Ages and Renaissance). 

    References and Further Information:

    • American Cetacean Society. Cetacean Curriculum.Accessed October 30, 2015.
    • Natural History Museum of Los Angeles County. Crustacean Glossary. Accessed October 30, 2015.
    • Perrin, W.F., Wursig, B. and J.G.M. Thewissen. Encyclopedia of Marine Mammals. Academic Press. p.1366.
    • St. Lawrence Global Observatory. American Lobster - Characteristics. Accessed October 30, 2015.
    • The Lobster Conservancy. 2004. Lobster Biology. Accessed October 30, 2015.
    • University of Bristol. Crustacea. Accessed October 30, 2015.