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The Otolith Sectioning Process

In 2003, the Age and Growth Lab processed over 19,000 otoliths from 23 species of fish and aged 9,500 from 4 different species. Read this article to find out how FWRI processes and uses otoliths for research.

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Otoliths, commonly referred to as earstones, are removed from the fish’s head either by entering through the top of the head, or by pulling back the gill. Once the sagittal otoliths are removed from the fish, they are cleaned, dried, and stored in a vial.

Thin, transverse sections (approximately 300 µm to 400 µm) are cut with a low speed saw. Generally, 4 blades, each separated by a 400 µm aluminum spacer, are used to yield 3 sections. The left otolith is processed for consistency. The right otolith is archived if the left is successfully processed and aged.

A few anatomical features are prominent on the otolith. A small groove or channel runs along the proximal face of the otolith. This is called the sulcus, and it generally runs parallel with and closer to the dorsal edge and then curves downward away from the dorsal edge. The narrow portion of the sulcus is called the cauda. The cauda joins a more open area called the ostium. It sometimes has an oval shape to it, but the shape varies among species. The cauda curves to the posterior of the fish, and the ostium points toward the anterior. Together, they resemble a tadpole. Always remember the head of the tadpole points in the same direction as the head of the fish, and the tail curves downward and toward the rear. The otolith that looks like a tadpole facing right is the left otolith.

To section the otolith, specimens are hot-glued to small pieces of tag paper, which are held to the saw arm by a small binder clip. The specimen is slowly lowered onto the spinning blades of the saw. The blades dip into a water bath for lubrication.

The newly cut sections are then rinsed in water and dried on blotting paper. Dry sections are mounted on a microscope slide and covered with Flo-Texx mounting media.

Otolith formation begins at the earliest stage in fish development. The “core” or “focus” of the otolith is the point at which the otolith originates. The exact location of the core of an adult specimen is different for every species. It is generally a thickening located behind the ostium. It is important to mark the core before cutting to ensure a core section is obtained. Sections that are anterior or posterior to the core are difficult to age. Figure 9 shows examples of anterior, core, and posterior section from a red drum, Sciaenops ocellatus. A perfect section is characterized by a sharp V-shape, with the point of the V ending at the core. Sections that are anterior to the core have an incomplete V that does not come to a point. A posterior section is distinguishable by a funnel or tornado-like shape. Notice the fading of the first annulus in the anterior and posterior sections.

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Vocabulary

Proximal. Near the center of the body.

Anterior. Near the head.

Posterior. Near the tail.

Dorsal. Near the back.

Ventral. Near the belly.