One of the facts of life for humans is the replacement of baby teeth with permanent adult teeth. Whether pulled out prematurely, wiggled loose by eager hands or naturally falling out unexpectedly, this occasion marks an important milestone in the maturation process that is shared amongst all vertebrates in some form. 

Imagine, though, losing and replacing a tooth every single day. For Pacific lingcod, this isn’t some fairy tale gone wrong or the unfortunate result of dismissing dentist recommendations, and is in fact their reality. In a paper recently published in the Royal Society, researchers from the University of Washington’s Friday Harbor Laboratories assessed over 10,000 teeth in 20 fish to find a daily replacement rate of about 3%.

Emily Carr studies lingcod using a florescent technique called pluse-chase
Emily Carr studies lingcod using a florescent technique called pluse-chase

Using a fluorescent technique called pulse-chase, University of South Florida undergraduate Emily Carr, University of Washington Ph.D. candidate Karly Cohen and professor Adam Summers designed an experiment to track the growth of teeth in these carnivorous fish. Using this method, the researchers were able to observe tooth growth and replacement while the organisms were still alive — which is not the case for other research methods such as CT scans.

The experiment involved transporting the fish from their normal saltwater tank into a tank filled with red dye, kept at a constant temperature and covered with a black tarp to block out light. The lingcod remained in this solution for 12 hours before they were returned to the normal tank. The red dye will stick onto anything hard or mineralized, and will remain in the mineralized tissue even after the organisms are transferred out of the tank containing the dye — allowing Carr, Cohen and Summers to mark off the teeth that were present in the lingcod’s mouth during their tenure in the red dye tank.

The fish stayed in the normal tank for 10 days, then were moved to a tank containing green dye for another 12 hours. At the end of this process, the researchers could tell which teeth were present for the first treatment if they were both red and green, whereas new teeth were just green. Using this information, they could then deduce how quickly lingcod lost and grew teeth, locate tooth replacement “hotspots,” identify tooth replacement patterns, and compare tooth replacement in feeding versus fasting fish.

Imaging showing teeth that are green (new growth) and red (old growth)
Fluorescent microscopy images of each toothed element showing old teeth (at least partial red fluorescence) and newly replaced teeth (green fluorescence): (a) vomer (includes palatine, surrounded by dashed line), (b) dentary, (c) upper pharyngeal jaw and (d) lower pharyngeal jaw. All toothed elements with the anterior towards the top right corner. Scale 1 mm.

After sitting in a dark room for days to count over 10,000 lingcod teeth, Carr found that lingcod replace approximately 20 teeth a day, with 500 total teeth in their mouths at any given time — which is approximately a 3.6% replacement rate. Much like in humans, tooth damage in lingcod doesn’t drive tooth replacement. When separated into feeding and non feeding groups, no significant differences were found in tooth replacement rates. Carr was able to find hotspots at regions of great replacement frequencies (60% or greater) towards the back of the mouth, signaling that the teeth experiencing greater stress had a higher replacement rate. Bigger teeth in less risky positions, such as those towards the front of the mouth, showed slower replacement rates. Much like in humans, tooth replacement is predetermined — teeth will be replaced by the same kind of teeth (e.g., molars will replace molars), and teeth don’t grow bigger over time.

“We know shockingly little and a lot about teeth,” said Cohen. “We know that the same genetic network is responsible for tooth development in fish, but we know very little about what controls tooth replacement. This study laid the groundwork for others to look at the evolutionary origins of tooth replacement, and allows us to ask bigger questions about its mechanics.”

This work was made possible thanks to the FHL Adopt-a-Student Program Fund, a Kozloff undergraduate endorsement fund to Carr, NSF and Seaver Institute grants to Summers, and the Stephen and Ruth Wainwright Endowment, Edwards Award, Wingfield-Ramenofsky Award and Orians Award to Cohen.