Many fishes like sharks are known for their ability to regrow sets of teeth. Regrowing teeth comes in handy if a tooth breaks or is worn down over time. Fishes also often have a second set of jaws – pharyngeal jaws – which hide in their throats. These jaws and the teeth upon them are used to crush food, in contrast to oral jaws, which grab morsels. Previous tooth replacement studies have focused on fishes with tooth shapes specialized to the specific food they consume. But little was known about tooth replacement in fish that eat most anything, such as Pacific lingcod (Ophiodon elongatus) with simple cone-shaped teeth. What are the tooth replacement rates in either set of jaws? Is a new tooth fated to replace another of the same size? And does feeding influence tooth replacement rates? To study these questions, Emily Carr from the University of South Florida, USA, and colleagues from the University of Washington, USA, compared tooth replacement rates and positions in the oral and pharyngeal jaws of Pacific lingcod.
The scientists used special fluorescent staining techniques, called pulse–chase, to monitor new versus old teeth so that new teeth would fluoresce as green and old teeth would fluoresce as red. First, the team separated the fish into two groups – one that was fed and a second that was fed nothing to determine whether feeding affects tooth replacement rates. The scientists then used a microscope and fluorescent lamp to monitor the replacement rates of the fishes’ teeth, as well as the size and location of new teeth, on both the oral and pharyngeal jaws.
It turns out that the size and locations of replacement teeth are fated. New teeth erupted as small cone-shaped structures next to the teeth that they were destined to replace, before growing to the size of the original teeth. In addition, small teeth were replaced more often than large teeth, as they broke more often. The team also found that the tooth replacement rate was mostly similar across the pharyngeal and oral jaws, except in the lower pharyngeal jaw, where the replacement rate was nearly twice as fast, possibly because food hits these teeth at many different angles thanks to the mobility of the jaw. Lastly, the team realised that whether a fish is actively feeding or not does not influence how often they replace their teeth.
So, it seems likely that all fishes with single simplified tooth shapes like the lingcod follow the replacement pattern that Carr and colleagues have found. Interestingly, neither wear nor the high cost of tooth replacement determines the replacement rate. Instead, the team suggests that replacement rates may be driven by a developmental network which controls when and where teeth are replaced. Future studies can investigate the signals that determine replacement tooth size and whether size is decided by the cells that surround the tooth. This work will help us to understand the diversity of tooth development and shape in fishes, which are the largest group of vertebrate animals on the planet.