The cerato-mandibular ligament: a key functional trait for grazing in damselfishes (Pomacentridae)

Introduction The success of a taxonomic group can be promoted by a key character that allows the group to interact with its environment in a different way and to potentially occupy new niches. The Pomacentridae possess a synapomorphic trait, the cerato-mandibular (c-md) ligament, which joins the hyoid bar to the inner part of the lower jaw. It has previously been shown that this ligament is a key trait in communication in damselfishes because it enables them to slam the oral jaws shut causing teeth collision and sound production. This specific behavior of mouth closing could, however, also be used for other tasks, such as feeding. Many territorial damselfishes are referred to as farmers, due to their ability to manage algal crops on which they feed. This study hypothesizes that the c-md ligament provides an advantage for grazing filamentous algae, and should thus be considered a key trait for farming behavior. Results The kinematic patterns associated with sound production and biting filamentous algae or attached animal prey are all based on the same mechanism and are associated with a slam of the oral jaws. We observed that transection of the c-md ligaments makes the fish unable to perform such actions. We also counted biting rates on filamentous algae in fish with and without the c-md ligament and observed a drop of more than 80% in the latter. Conclusion This study shows that the c-md ligament is a key trait both for sound production and for grazing activities in damselfishes. The buccal jaw slam enables the fish to perform accurate strikes on small filamentous algae. This kind of bite probably plays a major role in farming activity and allows grazing damselfishes to occupy distinct niches, possibly increasing their competitive evolutionary success.