Many ways to build an angler: diversity of feeding morphologies in a deep-sea evolutionary radiation

Almost nothing is known about the diets of bathypelagic fishes, but functional morphology can provide useful tools to infer ecological processes. Here we quantify variation in jaw and tooth morphologies across anglerfishes (Lophiiformes), a clade spanning shallow and deep-sea habitats. Deep-sea ceratioid anglerfishes are believed to be dietary generalists due to the presumed necessity of opportunistic feeding in the food-limited bathypelagic zone. However, we found unexpected diversity in the trophic morphologies of ceratioid anglerfishes. Ceratioid jaws span a functional continuum ranging from species with numerous stout teeth, a relatively slow but forceful bite, and high jaw protrusibility at one extreme (characteristics shared with benthic anglerfishes), to species with large fang-like teeth, a fast but weak bite, and low jaw protrusibility at the other (including a unique "wolftrap" phenotype). Our finding of high morphological diversity seems to be at odds with ecological generality, reminiscent of Liem's paradox (morphological specialization allows organisms to have broader niches). Another possible explanation is that diverse ceratioid morphologies may yield similar trophic success (many-to-one mapping of morphology to diet), allowing diversity to arise through neutral evolutionary processes. Our results highlight that there are many ways to be a successful predator in the deep sea.