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.