Bottom Feeding and Beyond: How the Premaxillary Protrusion of Cypriniforms Allowed for a Novel Kind of Suction Feeding

Deploy the proboscis!: Functional morphology and kinematics of a novel form of extreme jaw protrusion in the hingemouth, Phractolaemus ansorgii (Gonorynchiformes)

Premaxillary protrusion and the performance advantages it confers are implicated in the success of diverse lineages of teleost fishes, such as Cypriniformes and Acanthomorpha. Although premaxillary protrusion has evolved independently at least five times within bony fishes, much of the functional work investigating this kinesis relates to mechanisms found only in these two clades. Few studies have characterized feeding mechanisms in less-diverse premaxilla-protruding lineages and fewer yet have investigated the distinctive anatomy underlying jaw kinesis in these lineages. Here, we integrated dissection, clearing and staining, histology, micro-CT, and high-speed videography to investigate an isolated and independent origin of jaw protrusion in the hingemouth, Phractolaemus ansorgii, which employs a complex arrangement of bones, musculature, and connective tissues to feed on benthic detritus via a deployable proboscis. Our goals were to provide an integrative account of the underlying architecture of P. ansorgii's feeding apparatus and to assess the functional consequences of this drastic deviation from the more typical teleost condition. Phractolaemus ansorgii's cranial anatomy is distinct from all other fishes in that its adducted lower jaw is caudally oriented, and it possesses a mouth at the terminal end of an elongated, tube-like proboscis that is unique in its lack of skeletal support from the oral jaws. Instead, its mouth is supported primarily by hyaline-cell cartilage and other rigid connective tissues, and features highly flexible lips that are covered in rows of keratinous unculi. Concomitant changes to the adductor musculature likely allow for the flexibility to protrude the mouth dorsally and ventrally as observed during different feeding behaviors, while the intrinsic compliance of the lips allows for more effective scraping of irregular surfaces. From our feeding videos, we find that P. ansorgii is capable of modulating the distance of protrusion, with maximum anterior protrusion exceeding 30% of head length. This represents a previously undescribed example of extreme jaw protrusion on par with many acanthomorph species. Protrusion is much slower in P. ansorgii-reaching an average speed of 2.74 cm/s-compared to acanthomorphs feeding on elusive prey or even benthivorous cypriniforms. However, this reorganization of cranial anatomy may reflect a greater need for dexterity to forage more precisely in multiple directions and on a wide variety of surface textures. Although this highly modified mechanism may have limited versatility over evolutionary timescales, it has persisted in solitude within Gonorynchiformes, representing a novel functional solution for benthic feeding in tropical West African rivers.

Surfperches versus Damselfishes: Trophic Evolution in Closely Related Pharyngognath Fishes with Highly Divergent Reproductive Strategies

Surfperches and damselfishes are very closely related ovalentarians with large reproductive differences. Damselfishes are typical of most Ovalentaria in that they lay demersal eggs that hatch into small, free-feeding larvae. Surfperches are unusual among ovalentarians and most acanthomorphs in having prolonged internal development. They are born at an advanced stage, some as adults, and bypass the need to actively feed throughout an extended period of ontogeny. Damselfishes and surfperches possess the same modifications of the fifth branchial arch that allow them to perform advanced food processing within the pharynx. This condition (pharyngognathy) has large effects on the evolution of feeding mechanics and trophic ecology. Although the evolution of pharyngognaths has received considerable attention, the effects of different reproductive strategies on their diversification have not been examined. We compared head shape evolution in surfperches and damselfishes using geometric morphometrics, principal component analyses, and multiple phylogenetic-comparative techniques. We found that they have similar mean head shapes, that their primary axes of shape variation are comparable and distinguish benthic-feeding and pelagic-feeding forms in each case, and that, despite large differences in crown divergence times, their head shape disparities are not significantly different. Several lines of evidence suggest that evolution has been more constrained in damselfishes: Head shape is evolving faster in surfperches, more anatomical traits have undergone correlated evolution in damselfishes, there is significant phylogenetic signal in damselfish evolution (but not surfperches), and damselfishes exhibit significant allometry in head shape that is not present in surfperches.

The Evolution of Feeding Mechanics in the Danioninae, or Why Giant Danios Don't Suck Like Zebrafish

By linking anatomical structure to mechanical performance we can improve our understanding of how selection shapes morphology. Here we examined the functional morphology of feeding in fishes of the subfamily Danioninae (order Cypriniformes) to determine aspects of cranial evolution connected with their trophic diversification. The Danioninae comprise three major lineages and each employs a different feeding strategy. We gathered data on skull form and function from species in each clade, then assessed their evolutionary dynamics using phylogenetic-comparative methods. Differences between clades are strongly associated with differences in jaw protrusion. The paedomorphic Danionella clade does not use jaw protrusion at all, members of the Danio clade use jaw protrusion for suction production and prey capture, and members of the sister clade to Danio (e.g., Devario and Microdevario) use jaw protrusion to retain prey after capture. The shape of the premaxillary bone is a major determinant of protrusion ability, and premaxilla morphology in each of these lineages is consistent with their protrusion strategies. Premaxilla shapes have evolved rapidly, which indicates that they have been subjected to strong selection. We compared premaxilla development in giant danio (Devario aequipinnatus) and zebrafish (Danio rerio) and discuss a developmental mechanism that could shift danionine fishes between the feeding strategies employed by these species and their respective clades. We also identified a highly integrated evolutionary module that has been an important factor in the evolution of trophic mechanics within the Danioninae.

Does phenotypic plasticity initiate developmental bias?

The generation of variation is paramount for the action of natural selection. Although biologists are now moving beyond the idea that random mutation provides the sole source of variation for adaptive evolution, we still assume that variation occurs randomly. In this review, we discuss an alternative view for how phenotypic plasticity, which has become well accepted as a source of phenotypic variation within evolutionary biology, can generate nonrandom variation. Although phenotypic plasticity is often defined as a property of a genotype, we argue that it needs to be considered more explicitly as a property of developmental systems involving more than the genotype. We provide examples of where plasticity could be initiating developmental bias, either through direct active responses to similar stimuli across populations or as the result of programmed variation within developmental systems. Such biased variation can echo past adaptations that reflect the evolutionary history of a lineage but can also serve to initiate evolution when environments change. Such adaptive programs can remain latent for millions of years and allow development to harbor an array of complex adaptations that can initiate new bouts of evolution. Specifically, we address how ideas such as the flexible stem hypothesis and cryptic genetic variation overlap, how modularity among traits can direct the outcomes of plasticity, and how the structure of developmental signaling pathways is limited to a few outcomes. We highlight key questions throughout and conclude by providing suggestions for future research that can address how plasticity initiates and harbors developmental bias.

Anatomical Assessment of the Adult Skeleton of Zebrafish Reared Under Different Thyroid Hormone Profiles

Thyroid hormone (TH) directs the growth and maintenance of tissues throughout the body during development and into adulthood, and plays a particularly important role in proper ossification and homeostasis of the skeleton. To better understand the roles of TH in the skeletogenesis of a vertebrate model, and to define areas of the skeleton that are particularly sensitive to developmental TH, we examined the effects of hypo- and hyperthyroidism on skeletal development in zebrafish. Performing a bone-by-bone anatomical assessment on the entire skeleton of adult fish, we found that TH is required for proper ossification, growth, morphogenesis, and fusion of numerous bones. We showed that the pectoral girdle, dermatocranium, Weberian apparatus, and dentary are particularly sensitive to TH, and that TH affects development of skeletal element regardless of bone type and developmental origin. Indeed, the hormone does not universally promote ossification: we found that developmental TH prevents ectopic ossification in multiple thin bones and within connective tissue of the jaw. In all, we found that TH regulates proper morphogenesis and ossification in the majority of zebrafish bones, and that the requirement for the hormone extends across bone types and developmental profiles. Anat Rec, 302:1754-1769, 2019. © 2019 American Association for Anatomy.

The Role of Developmental Integration and Historical Contingency in the Origin and Evolution of Cypriniform Trophic Novelties

While functional morphologists have long studied the evolution of anatomical structures, the origin of morphological novelties has received less attention. When such novelties first originate they must become incorporated into an integrated system to be rendered fully functional. Thus, developmental integration is key at the origin of morphological novelties. However, given enough evolutionary time such integration may be broken, allowing for a division of labor that is facilitated by subsequent decoupling of structures. Cypriniformes represent a diverse group of freshwater fishes characterized by several trophic novelties that include: kinethmoid-mediated premaxillary protrusion, a muscular palatal and post-lingual organ, hypertrophied lower pharyngeal jaws that masticate against the base of the neurocranium, novel pharyngeal musculature controlling movement of the hypertrophied lower pharyngeal jaws, and in a few species an incredibly complex epibranchial organ used to aggregate filtered phytoplankton. Here, we use the wealth of such trophic novelties in different cypriniform fishes to present case studies in which developmental integration allowed for the origin of morphological innovations. As proposed in case studies 1 and 2 trophic innovations may be associated with both morphological and lineage diversification. Alternatively, case studies 3 and 4 represent a situation where ecological niche was expanded but with no concomitant increase in species diversity.

Functional diversity in biters: the evolutionary morphology of the oral jaw system in pacus, piranhas and relatives (Teleostei: Serrasalmidae)

Serrasalmid fishes form a highly specialized group of biters that show a large trophic diversity, ranging from pacus able to crush seeds to piranhas capable of cutting flesh. Their oral jaw system has been hypothesized to be forceful, but variation in bite performance and morphology with respect to diet has not previously been investigated. We tested whether herbivorous species have higher bite forces, larger jaw muscles and more robust jaws than carnivorous species. We measured in vivo and theoretical bite forces in 27 serrasalmid species. We compared the size of the adductor mandibulae muscle, the jaw mechanical advantages, the type of jaw occlusion, and the size and shape of the lower jaw. We also examined the association between bite performance and functional morphological traits of the oral jaw system. Contrary to our predictions, carnivorous piranhas deliver stronger bites than their herbivorous counterparts. The size of the adductor mandibulae muscle varies with bite force and muscles are larger in carnivorous species. Our study highlights an underestimated level of functional morphological diversity in a fish group of exclusive biters. We provide evidence that the trophic specialization towards carnivory in piranhas results from changes in the configuration of the adductor mandibulae muscle and the lower jaw shape, which have major effects on bite performance and bite strategy.

The evolution of jaw protrusion mechanics is tightly coupled to bentho-pelagic divergence in damselfishes (Pomacentridae)

Most species-rich lineages of aquatic organisms have undergone divergence between forms that feed from the substrate (benthic feeding) and forms that feed from the water column (pelagic feeding). Changes in trophic niche are frequently accompanied by changes in skull mechanics, and multiple fish lineages have evolved highly specialized biomechanical configurations that allow them to protrude their upper jaws toward the prey during feeding. Damselfishes (family Pomacentridae) are an example of a species-rich lineage with multiple trophic morphologies and feeding ecologies. We sought to determine whether bentho-pelagic divergence in the damselfishes is tightly coupled to changes in jaw protrusion ability. Using high-speed video recordings and kinematic analysis, we examined feeding performance in 10 species that include three examples of convergence on herbivory, three examples of convergence on omnivory and two examples of convergence on planktivory. We also utilized morphometrics to characterize the feeding morphology of an additional 40 species that represent all 29 damselfish genera. Comparative phylogenetic analyses were then used to examine the evolution of trophic morphology and biomechanical performance. We find that pelagic-feeding damselfishes (planktivores) are strongly differentiated from extensively benthic-feeding species (omnivores and herbivores) by their jaw protrusion ability, upper jaw morphology and the functional integration of upper jaw protrusion with lower jaw abduction. Most aspects of cranial form and function that separate these two ecological groups have evolved in correlation with each other and the evolution of the functional morphology of feeding in damselfishes has involved repeated convergence in form, function and ecology.