Functional Morphology of the Urohyal Shunt for Symmetrical and Asymmetrical Ventilation in the Flatfish, Isopsetta isolepis.

Flatfishes are benthic fishes that are well known for their ability to bury in the sediment, making the transition from above to below the sediment in a matter of seconds. Laterally flattened bodies allow flatfishes to lay flush against the substrate, a behavior facilitated by having an asymmetrical neurocranium with two eyes on one side of the head. Despite neurocranial asymmetry, their gill chambers are highly symmetrical. Additionally, most flatfishes have a uniquely shaped urohyal bone that forms passageway for water to travel ventrally between the "eyed-side" and "blind-side" gill chambers. Our study examines whether the kinematics and pressures generated by the gill chambers are also symmetrical during breathing above and below the sediment and during rapid burial in sediment. We studied Isopsetta isolepis individuals using sonomicrometry crystals to measure the changes in positions of the opercle bones relative to the urohyal and pressure transducers to record gill chamber pressures during burial. We conclude I. isolepis exhibit both symmetrical and asymmetrical breathing above and below the sediment. Pressures and movements were highly asymmetrical during burial jetting. We observed motions that indicate that the urohyal is an active shunt to allow passage of water between the eyed to the blind-side gill chambers.

Multifunctional Structures and Multistructural Functions: Integration in the Evolution of Biomechanical Systems.

Integration is an essential feature of complex biomechanical systems, with coordination and covariation occurring among and within structural components at time scales that vary from microseconds to deep evolutionary time. Integration has been suggested to both promote and constrain morphological evolution, and the effects of integration on the evolution of structure likely vary by system, clade, historical contingency, and time scale. In this introduction to the 2019 symposium "Multifunctional Structures and Multistructural Functions," we discuss the role of integration among structures in the context of functional integration and multifunctionality. We highlight articles from this issue of Integrative and Comparative Biology that explore integration within and among kinematics, sensory and motor systems, physiological systems, developmental processes, morphometric dimensions, and biomechanical functions. From these myriad examples it is clear that integration can exist at multiple levels of organization that can interact with adjacent levels to result in complex patterns of structural and functional phenotypes. We conclude with a synthesis of major themes and potential future directions, particularly with respect to using multifunctionality, itself, as a trait in evolutionary analyses.

Functional coupling in the evolution of suction feeding and gill ventilation of sculpins (Perciformes: Cottoidei).

Suction feeding and gill ventilation in teleosts are functionally coupled, meaning that there is an overlap in the structures involved with both functions. Functional coupling is one type of morphological integration, a term that broadly refers to any covariation, correlation, or coordination among structures. Suction feeding and gill ventilation exhibit other types of morphological integration, including functional coordination (a tendency of structures to work together to perform a function) and evolutionary integration (a tendency of structures to covary in size or shape across evolutionary history). Functional coupling, functional coordination, and evolutionary integration have each been proposed to limit morphological diversification to some extent. Yet teleosts show extraordinary cranial diversity, suggesting that there are mechanisms within some teleost clades that promote morphological diversification, even within the highly integrated suction feeding and gill ventilatory systems. To investigate this, we quantified evolutionary integration among four mechanical units associated with suction feeding and gill ventilation in a diverse clade of benthic, primarily suction-feeding fishes (Cottoidei; sculpins and relatives). We reconstructed cottoid phylogeny using molecular data from 108 species, and obtained 24 linear measurements of four mechanical units (jaws, hyoid, opercular bones, and branchiostegal rays) from micro-CT reconstructions of 44 cottoids and 1 outgroup taxon. We tested for evolutionary correlation and covariation among the four mechanical units using phylogenetically corrected principal component analysis to reduce the dimensionality of measurements for each unit, followed by correlating phylogenetically independent contrasts and computing phylogenetic generalized least squares models from the first principle component axis of each of the four mechanical units. The jaws, opercular bones, and branchiostegal rays show evolutionary integration, but the hyoid is not positively integrated with these units. To examine these results in an ecomorphological context, we used published ecological data in phylogenetic ANOVA models to demonstrate that the jaw is larger in fishes that eat elusive or grasping prey (e.g., prey that can easily escape or cling to the substrate) and that the hyoid is smaller in intertidal and hypoxia-tolerant sculpins. Within Cottoidei, the relatively independent evolution of the hyoid likely has reduced limitations on morphological evolution within the highly morphologically integrated suction feeding and gill ventilatory systems.