Adaptive Significance of Intra- and Interspecific Differences in the Feeding Repertoires of Cichlid Fishes

Intraspecific modulatory multiplicity in the feeding mechanismof cichlids as elucidated by electromyography has profound implicationson small-scale trophic events the sum of which is the very coreof such large-scale population and evolutionary phenomena asefficiency of trophic exploitation niche width and overlap competitionand adaptation. The greatest paradox emerging from the studyon intra and interspecific differences in feeding repertoiresof cichlid fishes is that the most specialized taxa are notonly remarkable specialists in a narrow sense but also jacks-of-all-trades.If specialists are simultaneously jacks of-all-trades how couldthey have evolved according to the widely accepted hypothesisthat broadening the range of usable resources prevents speciesfrom specializing on individual types? The organism can be considered to be composed of structuralelements and functional components that exert mutual influences.As a result, a network of interacting constraints is set up.The nature of the network determines the direction and rangeof evolutionary changes the possibilities of optimizing adaptationsand built-in variability of the trait. A change in the networkcan put a static trait into a different context to become dynamic.These changes in variability due to changes in the structureof the network cannot be explained in terms of simple adaptationto the external environment. Fitness is considered to dependon the nature of a network of interacting constraints. The possibilitiesof optimization the sensitivity of the phenotypic traits toenvironmental differences and the correlated evolutionary responsesof different traits are all related to this network of interactingconstraints between the elements and components of the organism.Thus the experimental and comparative studies of these networksof interacting constraints should become an increasingly moreimportant focus for morphologists as they attempt to refinetheir understanding of adaptation.     

Symposium Summary: Evolutionary Patterns in Actinopterygian Fishes

SYNOPSIS. The actinopterygian fishes are an exemplary cladefor the study of structural and functional evolutionary patterns.With over half of all vertebrate species, ray-finned fisheshave diversified into a wide variety of habitats, and considerableprogress has been made over the last fifteen years in understandingthe genealogical relationships of actinopterygians. This symposiumhas contributed to our understanding of phylogenetic patternsin actinopterygians and to knowledge of the major structuraland functional patterns in locomotor, auditory, trophic, andneural systems. A number of key areas for future research havebeen identified. (1) The relationships of "palaeonisciform"fishes, (2) the study of trends in feeding and locomotor systemswithin a phylogenetic context, (3) the identification of primitivepatterns of pharyngeal jaw movement and steady and unsteadylocomotor patterns in actinopterygians, (4) the homologies,identification, and functional significance of neural pathwaysin the telencephalon, and (5) the comparative study of form-functionrelations in the auditory system. The study of teleost fishbiology has proceeded at the expense of data on primitive actinopterygians(e.g., Polypterus, Polyodon, Aapenser, Lepisosteus, Amia) whichare especially important in the analysis of structural and functionalpatterns in ray-finned fishes.     

Respiratory Gas Bladders in Teleosts: Functional Conservatism and Morphological Diversity

SYNOPSIS. Respiratory gas bladders are found in the Osteoglossomorpha,Elopomorpha and Euteleostei and are absent in the Clupeomorpha.All teleosts with respiratory gas bladders share a common patternof air ventilation: during the transfer phase gas is transferredpassively from the gas bladder to the buccal cavity. Subsequently,gas is expelled during the active expulsion phase mediated byaction of the geniohyoideus muscle causing a positive pressurepulse in the buccal cavity. This is followed by an active intakephase by action of the sternohyoideus muscle creating a negativepressure pulse, which is succeeded by an extensive compressivephase by action of the geniohyoideus muscle forcing fresh airinto the gas bladder. Saltatory evolution of gas bladders andtheir buccal pumps seems to have proceeded by major transformationsin structural design without appreciable changes in the patternof neural control. The hypothesis of symmorphosis in gas bladderdesign is well corroborated by the independent evolution ofaccessory esophageal pumps in three unrelated lineages. Evolutionaryreversals (Primitive lung evolving into nonrespiratory hydrostaticswim bladder which subsequently reverts back to become a respiratorygas bladder) have occurred repeatedly. Such reversed shiftsare facilitated by the conserved neuromuscular pattern duringfunctional transformations. Experimental comparative evidenceis offered for the notion that evolutionary innovations mayinvolve the addition of entirely new functions (respiratory)of a structural complex (gas bladder) while the original functions(hydrostatic, hearing and sound production) are rigidly retained.The paucity in Elopomorpha and absence in Clupeomorpha of respiratorygas bladders reflect the lack of functional demands for newhabits in the environment rather than the absence of essentialpreexisting building blocks.     

Form and Function of Lungs: The Evolution of Air Breathing Mechanisms

SYNOPSIS. Structural evolution of the vertebrate lung illustratesthe principle that the emergence of seemingly new structuressuch as the mammalian lung is due to intensification of oneof the functions of the original piscine lung. The configurationof the mechanical support of the lung in which elastic and collagenfibers form a continuous framework is well matched with thefunctional demands. The design of the mammalian gas exchangecells is an ingenious solution to meet the functional demandsof optimizing maintenance pathways from nucleus to the cytoplasmwhile simultaneously providing minimal barrier thickness. Surfactantis found in the most primitive lungs providing a protectivecontinuous film of fluid over the delicate epithelium. As thelung became profusely partitioned, surfactant became a functionallynew surface-tension reduction device to prevent the collapseof the super-thin foam-like respiratory surface. Experimentalanalyses have established that in lower vertebrates lungs areventilated with a buccal pulse pump, which is driven by identicalsets of muscles acting in identical patterns in fishes and frogs.In the aquatic habitats suction is the dominant mode of feedinggenerating buccal pressure changes far exceeding those recordedduring air ventilation. From the perspective of air ventilationthe buccal pulse pump is overdesigned. However in terrestrialhabitats vertebrates must operate with higher metabolic demandsand the lung became subdivided into long narrow airways andprogressively smaller air spaces, rendering the pulse pump inefficient.With the placement of the lungs inside a pump, the aspirationpump was established. In mammals, the muscular diaphragm representsa key evolutionary innovation since it led to an energeticallymost efficient aspiration pump. Apparently the potential energycreated by contraction of the diaphragm during inhalation isstored in the elastic tissues of the thoracic unit and lung.This energy is released when lung and thorax recoil to bringabout exhalation. It is further determined experimentally thatrespiratory and locomotory patterns are coupled, further maximizingthe efficiency of mammalian respiration. Symmorphosis is exhibitedin the avian breathing apparatus, which is endowed with a keyevolutionary innovation by having the highly specialized lungcontinuously ventilated by multiple air sacs that function asbellows. Functional morphologists directly deal with these kindsof functional and structural complexities that provide an enormouspotential upon simple changes in underlying mechanisms.     

Aquatic Versus Terrestrial Feeding Modes: Possible Impacts on the Trophic Ecology of Vertebrates

On the basis of experimental work, a clear dichotomy in designof the feeding mechanism between aquatic and terrestrial vertebrateshas been found. The aquatic medium combined with suction andthe hydrodynamic tongue offer an unparalleled array of preycapture opportunities for aquatic vertebrates. In the terrestrialfeeding model, prey capture and prey processing require a precisefunctional design to match the nature of the prey. It is proposedthat this dichotomy in basic design would result in fundamentaldifferences in the aquatic and terrestrial paradigms in vertebrateecology. Four hypotheses and their respective supporting evidenceare presented: (1) Increased opportunism, more extensive preyswitching and food overlap are prevalent in aquatic systems;(2) Convergent evolution which is so pervasive in terrestrialfeeding systems is uncommon among aquatic vertebrate feedingsystems; (3) Competition is relaxed and character displacementis absent in aquatic vertebrate feeding systems; and (4) Patternsof diversity in aquatic vertebrate feeding systems defy theterrestrial paradigm.     

Biological Versatility, Evolution, and Food Resource Exploitation in African Cichlid Fishes

Increased potential versatility in form and function of thefeeding apparatus of cichlid fishes has led to a prodigiousproliferation in the number of possible functional solutionsto an increasing variety of biological problems. Optimal utilizationof every conceivable trophic resource in lacustrine environmentsby just one fish family, the Cichlidae, has been achieved byeruptive evolutionary radiation within the characteristicallycichlid body plan producing mechanisms which partition the diversefood resources with extraordinary efficiency therefore minimizingresource sharing. There is a direct relationship between theeffectiveness of trophic resource exploitation and the functionalintegration of the cichlid body plan in which a minimum numberof adaptive compromises are necessary to evolve optimal anatomicalsolutions by rapidly realizable changes Anatomical data presented here reveal that cichlids possessa specific kind of mosaic in which the basic percoid jaw apparatuspermits unparalleled optimal adaptations by simple morphogeneticchanges while unique and dramatically diverse patterns of muscularcoordination involving degrees of synchrony and extensive modulatingcapabilities of antagonistic muscle groups have been discoveredelectromyographically. At the same time the highly integratedpharyngeal jaw apparatus is sufficiently specialized providingcomplete freedom for the jaws to evolve into refined collectingdevices. The exceptional evolutionary success of lacustrine cichlidsdemonstrates how rare and very specific kinds of biologicallyversatile morphological mosaics represent the best preadaptationsfor the ancestors of major new taxa. Given identical ecologicalconditions and temporal factors, a group of organisms possessingsuch rare mosaics, in which optimal biological versatility isrealizable by simple evolutionary mechanisms, will dominatenewly formed environments to the detriment of taxa not so endowed.