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Linking Morphology and Motion: A Test of a Four-Bar Mechanism in Seahorses

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Abstract
Syngnathid fishes (seahorses, pipefish, and sea dragons) possess a highly modified cranium characterized by a long and tubular snout with minute jaws at its end. Previous studies indicated that these species are extremely fast suction feeders with their feeding strike characterized by a rapid elevation of the head accompanied by rotation of the hyoid. A planar four-bar model is proposed to explain the coupled motion of the neurocranium and the hyoid. Because neurocranial elevation as well as hyoid rotation are crucial for the feeding mechanism in previously studied Syngnathidae, a detailed evaluation of this model is needed. In this study, we present kinematic data of the feeding strike in the seahorse Hippocampus reidi. We combined these data with a detailed morphological analysis of the important linkages and joints involved in rotation of the neurocranium and the hyoid, and we compared the kinematic measurements with output of a theoretical four-bar model. The kinematic analysis shows that neurocranial rotation never preceded hyoid rotation, thus indicating that hyoid rotation triggers the explosive feeding strike. Our data suggest that while neurocranium and hyoid initially (first 1.5 ms) behave as predicted by the four-bar model, eventually, the hyoid rotation is underestimated by the model. Shortening, or a posterior displacement of the sternohyoid muscle (of which the posterior end is confluent with the hypaxial muscles in H. reidi), probably explains the discrepancy between the model and our kinematic measurements. As a result, while four-bar modeling indicates a clear coupling between hyoid rotation and neurocranial elevation, the detailed morphological determination of the linkages and joints of this four-bar model remain crucial in order to fully understand this mechanism in seahorse feeding.
Keywords
BEHAVIOR, FUNCTIONAL-MORPHOLOGY, TELEOSTEI, DEPRESSION, MOVEMENTS, SUCTION, MOUTH, PREY-CAPTURE KINEMATICS, SYNGNATHID FISHES, ECOLOGY

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Chicago
Roos, Gert, Heleen Leysen, Sam Van Wassenbergh, Anthony Herrel, Patric Jacobs, Manuel Dierick, Peter Aerts, and Dominique Adriaens. 2009. “Linking Morphology and Motion: A Test of a Four-Bar Mechanism in Seahorses.” Physiological and Biochemical Zoology 82 (1): 7–19.
APA
Roos, G., Leysen, H., Van Wassenbergh, S., Herrel, A., Jacobs, P., Dierick, M., Aerts, P., et al. (2009). Linking Morphology and Motion: A Test of a Four-Bar Mechanism in Seahorses. PHYSIOLOGICAL AND BIOCHEMICAL ZOOLOGY, 82(1), 7–19. Presented at the Symposium on Functional Consequences of Extreme Adaptations of the Trophic Apparatus in Craniates held at the 8th International Congress on Vertebrate Morphology.
Vancouver
1.
Roos G, Leysen H, Van Wassenbergh S, Herrel A, Jacobs P, Dierick M, et al. Linking Morphology and Motion: A Test of a Four-Bar Mechanism in Seahorses. PHYSIOLOGICAL AND BIOCHEMICAL ZOOLOGY. 1427 E 60TH ST, CHICAGO, IL 60637-2954 USA: UNIV CHICAGO PRESS; 2009;82(1):7–19.
MLA
Roos, Gert, Heleen Leysen, Sam Van Wassenbergh, et al. “Linking Morphology and Motion: A Test of a Four-Bar Mechanism in Seahorses.” PHYSIOLOGICAL AND BIOCHEMICAL ZOOLOGY 82.1 (2009): 7–19. Print.
@article{687037,
  abstract     = {Syngnathid fishes (seahorses, pipefish, and sea dragons) possess a highly modified cranium characterized by a long and tubular snout with minute jaws at its end. Previous studies indicated that these species are extremely fast suction feeders with their feeding strike characterized by a rapid elevation of the head accompanied by rotation of the hyoid. A planar four-bar model is proposed to explain the coupled motion of the neurocranium and the hyoid. Because neurocranial elevation as well as hyoid rotation are crucial for the feeding mechanism in previously studied Syngnathidae, a detailed evaluation of this model is needed. In this study, we present kinematic data of the feeding strike in the seahorse Hippocampus reidi. We combined these data with a detailed morphological analysis of the important linkages and joints involved in rotation of the neurocranium and the hyoid, and we compared the kinematic measurements with output of a theoretical four-bar model. The kinematic analysis shows that neurocranial rotation never preceded hyoid rotation, thus indicating that hyoid rotation triggers the explosive feeding strike. Our data suggest that while neurocranium and hyoid initially (first 1.5 ms) behave as predicted by the four-bar model, eventually, the hyoid rotation is underestimated by the model. Shortening, or a posterior displacement of the sternohyoid muscle (of which the posterior end is confluent with the hypaxial muscles in H. reidi), probably explains the discrepancy between the model and our kinematic measurements. As a result, while four-bar modeling indicates a clear coupling between hyoid rotation and neurocranial elevation, the detailed morphological determination of the linkages and joints of this four-bar model remain crucial in order to fully understand this mechanism in seahorse feeding.},
  author       = {Roos, Gert and Leysen, Heleen and Van Wassenbergh, Sam and Herrel, Anthony and Jacobs, Patric and Dierick, Manuel and Aerts, Peter and Adriaens, Dominique},
  issn         = {1522-2152},
  journal      = {PHYSIOLOGICAL AND BIOCHEMICAL ZOOLOGY},
  keyword      = {BEHAVIOR,FUNCTIONAL-MORPHOLOGY,TELEOSTEI,DEPRESSION,MOVEMENTS,SUCTION,MOUTH,PREY-CAPTURE KINEMATICS,SYNGNATHID FISHES,ECOLOGY},
  language     = {eng},
  location     = {Paris, FRANCE},
  number       = {1},
  pages        = {7--19},
  publisher    = {UNIV CHICAGO PRESS},
  title        = {Linking Morphology and Motion: A Test of a Four-Bar Mechanism in Seahorses},
  url          = {http://dx.doi.org/10.1086/589838},
  volume       = {82},
  year         = {2009},
}

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