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

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.

Extensive Jaw Mobility in Suckermouth Armored Catfishes (Loricariidae): A Morphological and Kinematic Analysis of Substrate Scraping Mode of Feeding

“…This study shows that these fishes are performing at extreme speeds (Roos et al 2009, in this issue) and exhibit highly specialized and unique cranial morphologies, such as narrow and elongated snouts, relative to basal gasterosteiforms. The study on shark biting forces by Huber et al and the paper on chisel digging using incisors in African mole-rats by Van Daele et al provide nice examples of systems where demands for extreme forces are apparent in the cranial morphology (Huber et al 2009, in this issue;Van Daele et al 2009, in this issue).…”

Kinematics of suction feeding in the seahorseHippocampus reidi

“…Even though syngnathid fishes [pipefish, seahorses and close relatives (Kuiter, 2003)] are specialised suction feeders (Bergert and Wainwright, 1997;de Lussanet and Muller, 2007;Van Wassenbergh et al, 2008;Roos et al, 2009) the hyoid apparatus is strongly reduced in size, despite its crucial role in buccal expansion in most fish. For example, the length of the hyoid apparatus in the seahorse Hippocampus reidi equals only 15% of the total head length (Roos et al, 2009); in other suction-feeding fish this proportion typically varies between 20 and 60% (Gibb, 1997).…”

Sound production in the longnose butterflyfishes (genusForcipiger): cranial kinematics, muscle activity and honest signals

“…Members of the seahorse and pipefish family Syngnathidae, however, produce cranial elevations during feeding that far exceed those measured during sound production in Forcipiger (Van Wassenbergh et al, 2008;Flammang et al, 2009;Roos et al, 2009). The relationship between suction feeding in fishes such as largemouth bass may be explained by both cranial kinematics (Svanbäck et al, 2002) and motor patterns of associated muscles (Grubrich and Wainwright, 1997).…”