The Swim Bladder of Butterflyfishes

Background

The swimbladder of all Chaetodon species examined is divided internally by a constriction of the tunica interna, which forms a narrow passage through which air can presumably travel. It appears that this constriction, or sphincter, is vascularized, and innervated, and appears to contain smooth muscle fibers, suggesting that the size of its central opening can be changed. This feature cannot be detected externally, but is clearly visible in radiographs, and in some CT images (see below).

Two distinct swimbladder morphologies are found among subgenera within the genus Chaetodon; species within each subgenus demonstrate the same swimbladder morphology. Interestingly, the gross morphology of the swim bladder is correlated with the type of LC present (e.g., Direct or Indirect); a source of variation in swim bladder morphology that has gone unnoticed until now. In species with a direct LC, the swim bladder sits behind a very thin (often pigmented) peritoneum; the fibrous tunica externa is relatively thick around the entire circumference of the body of the swim bladder. The swimbladder has a distinct “kink” it its longitudinal contour; the posterior end of the swimbladder is rounded and sits free from the body wall. In contrast, in species with an indirect LC, the swimbladder sits behind a thick peritoneum which has tight fibrous connections to the wall of the body cavity. The tunica externa is very thin dorsally; the longitudinal contour is smooth (without a “kink”), and the posterior end of the swim bladder is not visible in the body cavity; the swimbladder sits up behind the wall of the body cavity.

An examination of additional species of museum specimens has revealed, that with a only a few exceptions, gross morphology of the swim bladder is correlated with the type of LC that is predicted to be present by virtue of a species’ subgeneric placement. These results further suggest that the morphology of the swimbladder is functionally correlated with LC morphology, that the definition of the laterophysic connection be expanded to include the swimbladder, and that the laterophysic connection needs to be studied as a functional complex.

Histological Analysis of Swimbladder

Variation in swim bladder morphology noted through gross dissection appears to be due to differences in the number and thickness of collagen layers that compose the tunica externa. Quantitative analyses of this feature are ongoing.

CT Imaging of Swim bladders in Live, Anaesthetized Fishes

If swim bladder morphology is correlated with LC morphology, then it is likely very important for understanding the function of the LC. Thus, a method in which the morphology of the swimbladder and horns can be examined in live anaesthetized animals, without the artifacts introduced by fixation/preservation or even dissection, is essential. In collaboration with Dr. Darlene Ketten (Department of Otolaryngology, Harvard Medical School and Woods Hole Oceanographic Institution) we have shown that high resolution computerized tomography (CT) is an effective method for visualizing the morphology of the swim bladder and the anterior swim bladder horns, as well as their relationship to other tissues.

CT scans of over a dozen species of Chaetodon and one species of Forcipiger have revealed the detailed morphology of the anterior swim bladder horns (which are circular in profile, as predicted from histology) and the body of the swim bladder, which are being subjected to quantitative anlaysis. In addition, 3-D imaging of the volume of air in the swim bladder revealed that details of horn morphology are consistent with histological images. CT provides a quick (albeit costly) method for assessing swim bladder morphology in anaesthetized, especially in those with morphological specializations that might be altered by further dissection.

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