The Cypriniform Pharyngeal Complex: Making Sense of a Novel Type of Pharyngeal Processing
Open Access DepositedThe musculoskeletal diversity of fish feeding anatomy is unlike any other vertebrate clade. The most speciose order of freshwater fishes, Cypriniformes, is characterized by a suite of trophic morphological novelties. Specifically, the pharyngeal complex differs anatomically from the pharyngeal jaw apparatus of any other group. The cypriniform pharyngeal complex consists of the pharyngeal jaws, the pharyngeal process, and the palatal organ. Cypriniform pharyngeal jaws consist of bilaterally paired lower jaws that occlude against the pharyngeal process. This pharyngeal process, in addition to being an occlusal surface, provides osteological support to the novel palatal organ. The function of this muscular organ has yet to be established across cypriniform taxa but is hypothesized to aid in food selection via tasting and trapping food items with muscular protrusions. Much of the published literature investigating teleost pharyngeal jaws has focused on pharyngognathous fishes, such as cichlids, while neglecting the unique cypriniform pharyngeal complex. We know comparatively little about the pharyngeal jaws of cypriniforms as compared to cichlids, and this dissertation illuminates a complex, integrated anatomical system. Notably, we have previously lacked an understanding of the degree of integration and whether diet and niche influence the morphology of the pharyngeal complex in cypriniforms. Examinations of cichlids have revealed that diet significantly influences pharyngeal jaw shape and that pharyngeal jaw morphological disparity covaries with head shape. Here, I demonstrate that the pharyngeal jaws and pharyngeal process are integrated with one another and with head shape in leuciscids. Moreover, I show that the shape of the pharyngeal jaws, the pharyngeal processes, and the size of the palatal organ are significantly correlated with diet in catostomids. These results indicate a tight morphological and functional relationship within the pharyngeal complex and confirm the palatal organ’s role in feeding. The palatal organ’s specific function in feeding has been hypothesized but not established across the order, and the muscle ultrastructure has yet to be fully characterized. Here, I show that the cypriniform palatal organ does not obey the organizational rules of typical vertebrate skeletal muscle and forms an intricate network of the smallest muscle fibers recorded from any vertebrate striated muscle. Indeed, the thin muscle fibers, sarcoplasmic reticulum, and transverse tubule system of the palatal organ deviate far from typical skeletal muscle and instead appear functionally and morphologically convergent with the hydrostatic muscles found in cuttlefish dermal papillae. I find interspecific variation between the goldfish and zebrafish which reflect functional differences between them. I hypothesize that the goldfish palatal organ is specialized for chemosensation and fine movement and that the zebrafish palatal organ is used to generate suction within the oropharynx. The data presented here highlight the necessity for investigation of the anatomical, evolutionary, and ecomorphological implications of the cypriniform pharyngeal complex. This highly integrated and morphologically diverse system functions entirely differently from pharyngognathous fishes and further exploration of cypriniforms must be done to better inform our broad macroevolutionary conclusions about pharyngeal processing in teleosts.
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