Abstract
Respiration in fishes involves buccal pumping, which is characterized by the generation of nearly continuous water flow over the gills because of the rhythmic expansion/compression of the pharyngeal cavity. This mechanism is achieved by the functions of the vascular, skeletal, and muscular systems. However, the process by which the embryo establishes the mechanism remains a mystery. Morphological and kinematical observations on captive cloudy catsharks, Scyliorhinus torazame, have suggested that the embryo starts buccal pumping just before the respiratory slits open on the egg capsule. During the pre-opening period, the embryo acquires oxygen mainly via the external gill filaments. After slit opening, respiration of the embryo involves buccal pumping to pass water over the “internal gills.” The onset of buccal pumping accompanies four morphological changes: (1) regression of the external gill filaments, (2) development of blood vessels within the “internal gills,” (3) completion of the development of hyoid skeletal and muscular elements, and (4) development of the oral valve. A previous study showed that buccal pumping allows the embryo to actively regulate oxygen intake by changing the pumping frequency. Thus, establishment of buccal pumping in the egg capsule is probably important for embryo survival in the unstable oxygen environment of the egg capsule after slit opening.
Highlights
IntroductionChondrichthyes (sharks, batoids, and chimaeroids) are unique among fishes in producing exceptionally large embryos (e.g. [1,2,3])
Chondrichthyes are unique among fishes in producing exceptionally large embryos (e.g. [1,2,3])
The present study revealed that the gill and pharyngeal structures of the cloudy catshark embryos drastically change in association with the onset of buccal pumping
Summary
Chondrichthyes (sharks, batoids, and chimaeroids) are unique among fishes in producing exceptionally large embryos (e.g. [1,2,3]). Chondrichthyes (sharks, batoids, and chimaeroids) are unique among fishes in producing exceptionally large embryos Small embryos, like those of bony fishes, use the entire body surface as the main respiratory organ [5]. For large embryos with a small surface area-tovolume ratio, gas diffusion through the body surface may be insufficient, and some special adaptations may be required. The external gill filaments have been proposed as the organ that meets an embryo’s oxygen demands [4,6]. This structure is characterized by extremely elongated gill lamellae extending outside of the gill slits. External gill filaments are estimated to supply at least 50% of the entire oxygen requirement in skates [4]
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