The gas bladder of Heterotis niloticus (Cuvier, 1829)

Abstract

This work reports on the structural characteristics of the respiratory gas bladder of the osteoglossiform fish Heterotis niloticus. The bladder‐vertebrae relationships are also analyzed. A slit‐shaped orifice in the mediodorsal pharyngeal wall is surrounded by a muscle sphincter and serves as a glottis‐like opening to the gas bladder. The dorsolateral internal surface of the gas bladder is lined by a parenchyma of highly vascularized trabeculae and septa displaying an alveolar‐like structure. The trabeculae contain, in addition to vessels, numerous eosinophils probably involved in immune responses. The air spaces are endowed with a thin exchange barrier indicating a good potential for respiratory gas exchange. The ventral wall of the gas bladder is a well‐vascularized membrane that exhibits an exchange barrier in the luminal face and an inner structure dominated by the presence of a layer of richly innervated smooth muscle. This is suggestive of an autonomous adjustability of the gas bladder ventral wall. The trunk vertebrae show large transverse processes (parapophyses) and numerous surface openings that lead into intravertebral spaces that become invaded by the bladder parenchyma. Curiously, the caudal vertebrae show a regular teleost morphology with neural and hemal arches, but have similar surface openings and intravertebral pneumatic spaces. The African Arowana hence rivals the freshwater butterfly fish Pantodon in its exceptional role of displaying postcranial skeletal pneumaticity outside of Archosauria. The possible significance of these findings is discussed.

Air‐breathing behavior in Heterotis niloticus fingerlings: Response to changes in oxygen, temperature, and exercise regimes

“…The species has a suprabranchial organ that supplements oxygen demands by aiding in gas exchange at the air–water interface (Lefevre, Wang, et al ., 2014; Monentcham, 2009). Specifically, the species relies on a gas bladder that occupies the entire length of the coelomic cavity, with its dorsolateral walls extending to the lateral walls of the body cavity for aerial respiration (Icardo et al ., 2023). In the natural habitat of H. niloticus , they frequently encounter low dissolved oxygen conditions which they overcome with their air‐breathing capabilities (Adite et al ., 2006).…”

“…Although the scientific literature classifies H. niloticus as an obligate air‐breather, no published studies exist on their air‐breathing physiology or behavior. While the recent detailed description of the gas bladder (Icardo et al ., 2023) has improved our understanding of the air‐breathing mechanisms of the species, it still remains largely unclear how much abiotic factors (e.g., hypoxia and temperature) and physical activity affect the air‐breathing patterns of the species. Despite the species inhabiting tropical freshwater systems, and generally being tolerant to suboptimal oxygen levels, we hypothesized that exposures to hypoxia or hyperthermia will exert significant effects on air‐breathing frequencies and lead to behavioral compensations in fingerlings of H. niloticus .…”