Geographic distribution and environmental control of vertebral count in Ammodytes spp. along the northern Pacific coast of Japan

Leptin gene expression in the brain is associated with the physiological onset of estivation in western sand lance Ammodytes japonicus

Dormancy is an essential ecological characteristic for the survival of organisms that experience harsh environments. Although factors that initiate dormancy vary, suppression or cessation of feeding activities are common among taxa. To distinguish between extrinsic and intrinsic causes of metabolic reduction, we focused on estivation, which occurs in summer when the feeding activity is generally enhanced. Sand lances (genus Ammodytes) are a unique marine fish with a long estivation period from early summer to late autumn. In the present study, we aimed to elucidate the control mechanisms of estivation in western sand lance (A. japonicus), and firstly examined behavioral changes in 8 months including a transition between active and dormant phases. We found that swimming/feeding behavior gradually decreased from June, and completely disappeared by late August, indicating all individuals had entered estivation. Next, we focused on leptin, known as a feeding suppression hormone in various organisms, and examined leptin-A gene (AjLepA) expression in the brain that may regulate the seasonal behavioral pattern. AjLepA expression decreased after 7 days of fasting, suggesting that leptin has a function to regulate feeding in this species. The monthly expression dynamics of AjLepA during the feeding (active) and non-feeding (estivation) periods showed that the levels gradually increased with the onset of estivation and reached its peak when all the experimental fish had estivated. The present study suggests that the suppression of feeding activity by leptin causes shift in the physiological modes of A. japonicus before estivation.

Circadian Rhythm and Endocrinological Control on the Swimming and Sand Burrowing Behaviors of Japanese Sand Lances Ammodytes spp. (Uranoscopiformes, Ammodytidae)

In diurnal and nocturnal organisms, daily activity is regulated by the perception of environmental stimuli and circadian rhythms, which enable organisms to maintain their essential behaviors. The Japanese sand lances genus Ammodytes are coastal marine fish that exhibit unique nocturnal sand burrowing behavior. To elucidate the extrinsic and intrinsic regulation of this behavior and its endocrinological basis, we conducted a series of rearing experiments under various light conditions and hormone administrations. Under a light-dark photoperiod, the fish showed three types of behavior: sand buried, head-exposed from sand, and swimming/feeding. During the transition from dark to light periods, the fish first showed head exposure, followed by swimming and foraging, and buried themselves in the sand immediately after shifting to the dark period. Under constant light conditions, fish exhibited swimming behavior during the period corresponding to the acclimated light period. In addition, swimming did not occur under constant dark conditions but head exposure was observed at the time of the dark-light transition during acclimation. These observations indicate that the essential behavior of sand lances is regulated by both light and circadian rhythms. Subsequently, a melatonin-containing diet promoted the onset of burrowing in 10 to 120 min in a dose-dependent manner at 0.3-128 µg/g-diet, suggesting the direct behavioral regulation by this hormone. These findings suggest that the behavior of sand lances is strictly regulated by an intrinsic mechanism and that melatonin is a regulatory endocrine factor that induces burrowing behavior.

Effects of Myostatin b Knockout on Offspring Body Length and Skeleton in Yellow Catfish (Pelteobagrus fulvidraco)

Based on obtaining mstnb gene knockout in Pelteobagrus fulvidraco, a study on the effect of the mstn gene on skeletal morphology and growth was performed by comparing the number and length of the vertebrae of mutant and wild-type fish in a sibling group of P. fulvidraco, combined with the differences in cells at the level of vertebral skeletal tissue. It was found that mstnb gene knockdown resulted in a reduction in the number of vertebrae, the length, and the intervertebral distance in P. fulvidraco, and these changes may be the underlying cause of the shorter body length in mutant P. fulvidraco. Further, histological comparison of the same sites in the mstn mutant and wild groups of P. fulvidraco also revealed that the number and density of osteocytes were greater in mstnb knockout P. fulvidraco than in wild-type P. fulvidraco. Our results demonstrated that when using genome editing technology to breed new lines, the effects of knockout need to be analyzed comprehensively and may have some unexpected effects due to insufficient study of the function of certain genes.