Nagasaki University reveals complex courtship strategy of male black rockfish

A research group led by Assistant Professor Takafumi Amagai, Researcher Yo Yamaguchi (JSPS Research Fellow PD), and Professor Kiyoshi Soyano at the Institute for East China Sea Research at Nagasaki University, has revealed for the first time in the world that male black rockfish (Sebastes ventricosus) court females during the breeding season using both urine and sound. Their findings were published in the Marine Ecology Progress Series.

Fish courtship behaviors occur through a diverse set of sensory modalities including visual, acoustic, and chemical signals. These are important ecological factors that greatly influence reproductive success. In several freshwater fish species, it is known that males and females release sex pheromones through urine to communicate their physiological state and reproductive readiness, leading to synchronization of breeding periods and induction of spawning behavior. In contrast, typical marine fish's urine production is considered to be limited due to the mechanisms of osmoregulation in the body. The conventional wisdom has been that urine does not function as a pheromone transmission medium in marine fish. Therefore, there has been very little empirical research on the role of urine and its release behavior during the breeding season in marine fish.

In fish of the genus Sebastes (rockfish), courtship behavior in which males bring their urogenital papilla close to the female's nose has been reported, suggesting the possibility of some form of information transmission through urine. However, "whether urine is actually released" during such behavior had not been confirmed because urine is difficult to visualize in seawater.

"Sound signals" in fish courtship have been known for a long time. While it was known that some rockfish species produce sounds, the relationship between this signaling and reproductive behavior has remained unclear.

The research group used the black rockfish, a marine fish, to visualize "urine release behavior during courtship" for the first time through injecting dye into the urinary bladder. This remained visible even in seawater. Simultaneously, through recording and analysis of underwater sounds, they revealed the presence and characteristics of sounds produced during courtship behavior.

This is the first report to empirically demonstrate the existence of chemical and acoustic communication in rockfish, holding significance for reforming the existing academic understanding of fish reproductive behavior. Notably, the research presented counterevidence from both behavioral observation and histological analysis against the conventional physiological wisdom: "marine fish do not release pheromones through urine." This strongly suggests that urine may function as a means of pheromone transmission rather than merely as waste.

Furthermore, through sampling of wild individuals and breeding experiments, it was found that males with higher social status showed more developed testes and increased bladder development and urine retention capacity. This indicates urine may convey information not only about physiological state but also about social status. This is a new discovery revealing the role of sociality and dominance in reproductive strategy.

Moreover, the fact that sounds produced by males occurred frequently during the initial timing of courtship behavior, specifically "during approach," suggests that sound may play a role in preliminarily arousing courtship behavior. On the other hand, male urination always occurred at the end of multiple consecutive courtship behaviors, around 2 to 4 times.

Together, this revealed the existence of a complex courtship strategy which integrates multiple sensory modalities of sound and smell. According to the research group, there may be a strategy by which rockfish, being marine fish, avoid wasteful release of their precious urine.

Journal Information
Publication: Marine Ecology Progress Series
Title: Urination and sound production during courtship behavior of male Japanese common rockfish Sebastes ventricosus
DOI: 10.3354/meps14870