Does the expression “silence of the fishes” really represent the truth? We have all grown up believing that the deep is silent. But this is far from true, and many aquatic/marine animals (and even plants) produce sounds and some of them are very loud!
Recently, researchers at the Institute of Marine Biology, Biotechnology and Aquaculture (IMBBC) of the Hellenic Centre for Marine Research (HCMR) completed experiments monitoring the reproductive behaviour of meagre (Argyrosomus regius), a marine fish that produces very loud sounds (Photo 1). Meagre belongs to the family of the Sciaenidae, which includes several species of sound-producing fish, called “drums” or “croakers”. Other Mediterranean species in this family are the shi drum (Umbrina cirrosa) and the brown meagre (Sciaena umbra).
Meagre is equipped with special «sound» muscles along the inner tissue surrounding the abdomen, which is in direct contact with the swim bladder, a “bubble” organ whose main function is to control the buoyancy of the fish. During various activities of the fish, which are still under investigation, these muscles contract very rapidly, causing a low-frequency (usually less than 1 KHz) and high-intensity pulse. These muscles are among the fastest contracting striated muscles in vertebrate animals.
Within the framework of NewTechAqua, Dr. Constantinos Mylonas from HCMR studies reproductive dysfunctions of meagre under rearing conditions and is investigating at ways to overcome them. In aquaculture, this species does not reproduce spontaneously but requires a special treatment to mature and release its eggs (i.e. to spawn). The reason for this difficulty to spawn is believed to be related to the fish’s reproductive behaviour. In natural conditions, this species lives in the sea throughout the year but breeds in estuaries and rivers in late spring. In aquaculture farms, the fish is always maintained in seawater, so it is not exposed to the natural stimuli that trigger reproductive maturation and spawning.
Thus, postdoctoral associate Dr. Ioannis Fariadis from IMBBC and M.Sc. student of the University of Crete Christos Siapazis (Photo 2), study the reproductive behaviour of meagre at HCMR’s facilities in Crete, Greece, and film its behaviour and record the sounds it produces during reproduction. Reproduction is induced after treatment with gonadotropin releasing hormone agonist (GnRHa), the hormone produced by the fish brain – but also by the brain of all vertebrates, including humans – that controls the release of the Luteinizing hormone (LH) causing oocyte maturation, ovulation and spawning. The aim of their studies is to simultaneously record sound and images during spawning, and to comprehensively, and for the first time, record the reproductive behaviour of meagre, and its relationship with sound production. This was done this year between 9 and 16 May 2022, using hydrophones and underwater cameras mounted in two tanks. Three males and two females were placed in each tank after checking their reproductive status and administering GnRHa. External tags were placed on the skin of the fish so that they could be visually identified.
For three consecutive nights, the fish in each tank laid a total of 2.5 million eggs (Video 1, at the end of the video, the cloudiness is the eggs and sperm released), and very loud sounds were recorded (Video 2, the cloudiness is due to the eggs and sperm released) that could be heard by the researchers throughout the building, even without the use of hydrophones! The data collected on the reproductive behaviour of the fish, and genetic analyses on the involvement of male fish in the fertilization of the eggs produced, will be correlated with the video and audio data to describe as accurately as possible the involvement of different fish in each reproductive event, and to highlight any possible hierarchy. In addition, capturing behaviours associated with the courtship and spawning period of fish will also contribute in part to deciphering the role of the sounds emitted by meagre during its reproductive behaviour.
The ultimate goal is to create a future “enhancing” environment for the reproductive maturation and spawning of meagre by playing back recorded sound clips from the same period, so that the fish can spawn without the need for GnRHa therapy. So, in the future, in addition to water pumps and lights that simulate the seasonal changes in day length, fish hatcheries may also have stereo system to provide the right “sonic” environment for fish to spawn.
Video 1 Meager fishes monitored in tanks releasing eggs and sperm
Video 2 Meager fisges monitored in tanks causing cloudiness due to eggs and sperm release
Photo 1 Meager fish used example in aquaculture
Photo 2 Research team at HCMR premises studying the reproductive behaviour of meager