5:15 PM - 7:15 PM
[HTT16-P07] Size dimorphism and alternative migration pattern in males of Carassius buergeri grandoculis endemic to Lake Biwa
Keywords:Lake Biwa, endemic species, migration, strontium isotope, Carassius buergeri grandoculis
Reproductive and life history dimorphism is universally observed in fish. Recently, dwarf males that reach sexual maturity at about the same body size as juveniles were observed at low frequency in the Carassius buergeri grandoculis, an endemic species of Lake Biwa, suggesting the existence of size dimorphism in the males. They usually spawn on floodplains during the flooding season from April to June, but rice paddies also serve as major spawning grounds. Juveniles born in the rice paddies descend through the channels as they grow, and reaching sexual maturity after several years of offshore migratory life.
Hypotheses that explain the evolution of size dimorphism include alternative reproductive strategies (tactics) and alternative life history strategies (tactics). Alternative reproductive strategies are defined as the phenomenon in which individuals with larger body size and superior same-sex competition and individuals with smaller body size and inferior competition enhance reproductive success in different ways, and the cases of ranged and sneaker males are well known. Sneaker males are characterized by more developed gonads relative to body size compared to roosting males.
Alternative life history strategies refer to the dimorphism found in life history traits such as age at which breeding begins and body size among individuals. There are two types: those that begin breeding late after they have high reproductive performance and those that begin breeding early after they have low reproductive performance and small body size. Two types are well known in migratory fishes: the large, late-maturing migratory type and the small, early-maturing remnant type.
The objective of this study is to verify what strategies (tactics) led to the evolution of male size dimorphism in Carassius buergeri grandoculis. We introduced a combined approach of an anatomical study to analyze the relationship between body size, age, and gonadal investment, and an earth science study to reconstruct the migration history of individuals through time series analysis of strontium stable isotope ratios (87Sr/86Sr) in otolith micro-cores. In the collection surveys conducted in paddy field drainages in the Tazuke area of Hikone City and the Enshoji area of Nagahama City, Shiga Prefecture, large males , dwarf males, juveniles, and juvenile fish, a total of 107 fish were captured. Length, weight, and gonad weight of the collected individuals were recorded, and scales and otoliths were collected. While the standard length of large males ranged from 90-250 mm, dwarf males and juveniles reached 40-90 mm in April, early in the spawning season, and were therefore considered overwintering individuals born the previous year, distinguishing them from fry (-40 mm) that appear late in the spawning season. Sexual maturity by large and dwarf males was determined based on the presence or absence of sperm release by abdominal pressure.
Large males were assessed as 2+ years old or older, and all dwarf males were assessed as 1+ years old, born the previous year based on the number of scale rings. Comparison of GSI (100 x gonad weight/body weight) between large and dwarf males showed no significant differences, thus rejecting the possibility of alternative breeding strategies. Subsequent time-series analysis of otolith microcores 87Sr/86Sr showed that the isotopic ratios of otolith nuclei formed in the juvenile stage of large males overlapped with those of the environmental water in the collecting channel and those of juveniles in the same channel, and that otoliths formed during growth were similar to those of lake water in Lake Biwa, suggesting that otoliths formed in the spawning run The otoliths formed during growth overlapped with those of Lake Biwa, suggesting that the juveniles were born at their spawning run and migrated offshore from Lake Biwa before returning to their birthplace. 87Sr/86Sr in the otolith nucleus and otolith outer margin of dwarf males overlapped with the environmental water in the sampling channel and the otolith isotope ratios of juveniles in the same channel, suggesting that they did not migrate and remained at their natal area. Therefore, we conclude that there are two alternative life history strategies (tactics) in Carassius buergeri grandoculis: large males that begin reproduction after a long migration offshore and dwarf males that remain in the natal area and begin reproduction at an early stage.
Hypotheses that explain the evolution of size dimorphism include alternative reproductive strategies (tactics) and alternative life history strategies (tactics). Alternative reproductive strategies are defined as the phenomenon in which individuals with larger body size and superior same-sex competition and individuals with smaller body size and inferior competition enhance reproductive success in different ways, and the cases of ranged and sneaker males are well known. Sneaker males are characterized by more developed gonads relative to body size compared to roosting males.
Alternative life history strategies refer to the dimorphism found in life history traits such as age at which breeding begins and body size among individuals. There are two types: those that begin breeding late after they have high reproductive performance and those that begin breeding early after they have low reproductive performance and small body size. Two types are well known in migratory fishes: the large, late-maturing migratory type and the small, early-maturing remnant type.
The objective of this study is to verify what strategies (tactics) led to the evolution of male size dimorphism in Carassius buergeri grandoculis. We introduced a combined approach of an anatomical study to analyze the relationship between body size, age, and gonadal investment, and an earth science study to reconstruct the migration history of individuals through time series analysis of strontium stable isotope ratios (87Sr/86Sr) in otolith micro-cores. In the collection surveys conducted in paddy field drainages in the Tazuke area of Hikone City and the Enshoji area of Nagahama City, Shiga Prefecture, large males , dwarf males, juveniles, and juvenile fish, a total of 107 fish were captured. Length, weight, and gonad weight of the collected individuals were recorded, and scales and otoliths were collected. While the standard length of large males ranged from 90-250 mm, dwarf males and juveniles reached 40-90 mm in April, early in the spawning season, and were therefore considered overwintering individuals born the previous year, distinguishing them from fry (-40 mm) that appear late in the spawning season. Sexual maturity by large and dwarf males was determined based on the presence or absence of sperm release by abdominal pressure.
Large males were assessed as 2+ years old or older, and all dwarf males were assessed as 1+ years old, born the previous year based on the number of scale rings. Comparison of GSI (100 x gonad weight/body weight) between large and dwarf males showed no significant differences, thus rejecting the possibility of alternative breeding strategies. Subsequent time-series analysis of otolith microcores 87Sr/86Sr showed that the isotopic ratios of otolith nuclei formed in the juvenile stage of large males overlapped with those of the environmental water in the collecting channel and those of juveniles in the same channel, and that otoliths formed during growth were similar to those of lake water in Lake Biwa, suggesting that otoliths formed in the spawning run The otoliths formed during growth overlapped with those of Lake Biwa, suggesting that the juveniles were born at their spawning run and migrated offshore from Lake Biwa before returning to their birthplace. 87Sr/86Sr in the otolith nucleus and otolith outer margin of dwarf males overlapped with the environmental water in the sampling channel and the otolith isotope ratios of juveniles in the same channel, suggesting that they did not migrate and remained at their natal area. Therefore, we conclude that there are two alternative life history strategies (tactics) in Carassius buergeri grandoculis: large males that begin reproduction after a long migration offshore and dwarf males that remain in the natal area and begin reproduction at an early stage.