A Preliminary Study On Reproductive Biology Of Gigantidas Platifrons In Formosa Cold Seep Of The South China Sea

The genus Gigantidas belongs to one of the subfamilies,Bathymodiolinae,in the family Mytilidae of molluscan Bivalvia.The deep-sea mussel Gigantidas platifrons(previously named Bathymodiolus platifrons),widely distributed in both hydrothermal vents in the Okinawa Trough,and cold seeps in Sagami Bay of Japan and the Taixinan Formosa of the South China Sea(SCS),is one of the dominant macrofauna in these extreme ecosystems.The most significant characteristics of the deep-sea mussel G.platifrons is that they rely primarily on chemoautotrophic endosymbionts for nutrition and energy.Thus,the mussel‘s spatial distribution and population variation are subject to the surrounding environment they habitat.Because of the difficulty in deep-sea survey sampling,there is no systematic understanding of the population dynamics and supplementation mechanism of the deep-sea mussel G.platifrons.In this paper,the reproductive biology of the deep-sea mussel G.platifrons from the Formosa cold seep area in southwest Taiwan was mainly studied.In details,the study contents included the reproductive characteristics,the frequency of population replenishment and the population genetic structures of G.platifrons,the acquisition ways of endosymbiotic methane-oxidizing bacteria(MOB)and the distribution and transfer model of symbiotic bacteria in the growth of G.platifrons.The main results are as follows:G.platifrons is a functionally dioecious species given the extremely low(0.76%)percentage of hermaphrodites found in the sampled population.The reproduction of G.platifrons is discontinuous with the maturity peak around the latter half of the year.Given the small oocyte size(48.99–70.14 ?m)comparable to coastal mussels,and the shell length of the second Prodissoconch(PII)was much longer than the first Prodissoconch(PI),we proposed that G.platifrons developed via a free-living,planktotrophic larvae stage before its settlement.The replenishment of juvenile G.platifrons is once a year through one peak in the plot of juvenile shell length frequency distribution in one year.The supplementary source of the dense population was reflected by the 2b-RAD genome sequences of different shell length populations,and the results showed that the genetic structure of G.platifrons population is stable in the intergenerational and the mussel population mainly is self-replenishing in Formosa cold seep.Research on the distribution and transfer model of symbiotic bacteria showed that there was no symbiotic bacteria in gonad tissue of both male and female mussels,but symbiotic bacteria were found in majority organs of juvenile mussels,suggesting that G.platifrons should obtain symbiotic bacteria by horizontal transmission from environment rather than vertical transmission by gametes.Furthermore,the bacteriocytes were widely distributed in the epithelial cells of gill,mantle,kidney and foot tissues of the juvenile mussels,but limited in the gill epithelial cells of adult mussel.These results confirmed an ontogenetic shift in symbiont colonization from indiscriminate infection of a wide range of epithelial tissues in early life stages to spatially restricted colonization of gills in later developmental stages in G.platifrons.Further research on the translocation mechanism of symbiont with G.platifrons development is required.During the long-term on-board culture experiment of G.platifrons,the abundance of the symbionts was found to be gradually decreased along with the content of lysosomes.We proposed that the lysosomes functioned to digest symbionts and eliminate the senile or premortal symbionts actively,but when there were no symbiotic bacteria to digest and the lysosomes would gradually disappear.This conclusion is further supported by the suppressed lysosomes after the loss of the symbionts.Although the deep-sea mussel G.platifrons could survive for more than one year in our circulating water systems,we could not breed the larva of G.platifrons,presumably because the mussels were not in the ir best physiological state in the on-board culture system.