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Factors causing extinction of a freshwater pearl mussel, Margaritifera laevis in Japan (Bivalvia: Unionoida)

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Title: Factors causing extinction of a freshwater pearl mussel, Margaritifera laevis in Japan (Bivalvia: Unionoida)
Other Titles: 日本におけるカワシンジュガイMargaritifera laevisの絶滅要因(二枚貝綱イシガイ目)
Authors: Akiyama, Yoshihiro1 Browse this author
Authors(alt): 秋山, 吉寛1
Issue Date: 25-Sep-2007
Abstract: Freshwater bivalves are endangered nowadays the world over. One group of freshwater bivalve, Margaritiferidae is a family that is particularly endangered and is devised for conservation measure in many countries. Life history of unionoida including Margaritiferidae is very unique and their larvae known as glochidia are parasitic to fish and/or amphibians. Accordingly, host and population dynamics of Margaritiferidae are closely related. The major objective of the present study was to clarify the mechanism and cause of extinction of a Margaritiferid, Margaritifera laevis. For the purpose of age estimation, marked mussels were reared in situ for one year and it was confirmed that internal and external growth bands of the shell increased annually. Both growth rings on the shell surface and growth bands on the cross-sectional surface of shell ligament could be used for determination of mussel age. For comprehending the status of growth and recruitment success, shell sizes and ages were examined for 14 populations in Japan. Irrespective of their reproductive potential, some populations lacked juveniles. These populations would become extinct if environment of habitat was not improved. The Lack of juveniles in mussel populations as described above is observed worldwide, and has been considered as the major cause of margaritiferid extinction. It was implied that the lack of juveniles in mussel populations was associated with dam and eutrophication. For clarifying the mechanism and cause of the lack of juveniles, I compared two contrasting populations, i.e. the population in the Chitose River which consists of adult and juvenile mussels, and the population in Abira River which consists of adult mussels only. Adult mussels in the Abira River had full reproductive potential indicated by glochidial release from exhalant siphon, parasitism on host fish and growth of parasitic glochidia. Accordingly, the reproductive potential was not the cause of the lack of juveniles. Survival rate in the early life stage from free-living glochidia to juveniles was lower in the Chitose River. Free-living glochidia cannot survive without infection to hosts. Although viability in the free-living glochidial stage was superior in the Abira River, it was suggested that the survival rate of free-living glochidia in the Abira River was inferior due to a low density of host fish and a low rate of overlap of distribution between mussels and host fish. In the Abira River, the rate of glochidial infection to host was higher and the mean number of attached glochidia per host was larger. However the water temperature in the Abira River was colder than that in the Chitose River, which extended the duration of parasitic stage exposing attached glochidia to immune attack by host fish. In rearing juveniles in the rivers survival times were shorter in the Abira River, irrespective of the origin of glochidia. This suggested that the environment in the Abira River was not appropriate for juvenile mussels. Therefore, the lack of juveniles in M. laevis was caused by the low survival rate in early life stages irrespective of the mussel’s reproductive potential. Furthermore, the population dynamics of host fish was closely related to that of M. laevis. In Margaritiferidae, sizes of glochidia were smaller than those in the other Unionoida and glochidium does not have a hook. Accordingly their host species and the suitable parts for their glochidia to attach on the host are extremely restricted. Survival time in free-living glochidia in Margaritiferidae is also shorter than that in the other Unionoida. These traits in Margaritiferidae make success rate of parasitism to keep low. The duration of the parasitic stage in Margaritiferidae is typically longer than that in the other Unionoida. In Margaritiferidae, attached glochidia are originally easy to be excluded by host immunity and their survival rate is typically low. In general, the survival of Margaritiferidae during the early life stages strongly depends on the distribution, population dynamics and physiological response of host fish.
Conffering University: 北海道大学
Degree Report Number: 甲第8382号
Degree Level: 博士
Degree Discipline: 地球環境科学
Type: theses (doctoral)
Appears in Collections:学位論文 (Theses) > 博士 (地球環境科学)

Submitter: 秋山 吉寛

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