Reproduction of freshwater mussels in the Tocantins river, Brazil, and implications for sustainable harvesting

Freshwater mussels in the mouth of the Tocantins River in northern Brazil are collected by hand by 8 - 10 local families, who dive without any special equipment from small boats. The mussel shells are cleaned, dried and sold to a nearby factory for the manufacture of buttons from the mother of pearl layer.

The species collected is predominantly Paxyodon syrmatophorus (Meuschen, 1781); Triplodon corrugatus (Lamarck, 1819) and Castalia ambigua (Lamarck, 1819) appear to be less important. Recently, however, fewer mussels have been harvested and in some populations many shells are too small for use. The shell harvesters wish to manage the mussel beds sustainably by taking an optimal harvest, i.e. taking as many individuals as possible without exploiting the population and causing it to decline. Practical conservation and species management strategies need to be based on sound scientific knowledge, and the identification of the timing and duration of reproductive events of the freshwater mussels is important in the development of such a strategy. The timing of gametogenesis and larval brooding, and size at sexual maturity were therefore investigated with the aim of suspending harvesting during peak times to maximise fertilization and the number of glochidia released.

Twenty P. syrmatophorus ranging in size from 20 to 81 mm were sampled each month, between September 1997 and August 1998. 132 were male, 93 female, 2 hermaphrodite and 13 indeterminate. Gametogenesis was evaluated quantitatively from histological sections using Haggerty et al.'s (1995) method. Demibranchs of females were dissected and examined for glochidia.

Gametogenesis occurs throughout the year. Males appear to spawn in May - July and again September - November, judged by the decline in sperm numbers. Oocyte numbers per follicle declined between July and August and again in November, indicating the movement of oocytes into the gills. Fertilization probably takes place between July and August following the first spawning, and again in December-January. The peak brooding period was between April and May. Embryos were found in February, and embryos and mature glochidia between April and September.

Collecting of mussels is most frequent during the dry season from June to December; little collecting is carried out in the rainy season - January to May - due to poor visibility and strong currents. Spawning and fertilization occur mainly between May and December. Collecting could be reduced during spawning episodes, and suspended in the periods of fertilization (July-August and December-January). This would help maximise the number of oocytes fertilized and also the number of glochidia released. Shell-collecting activities in the region vary in intensity due to fluctuating demands from the factory (no collecting was carried out between September 1997 and June 1998, nor from November 1998 up to May 1999), and mussel populations may recover during slack periods.

Water temperature varied little over the year, and there was no clear relationship between the measured environmental parameters and gemetogenesis. However, the September-December spawning was associated with a marked rise in water depth between September and November, and water levels were also high during the June - August spawning period. Fluctuations in water level and thus current speed and suspended solids may act as environmental cues for freshwater mussels in areas where temperatures are relatively constant.

Timing of reproduction of P. Syrmatophorus

Collectors typically select shells larger than 50 mm, as these are preferred by the button manufacturers. The smallest male mussels containing mature gametes were 23 mm, and females as small as 32 mm can be gravid. Some reproductively mature individuals must be left to allow breeding to continue, but removal of large individuals might in time result in a population of small adults. Moreover, smaller individuals might, by producing fewer gametes, lower the numbers of offspring.

I am grateful to the Malacological Society of London for the grant of a Centenary Award. This study was also in part supported by a Fellowship from the Conselho Nacional de Desenvolvimento Científico e Tecnológico, Brazil. I would like to thank the community of Ihla Grande, especially 'Gel' (Luiz de Jesus Gonçalves Brito) and his family, and Arlindo Pinto de Souza Júnior of the Universidade Federal do Pará, for their support during fieldwork.

Reference:

Haggerty T, Garner J, Patterson G, and Jones L C. (1995) A quantitative assessment of the reproductive biology of Cyclonaias tuberculata (Bivalvia: Unionidae). Canadian Journal of Zoology, 73, 83-88.

Colin R Beasley, Universidade Federal do Pará, Brazil

Captions:

Drilled shells returned from the factory in sacks. Coin diameter is 22 mm.

Timing of reproductive events in P. syrmatophorus