Date of Graduation

Summer 2011


Master of Science in Biology



Committee Chair

M. Chris Barnhart


unionoidea, mussel, byssus, glochidia, dispersal, plantigrade postlarva

Subject Categories



Freshwater mussels (Order Unionoida) are a widespread and ecologically significant group of bivalve mollusks that are particularly diverse in North American rivers. Mussels are patchily distributed, but can be very abundant in suitable habitat and dense multispecies aggregations known as "mussel beds” form in certain areas. A particularly important adaptation affecting juvenile dispersal, drift and settlement is the production of byssus. Byssus is proteinaceous thread secreted by a gland in the foot of bivalve mollusks. I investigated the occurrence of byssus among taxa and tested aspects of byssus function in several species. A review of published and unpublished sources revealed that 71 of the 299 species of North American mussel species produce byssus, mainly as early juveniles. Byssus appears to serve at least two functions: as drag lines during locomotion in early post-larval stages, and for semi-permanent attachment of older juveniles. In other bivalves, byssus can also serve to enhance drift in planktonic post-larval stages. Unattached byssus slowed sinking rate in very young mussels but its use for dispersal has not been demonstrated in Unionoida. Byssus attachment to solid surfaces and to the byssus of other mussels was evident in several species when they exceeded about 1.5 months of age and 1 mm shell length. Mussels were more likely to form attachments in the presence of other mussels, particularly conspecifics, and this behavior could play a role in the distribution of mussels in nature. Byssus attachment was strongly size-dependent among same-aged mussels that differed in size, and was not evident beyond shell lengths of 30 mm.


© Megan Elizabeth Bradley

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