Juvenile Resettlement

 

Young mussels have shown unique settlement patterns, first described by Bayne in 1964. Mussel larvae are pelagic for two or three months before settling. Once larvae have secreted their shell they initially settle on filamentous algae near, but not within, adult mussel beds. When they first settle they are on average 300μm long, and are often referred to as early plantigrades. After they have more or less doubled in size they undergo a secondary settlement and this time they settle with adult mussels, these settlers are referred to as late plantigrades. Gilg and Hilbish (2000) have used the definition of <500μm in shell length for early plantigrades and between 500μm and 1500μm for late plantigrades. Plantigrades have a relatively long ciliated foot that they frequently protrude, increasing the surface area exposed to water friction and this combined with the beating cilia helps transport them in water with very little current (Bayne 1964). Plantigrades can also attach themselves to the surface film of water by secreting byssus threads and with the tentacles of their inhalent siphon. Once settled, until they reach about 2 cm in length, mussels also have quite a bit of mobility via their byssal threads. At this size they can use their byssal threads like climbing rope, reattaching their threads and pulling themselves forward in successive movements (Bertness 2000). Using this mechanism they can place themselves in a niche within the adult mussel bed and settle down. It is quite fascinating that young mussels have so many modes of transportation. Once they get much larger than 2 cm, they are essentially sessile, except they always retain the ability to regenerate and reattach their byssal threads should they get detached.

 

            Primary settlement away from adults is an adaptive mechanism that provides many advantages for the survival of mussels. The initial growth away from adult mussels allows them to obtain food and oxygen without the competition of adults until they achieve a size more worthy of competing. Also, larvae swimming too close to adult mussels can be taken in to the inhalent current. Some still may be exhaled alive, but chances are they will become entangled in the mucus exhaled with them and die (Bayne 1964).

 

            It is this movement exhibited by juvenile mussels that I was interested in studying when I went to Nahant the first time. I was interested in documenting this movement and finding out how far they traveled. I was also interested in the musselís zonation pattern and determining how this might change as the secondary settlement took place. Go to the section describing my experiment to find out about the experimental design, everything that went wrong and why, and what results I did get.

 

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© 2002 Laura Brentner, for educational purposes only