In regard to long-lived mollusks/annular ring question, I offer my
experiences. In Sept. 1996 we translocated 6100 unionids from a large
Mississippi River side channel to another nearby side channel. Over 80 of
the unios were Obovaria olivaria, hickory-nut mussel, most of which were ~ 25
mm long, and thus easy to age (2-4 yrs of age).
We followed up on 1/2 of these numbered specimens after 1 yr. Some
unios put down a "disturbance ring" after the September translocation, then
grow 5mm before putting down another "rest ring" for the winter, then they
grew another 5 mm during the 2nd summer. Other specimens only put down the
one winter annular ring, followed by 5 mm of growth the following summer.
Some specimens barely grew, and did not put down a winter ring.
We also found young specimens of other species, a few of which had
obvious disturbance rings (usually indicated by incomplete annular rings). I
was baffled as to why this had occurred because they were in 10-15 feet of
water, and shouldn't have been subject to any type of disturbance from
fishing boats, swimmers etc (no commercial barge traffic in this area).
I came to the conclusion (after this and other projects) that annular
rings are useful to indicate age, but if the "periodicity" seems out of
whack (too little growth for the expected age of the shell), check for
incomplete rest rings, and don't count that ring. Some species are much
easier than others to age---Lampsilines are sometimes difficult to age as
they often have an "indentation" on the shell rather than a darkened ring
(esp. Lampsilis higginsi).
I have one large washboard mussel (Megalonaias nervosa) that I've aged
a number of times over the years --I always get at least 60 yrs of age. 3 of
us field-aged a large old Cumberlandia monodonta (spectaclecase mussel), and
we all got about the same age (62, 70, and 72 yrs of age).
Happy New Year to all.
Marian
Marian E Havlik
Malacological Consultants
1603 Mississippi Street
La Crosse, WI 54601-4969
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P/F: 608.782.7958
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