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Date: Tue, 16 Mar 1999 15:12:59 -1000
From: "Wesley M. Thorsson" <[log in to unmask]>
Organization: Internet Hawaiian Shell News
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To: NABIH SOBHY ABDELHAMID <[log in to unmask]>
Subject: Juvenile shells
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Nabih:
There was no answer to your question about juveniles, so we can guess
that there is no publication dedicated to that subject. As intimated,
the information is widely distributed, often with literature about a
family.
Generally, there are several groups of molluscs defined by their growth
patterns. One group keeps the same form once it develops from the
protoconch till full adulthood. This is true of a great many molluscs
and certainly with bivalves. Each species has a final size in its
genetic structure. Some keep growing for a hundred years as in
Tridacnidae. Growth in bivalves usually involves adding shell material
to the entire lip to lengthen and widen the shell and its hinge. In
Arcidae, in increasing hinge length, the number of teeth are also
usually increased. I once made a plot of one species of Arca and the
number of teeth was directly proportional to the width of the shell and
was the most smooth line of any natural data I have ever plotted..
Bivalves with cardinal and lateral teeth usually would only increase the
size of these teeth, not the number. Other molluscs such as Conus that
maintain constant shape may slow growth dramatically at some size and
thereafter add materially increasingly more slowly.
A second major group of molluscs, such as Cypraea, reach a maximum size
and then add a final varix. In the case of Cypraea, this final varix is
the base on both sides (Lip and columella) that almost closes the shell
except for a narrow central aperture that extends for almost the entire
shell length. In Strombidae, the final varix is turned outward.
A third group of molluscs have varices that repeat uniformly or
randomly. In Bursidae, these varices are lateral and linear. These
varices occur during what is loosely termed rest stages. While the
shell is between fully complete varices, it is usually called in a
juvenile stage. Where the posterior and anterior canals are important
in determining the family, this can cause confusion when dealing with a
juvenile shell. Many Turrids are an example. Some examples are
included in the current series on small Turrids now running in the IHSN.
Another group of molluscs such as Costellaria have labial lirae. This
can be confusing as you very often find juvenile or juvenile-stage
Costellaria without these lirae.
In Triphoridae, there is a lot of room for confusion. Some species are
conic with straight sides while juvenile, but with further growth the
anterior portion is cylindrical. Some have characteristic canals and
posterior canal notches only in the adults. This is not often indicated
in books for individual species, which invariably illustrate adults.
In studying changes from juveniles to adult shells, the color pattern
and even basic colors change in some species. Conus vexillum comes to
mind where the animal also changes color with growth. Absolute spot
size on shells changes with growth as well.
There have been a number of articles o these subjects on individual
genera or species in the IHSN and HSN. It would be very desirable to
have a more far reaching article illustrating the above for as many
families as possible. I may be able to do this some time in the future.
For now, separating juveniles from adults depends on your experience.
Unless you object, I am using your CONCH-L posting and this reply in the
May, 1999 Internet HSN.
--
Aloha from Wesley M. Thorsson
Editor of Internet Hawaiian Shell News, a monthly Internet Publication
122 Waialeale St, Honolulu, HI 96825-2020, U.S.A
http://www.hits.net/~hsn [log in to unmask]