Andrew and others on this thread,

Two species within what we consider to be Vitta usnea?  You bet!  I think
the recent years of DNA work have shown us that there are more rather than
fewer species out there.  It could be that V usnea is in the process (and
has been for the last few million years) of speciating between a brackish
water species and a totally freshwater species.  Or, it could be that this
species is just extremely wide-ranging in habitat selection.  There are
absolutely no absolutes!  The more I work with this family the more confused
I get.  Species you think are fairly standard in appearance can turn out
anything but.  The common, emerald-green Smaragdia rangiana has a brown
variant as well as a red variant!  Same species.  On the other hand,
Neripteron violacea and Neripteron cornucopia are very similar in appearance
(except the color of the aperture), yet they are definitely separate species
(proven by habitat selection, salinity tolerance, and DNA).  There is a lot
of work to be done with Vitta usnea (and many, many other mollusks).

There does not seem to be any correlation between egg capsule size and
direct development, but rather between shell size and capsule size.  In
general, the larger the shell (or more properly, the larger the adult size
of the species); the larger the egg capsule.  Interestingly, shell size and
number of eggs per capsule is not linked with some small nerites having
fewer than 6 (Theodoxus fluviatilis) to 15 (Puperita pupa) "fertile" eggs
per capsule; while the very small Smaragdia viridis has 80 eggs per capsule
and Vitta virginea has over 200 eggs per capsule.  Then we have Andrew's own
fairly large (for a nerite) New Zealand Nerita atramentosa with only 20 - 40
eggs per capsule while Nerita tesselata of the Caribbean has over 100.  The
"quotes" around fertile for the eggs of Theodoxus are because this genus may
actually have 50 to 150 eggs (dependant upon the salt concentration of the
water, more in freshwater), but only 1 - 6 are viable, the rest serve as
nurse eggs that the growing embryo feeds on.  This allows the direct
developing hatchling to basically spend its active feeding veliger stage
inside the egg capsule, nourished and growing.  It is not known if Vitta
usnea also has nurse eggs.  There have been papers linking protoconch
sculpture to direct development.  According to Bandel, direct developing
nerite embryos have a number of axial ridges on the protoconch while species
with a free swimming veliger stage have a smooth protoconch.

The habit of laying egg capsules on other members of the same species is
pretty much limited to the freshwater (and brackish) nerites.  These species
tend to remain submerged while many of the salt water nerites spend quite a
bit of time out of the water - not a good situation for developing eggs.  In
the freshwater environment, eggs are often laid most heavily during times of
flooding.  Laying them on another member of your species means they will
remain submerged, even if the stream is reduced in size.

Tom E.

>
> I have noticed that old eggcapsule bases are abundant on freshwater
> neritids in Fiji, and saw them on images of V. usnea also (I didn't
> see any on the V. usnea that I collected, which were WELL outside
> tidal reach).
>
> If they develop directly, are their capsules larger than those of
> other spp?
>
> Is it possible that there are two cryptic species being labelled V.
> usnea? This could be tested by... DNA analysis!
>

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