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Posted: Jun 5 2009, 01:44 PM
Member No.: 2
Joined: 12-November 05
This discussion is restricted to Gastropods and applies to 95%+ of what you might encounter. When it comes to the diversity of living things, there’re always exceptions.
A gastropod shell is herein presented as composed of three growth parts:
3. Adult whorl (final whorl of the teleoconch)
(NOTE: The distinction of teleoconch and adult whorl is solely for purposes of this discussion. The adult whorl is just the final whorl of the teleoconch, which is commonly defined as all growth subsequent to the protoconch.)
The Protoconch - A shell begins with the fertilized egg. The growth that occurs after fertilization is called the larval stage. The shell that forms during this very early stage of growth is the protoconch. At a point during this early stage of development, which may not occur until after several whorls of shell growth, the animal will go through a relatively abrupt change in form or organ function as a result of cell growth and/or differentiation that causes dramatic changes in subsequent development, including changes in shell sculpture (and often in growth orientation). This point in development when this dramatic change occurs is the end of the larval stage. The place on the young shell where this change in sculpture occurs is where the protoconch ends and the teleoconch begins. Quite often there is a visible line of demarcation where the protoconch ends and the teleoconch begins, with a noticeable change in sculpture, or a sudden appearance of sculpture at that point.
The Teleoconch - All the growth of whorls from the place where the protoconch ends to the beginning of the adult whorl is called the teleoconch. Some discussions of the protoconch refer to the protoconch as the “juvenile” stage. The definitions herein consider both the protoconch and the teleoconch to be “juvenile” shells. A shell is considered “juvenile” until the final whorl is in place and spiral growth stops.
The Adult Whorl - The final whorl of the teleoconch when spiral growth has stopped. This does not mean the animal no longer manufactures shell material or that additional shell sculpture is not created. It just means spiral growth of the shell has stopped. In some shells (notably those with flared lips) the adult whorl initially will look like the growth stage of the preceding teleoconch whorls until the lip begins to differentiate; such as the flaring lips of Strombus shells. In some shells spiral growth will pause several times, the lip will thicken and it may appear spiral growth has stopped. But, then spiral growth begins again. This creates what appears as thickened axial ridges (may include complex ruffles, frills, spines or lamellae) on earlier whorls. An axial ridge created via this mechanism is called a varix (varices). I mention this because many shells with thickened or sculpturally developed lips may appear to be “adult,” but actually are “juvenile” until the final whorl is in place and spiral growth ceases.
mmmmmmmmmmmmmmmMore about the Protoconch
Development during the larval stage of marine univalves can occur in one of three processes – direct, lecithotrophic or planktotrophic. Each generally has a differing impact on protoconch morphology.
Direct developers complete the larval stage in the egg and feed off of the yolk sac. These are usually species where egg masses or casings are affixed, when larval development is completed the animal/shell looks like the adult form, and upon “hatching” crawl away from the egg mass. These larvae are also known as “crawl-away.”
Lecithotrophic developers are dispersed into the aquatic environment before the larval stage is completed, usually with a yolk sac to provide nutrition for the completion of larval development, but may be able to feed. The period spent in the water column is short and by the end of the larval stage the shell has settled to the seafloor.
Planktotrophic developers are dispersed into the aquatic environment before the larval stage is completed, feed while in the water column, and have fairly long free swimming durations (2-3 months) during which larval development is completed. These shells are likely to have distinctive protoconch 1’s and 2’s (see discussion, below). Planktotrophic development is the most common type of larval development among benthic mollusks.
Paucispiral protoconch. Paucispiral means relatively few whorls, usually of rapidly increasing width. Shells with short larval stages (direct /crawl-away and lecithotrophic developers) usually have larger (bulbous), heavier paucispiral protoconchs. This probably reflects an adaptive strategy related to predator protection.
Planktotrophic protoconch. Since planktotrophic developers spend a long time in the water column adding protoconch growth until the larval stage is completed, their protoconchs will have more numerous whorls. These protoconchs tend to be tiny, light-weight and translucent, which makes them hard to see and permits them to remain in suspension. Again, this probably reflects an adaptive strategy related to predator protection.
The protoconchs of planktotrophic developers often have two parts. The first part (which is formed within the embryonic egg capsule) is called protoconch 1, and the part formed after the larva has been dispersed into the water column is called protoconch 2. There is most often an absence of sculpture or ornamentation on protoconch 1, but protoconch 2 frequently displays sculpture or some change in appearance.
The protoconchs of planktotrophic developers are fragile and as a result in many shells are broken off or eroded away after the post-larval stage begins on the ocean floor. Early shell growth often becomes encrusted and protoconchs are broken off when the shell is cleaned. The absence of a protoconch can be quite problematic since the structure of the protoconch has been widely used as a discriminating feature in separating gastropod species. Protoconch development is driven almost entirely by genetic instruction and is immune to variability driven by environment, since it is not exposed to the environment, or, in the case of planktotrophic developers, only protoconch 2 is influenced by minimal, short duration environmental exposure. Lecithotrophic developers remain in the water column so briefly that protoconch 2 differentiation is uncommon.