The protoconch (the first or earliest portion of the shell) is the embryonic or larval shell of a gastropod. The number of whorls of a protoconch is a function of the length of the larval stage and may consist of barely one or several whorls. At the conclusion of the larval stage 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. The portion of the shell after this change (subsequent to the end of the larval stage) is called the teleoconch. Quite often there is a visible line of demarkation where the protoconch ends and the teleoconch begins (often with a sudden and dramatic appearance of sculpture), or there may be a transitional area with a noticeable, but not dramatic change in sculpture that rapidly becomes more pronounced and reflective of the sculpture of the teleoconch.
The transition from protoconch to teleoconch may be:
1. Abrupt - Essentially instantaneous change
2. Sinusigera - change is abrupt and characterized by a curved edge or sinus
3. Transformation - a transitional area of changing sculpture between the initial larval sculpture and the teleoconch sculpture.
The protoconch counting method used by Marlo starts on the anterior suture at the transition and counts the number of whorls to arrive at the first appearance of the suture at the apex.
Example of Abrupt:
Examples of Sinusigera: (D = 2.5 whorls; E = 2.25 whorls; F = 2.33 whorls)
Example of Transformational. The exact point at which the transformation fully represents the sculpture of the teleoconch and is the "transition" can be subjective and may vary from individual-to-individual specimens within a species. In the below example the first three whorls of the protoconch are smooth, there then appear numerous faint, curved, opisthocline axial riblets that become more pronounced and with faint spiral swellings that at the transition become distinct spiral cords. With the transformational region, this protoconch would be described as having 3.25 whorls.