AtlantaRoger R. Seapy
This tree diagram shows the relationships between several groups of organisms.
The root of the current tree connects the organisms featured in this tree to their containing group and the rest of the Tree of Life. The basal branching point in the tree represents the ancestor of the other groups in the tree. This ancestor diversified over time into several descendent subgroups, which are represented as internal nodes and terminal taxa to the right.
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An atlantid with:
- Shell and keel calcareous
- Larval shell becomes the spire in the adult shell
- Eye morphology of three types: a, b and c (see Atlantidae page)
- Operculum morphology of three types: a, b, c (see Atlantidae page)
- Radular morphology of two types: I and II (see Pterotracheoidea page)
- The adult shell consists of three sequentially-deposited regions; a dome-shaped embryonic shell (protoconch I) of about one whorl, a larval shell (protoconch II) of a variable number of whorls, and the adult shell (teleoconch) also of a variable number of whorls.
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Figure. Scanning electron micrographs of the embryonic portion (protoconch I) of the larval shell (left), the larval shell (protoconch II; middle), and the adult shell (right) in Atlanta plana. In the last SEM the protoconch is shaded blue to distinguish it from the teleoconch. Scale bars = 100 µm (middle) and 0.5 mm (right). © 2005
- The number of whorls comprising the spire on the right side of the shell is a character that can be very useful in distinguishing species. However, the method of determining the number of spire whorls can be difficult to conceptualize. The total number of whorls that make up an adult shell is usually easy to determine when the shell is viewed under a dissection microscope. In the example of Atlanta plana above, the adult shell can be seen to possess a total of 4-1/4 whorls. Although this total whorl count may be useful as a taxonomic character for describing fully-grown adults, it is variable in all other individuals since the number increases with shell growth. The number of whorls comprising the shell spire, however, is not variable, but rather is predictable because it represents only the larval portion of the adult shell. In late-stage larvae (e.g., see the A. plana larval shell above), the last 1/4 whorl increases noticeably in width by comparison with the preceding whorls. In post-metamorphic shells with raised sculpture on the spire, the junction between the larval shell (protoconch) and the beginning of the adult shell (teleoconch) is often marked by the termination of, or a change in, the sculpture pattern (e.g., see the SEM of the tilted shell spire on the A. brunnea page). Even without the ability to distinguish this point of transition (as is often revealed clearly in SEMs), the point at which the whorl width increases markedly is visible under the dissection microscope. The sketch below of the whorl sutures from an adult A. inflata is used here to describe the method for determining the number of whorls comprising the spire. Note that in practice, the shell is first oriented so that the central embryonic part is directed upward in the field of view. Then, the whorls are counted outward using an imaginary line directed upward (as shown below). In the case of A. inflata, the number of spire whorls at the point of rapid increase in whorl width is 4-1/2. Examples of whorl sketches for two other species, A. lesueurii and A. peronii (from Richter and Seapy, 1999), are shown below for comparison. Spire whorl numbers for the nineteen species of Atlanta (included in the table below) range from two and one-half (in A. lesueurii and A. oligogyra) to six (in A. gibbosa).
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Figure. Sketch of the whorl sutures from Atlanta i