Evolution & Systematics
The name ‘pteropoda’, which literally translates to ‘wing-foot’, was originally assigned in 1804 to describe a large group of ‘flying’, holoplanktonic marine gastropods. This broad grouping, which is no longer considered a scientific taxonomic term, is still used colloquially for convenience. In 1824, pteropoda was divided into those with a shell (Thecosomata) and those without a shell (Gymnosomata). Click here to see a video of some examples of the diversity of ‘pteropods' found in the waters surrounding Heron Island on just one day of sampling, including Thecosomes, Gymnosomes and Heteropods. C. acicula is placed within the order Thecosomata.
Order: Thecosomata de Blainville, 1824
Euthecosomata retain the transparent calcareous shell that is characteristic of this clade throughout the life-cycle, while in Pseudothecosomata the shell is only present in part of the lifecycle, either adult or larval. Klussmann-Kolb & Dinapoli (2006) present an apomorphic gland situated along the inner mantle wall, which is believed to be used for gathering food. This gland is described for both Euthecosomata and Pseudothecosomata (Meisenheimer 1905 as cited in Klussmann-Kolb & Dinapoli 2006) and is therefore believed to be synapomorphic for all Thecosomata. C. acicula is euthecosomatous, within the family Cavoliniidae.
Family: Cavoliniidae Fisher, 1883
Cavoliniids are bilaterally symmetrical pteropods with large wings and an uncoiled shell that lacks an operculum (Newman 1990). The form of the shell can be quite variable, and the three subfamilies are differentiated based on the shell shape. Cuvierininae have a bottle-shaped shell and Cavoliniinae have rounded bivalve-like shells. C. acicula is within the subfamily Clioninae, which possess a distinctive needle-like or triangular shell that tapers posteriorly. Cavoliniidae is the most abundant holoplanktonic gastropod found in tropical waters (Newman 1990).
Genus: Creseis Rang, 1828
The genus Creseis is composed of four species; C.acicula, C. chierchiae, C. bulgia and C. virgula (Figure 1). Creseis virgula is the type. The shell is generally straight, though some species and formae show slight curvature. Klussmann-Kolb & Dinapoli (2006) placed the genus Creseis as basal to other taxa within the family Cavoliniidae, using molecular systematics (Figure 2). This was based on a “round opening of the shell, non-lobed parapodia, and the position of the kidney and heart on the right side”(Klussmann-Kolb & Dinapoli 2006). In more derived genera of Cavoliniidae, the opening of the shell flattens and the kidney and heart are found on the left (Klussmann-Kolb & Dinapoli 2006).
Figure 1: The four species of the genus Creseis. A- C. acicula; B- C. chierchiae; C- C. bulgia; D- C. virgula. Drawing adapted from ETI - World Biodiversity Database.
Figure 2: Bayesian inference cladogram for the extended 28S rRNA dataset; Bayesian posterior probabilities provided at the branches; asterisk denotes clade include Thecosomata and Gymnosomata. From Klussmann-Kolb & Dinapoli (2006). |