There is a generalization that among closely related, especially congeneric, marine snails, the species with smaller protoconchs have planktonic larvae that go thru a free-swimming stage, while those with larger protoconchs have direct developing larvae that hatch out of their eggs as tiny crawling snails. The idea seems to go back at least to Verduin (1977) with possibly even earlier versions.
To compare the sizes of the protoconchs of related species, Verduin (1977) measured the following 2 dimensions of a protoconch, where Dn is the diameter of the nucleus of the protoconch and D1/2 is the diameter of the 1st half whorl.
Since Dn is within D1/2, the 2 measurements are tightly, in fact, linearly, correlated. Nevertheless, a plot of Dn versus D1/2 is a useful way to separate groups of supposedly planktonic versus supposedly direct-developing species as Verduin (1977) showed to be the case with the species in the genus Alvania.
Recently, Aartsen (2008) noted that the application of Verduin's method to the Atlantic and Mediterranean species of Assiminea revealed the existence of 2 groups. However, he did not present a plot. So I added my own measurements of Assiminea succinea to Aartsen's measurements and did a Verduin plot.
To illustrate the intrinsic variability in the dimensions of such traits, I show here the measurements of 4 specimens rather than the mean value.
As far as I know, among these species, life history information is available only for A. grayana, which has planktonic larvae and for A. succinea, which has direct developing larvae. In the plot, the protoconchs of the former are smaller than those of the larger. So at least with those 2 species, we have agreement with the generalization that direct developing larvae are larger than planktonic larvae.
Aartsen. 2008. Basteria 72:165.
Verduin. 1977. Basteria 41:91.