C-Type Protoconchs and Planktotrophy in Small Eastern North AmericanFargoa(Pyramidellidae)

C-type protoconchs have been thought always to reflect a “direct” or short pelagic development, an idea deriving from Thorson's (1946) study of the early development (besides planktonic veligers) of only one pyramidellid species. The small-shelled genus Fargoa Bartsch (1955), some ectoparasitic on serpulid polychaetes such as Hydroides, apparently uniformly has C-type protoconchs. The eggs and early larval development of Fargoa dianthophila (Wells and Wells, 1961) (shells < 1.8 mm) and F. bartschi (Winkley, 1909) (< 4.0 mm) were studied in the laboratory at Woods Hole, Massachusetts. The eggs were only 53 to 57 µm and 55 to 60 µm in diameter, respectively, and as would be expected from eggs this size both species are definitely planktotrophic. Few other pyramidellids or other “opisthobranchs” lay eggs this small, and the single Brachystomia (Monterosato, 1884) species studied by Thorson (1946) was also planktotrophic. The protoconch diameters of F. dianthophila and F. bartschi are, respectively, about 260 to 315 µm and about 210 to 230 µm, i.e., surprisingly large. This variation parallels neither the nearly uniform egg size nor the disparate adult shell sizes. No attempt was made to rear the larvae in the laboratory because there were only a few newly hatched veligers and these protoconchs indicated much growth in the plankton. These Fargoa veligers may remain planktonic for longer times than planktotrophic Boonea Robertson (1978). One protoconch, an egg capsule containing a late blastula, and a newly hatched swimming veliger are illustrated. Small pyramidellids appear to lay the smallest eggs, but this seems not to be true of other “opisthobranchs.”