Early development of Monoplex pilearis and Monoplex parthenopeus (Gastropoda: Cymatiidae): biology and morphology

Abstract

Members of family Cymatiidae have an unusually long planktonic larval life stage (veligers) which allows them to be carried within ocean currents and become distributed worldwide. However, little is known about these planktonic veligers and identification of the larval state of many Cymatiidae is challenging at best. Here, we describe the first high-quality scanning electron microscopy images of the developing veliger larvae of Monoplex pilearis and Monoplex parthenopeus (Gastropoda: Cymatiidae). The developing shell of Monoplex veligers was captured by SEM, showing plates secreted to form the completed shell. The incubation time of the two species was recorded and found to be different; M. parthenopeus took 24 days to develop fully and hatch out of the egg capsules, whereas M. pilearis took over a month to leave the egg capsule. Using scanning electron microscopy and geometric morphometrics, the morphology of veliger larvae was compared. No significant differences were found between the shapes of the developing shell between the two species; however, it was found that M. pilearis was significantly larger than M. parthenopeus upon hatching. Although statistical analysis did not find morphological differences, this study concludes biological differences do exist between these two closely related species of Monoplex.