Raising predatory babies: development from fertilization to sexual maturity of the hoplonemertean Emplectonema viride (Nemertea)

Marine ribbon worms in the class Hoplonemertea were presumed to have lecithotrophic development until the recent discovery of predatory larvae in several species, including Emplectonema viride Stimpson, 1857: a common NE Pacific intertidal nemertean. This study reports the complete life cycle of E. viride in the laboratory from fertilization to sexual maturity, which took ~ 9.5 months at 12 − 15 °C. Predatory larvae of this species were successfully raised to metamorphosis on a diet of planktonic crustaceans, including barnacle nauplii and cyprids, and an occasional calanoid copepod. The larvae swam and grew for 3−4 months in lab culture with abundant food, before settling as juveniles and starting to feed on adult barnacles. Settlement was accompanied by a subtle but definite metamorphosis, which included shortening of epidermal cilia, loss of the caudal ciliary cirrus, and behavioral changes. Larvae were positively phototactic, whereas juveniles were negatively phototactic. A pelagic larval duration of several months provides abundant opportunities for dispersal, and likely results in high genetic connectivity among populations. Population genetic studies on other hoplonemertean species reveal higher-than-expected gene flow, suggesting that planktotrophic macrophagy (predatory larvae), as described here for E. viride , may be widespread within the class.