If I were to choose a favorite larval type, it would certainly be the veliger. While photos of these baby mollusks are lovely, their true charm can only be fully appreciated when you see them in motion.
Mollusks pass through an earlier larval stage, the trochophore, which is a larval type also seen in annelids. Although trochophores are cute in their own right, the lively action of the cilia on the margin of the velum and their tiny shells make veligers a special treat to run across in a plankton sample. Even nudibranchs, which lack a shell as adults, pass through a veliger stage.
Veligers have been famously celebrated in verse by Walter Garstang whose “Ballad of the Veliger” not only described the larvae, but also proposed an adaptive explanation for torsion. During development, molluscan larvae undergo a 180-degree twist of the body, which brings the gills and mantle cavity from where they start at the back of the body into position directly over the head. Perhaps, Garstang suggested in his poem, the mantle cavity provides a space for the larval head to retract into when threatened by enemies, which would make post-torsional larvae less vulnerable to predation. Almost six decades later, Pennington and Chia performed a series of feeding experiments and found no evidence to support this hypothesis, as a variety of predators, including hydromedusae, ctenophores, and crab larvae happily gobbled up abalone larvae whether they had undergone torsion yet or not. A complete understanding of the evolution of the gastropod body plan is likely to be rather more complicated than can be captured in verse.
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