For benthic invertebrates, the ability to disperse is usually limited to the short time period when they are larvae. A common way is to produce huge masses of tiny planktonic larvae that float and feed among the plankton, until they reach a big enough size to metamorphose to adult form and settle back to benthos. By that time, they might have floated quite far from their population of origin, increasing gene flow between distant populations. Despite the advantages of producing high-dispersing larvae, some species produce larvae with limited capabilities to disperse. This strategy is characterised by the production of much smaller numbers of larvae that do not feed on prey after hatching, but rely on the yolk or specific nursing eggs on their nutrition, and are therefore called lecithotrophic larvae. These larvae typically look similar to the adult individuals and do not need to metamorphose to mature, only grow in size.

Typically, each species relies on a single strategy, but a few species are capable of both, a phenomenon known as poecilogony. This extraordinary startegy is found in some polychaete annelids and sacoglossan gastropods, where different larval developmental modes have been observed in the same species or even within the same population.

In some cases, the variation in larval type is a ‘plastic’ trait in that it is environmentally mediated. One such example is the sea slug Alderia willowi that produces more yolk-feeding larvae during the summer, and more planktonic-feeding larvae in the winter. In other cases, the larval type is genetically determined, such as in the polychaete Streblospio benedicti, where within same population some mothers release small planktotrophic eggs and other mothers release large lecithotrophic eggs (Figure 1). However, in most cases the underlying mechanisms are not excatly known. One such example is one of my study species, the polychaete Pygospio elegans (Figure 2), that exhibits larval developmental mode variation between and sometimes within populations but neither a genetic nor an environmental basis for it has been determined.

What makes poecilogony interesting is because it can have major ecological and evolutionary consequences on species exhibing it: poecilogonous species provide sort of a living laboratory for studying how developmental modes evolve and the genetic basis of such variation, as well as the ecological pressures that shape life cycles in marine invertebrates. For example, variable dispersal ability can affect gene flow, lead to local patchiness and affect population densities. Sometimes poecilogony might raise taxonomic challenges, when different larval forms can look like separate species, but genetics might reveal they’re one species. Alternatively, many species thought to be poecilogonic have in fact revelaed to be cryptic species.

References

Kesäniemi, J.E., Mustonen, M., Boström, C. et al. 2014. Temporal genetic structure in a poecilogonous polychaete: the interplay of developmental mode and environmental stochasticity. BMC Evol Biol 14, 12. https://doi.org/10.1186/1471-2148-14-12

Knott, K.E. & McHugh, D. 2012. Introduction to Symposium: Poecilogony—A Window on Larval Evolutionary Transitions in Marine Invertebrates, Integrative and Comparative Biology, 52(1): 120–127, https://doi.org/10.1093/icb/ics037

Nagai, Y., Hamamatsu, I., Kishi, S. et al. 2025. Reproduction, larval development, and population dynamics of Polydora websteri (Annelida, Spionidae) inhabiting wild oyster shells from Gamo lagoon, Northeastern Japan. Mar Biol 172(134). https://doi.org/10.1007/s00227-025-04698-1

Thonig A., Kesäniemi J., Knott K.E., Winding Hansen B. & Banta G. 2016. Population and reproductive dynamics of the polychaete Pygospio elegans in a boreal estuary complex. Invertebrate Biology 135(4): 370–384.

Zakas, C. 2022. Chapter Seventeen – Streblospio benedicti: A genetic model for understanding the evolution of development and life-history, Eds: Bob Goldstein, Mansi Srivastava, Current Topics in Developmental Biology, Academic Press, Volume 147, p 497-521. https://doi.org/10.1016/bs.ctdb.2021.12.021

Cover image: Two developmental modes of Polydora websteri larvae from Nagai et al. 2025.