Only a handful of species produce both feeding and non-feeding larvae, a reproductive polymorphism termed poecilogony. We study two poecilogonous sea slugs, the Californian endemic Alderia willowi and the Caribbean Costasiella ocellifera. Identifying why populations switch from dispersing to non-dispersing larvae should yield insight into how natural selection drives the evolution of alternative larval strategies, an outstanding question in life-history evolution.

I previously reported that A. modesta produced feeding larvae throughout the Atlantic and Pacific, but in California also produced short-lived, non-feeding larvae in summer (Krug 1998). Recently, we collected molecular, morphological and developmental data for Alderia from 17 estuaries in the Pacific and Atlantic, and detected a cryptic phylogeographic breakpoint at Tomales Bay, CA. Slugs to the north fit the description of A. modesta, but southern slugs comprised a cryptic sister species that seasonally switches between larval morphs (Ellingson & Krug 2006). We described the poecilogonous species as A. willowi (Krug et al., in press); the name is a tribute to my grandmother, and to the character of Willow from Buffy the Vampire Slayer who displayed a similar flexibility in sexual behavior. Alderia willowi may produce non-feeding larvae in summer as an adaptation to closure of estuaries during the dry season, and switch to dispersing larvae in winter when rainfall opens estuaries to the coast.

Population genetic studies indicate that larval exchange maintains gene flow between estuaries in the Pacific; only the remote Morro Bay showed evidence of genetic isolation from other sites. In contrast, Atlantic and Pacific populations of A. modesta have been genetically isolated for 1.7 million years, when Pleistocene glaciation disrupted trans-Arctic gene flow; these populations may represent distinct species, but further sampling in the Atlantic will be needed to test this.

Field populations of A. willowi seasonally switch from feeding to non-feeding larvae, and individual slugs can toggle between development modes by changing the size of their eggs, which has never been reported for any other species. We are studying the cues that trigger such changes in development to better understand the adaptive value of this dispersal polymorphism, and the biochemical mechanisms that underlie it.

Variable Development - references:

Krug, P.J. 2007. Poecilogony and larval ecology in the gastropod genus Alderia. Am Malacol Bull 23, in press.

Krug, P.J., Ellingson, R.A., Burton, R.A. and Á. Valdés. 2007. A new poecilogonous species of sea slug (Opisthobranchia: Sacoglossa) from California: Comparison with the planktotrophic congener Alderia modesta (Lovén, 1844).J Moll Stud 73: 29-38. PDF

Ellingson, R.A. and Krug, P.J. 2006. Evolution of poecilogony from planktotrophy: Cryptic speciation, phylogeography and larval development in the gastropod genus Alderia. Evolution 60: 2293-2310. PDF

Krug, P.J. and Zimmer, R.K. 2004. Developmental dimorphism: Consequences for larval behavior and dispersal potential in a marine gastropod. Biol Bull 207: 233-246. PDF

Krug, P.J. 2001. Bet-hedging dispersal strategy of a specialist marine herbivore: A settlement dimorphism among sibling larvae of Alderia modesta. Mar Ecol Prog Ser 213: 177-192. PDF

Krug, P.J. and Zimmer, R.K. 2000. Developmental dimorphism and expression of chemosensory-mediated behavior: Habitat selection by a specialist marine herbivore. J Exp Biol 203: 1741-1754. PDF

Krug, P.J. 1998. Poecilogony in an estuarine opisthobranch: Planktotrophy, lecithotrophy, and mixed clutches in a population of the estuarine opisthobranch Alderia modesta. Mar Biol 132: 483-494. PDF

Planktotrophic (top) versus

lecithotrophic (bottom)

egg masses of A. willowi.

Phylogenetic analysis of 233 specimens of Alderia from 15 north-Pacific estuaries shows divergent clades 18-24% different at the COI gene. The northern A. modesta (blue) ranges west to Russia and into the Atlantic, while the recently described A. willowi (red) occurs south of Tomales Bay, CA. Unmarked haplotypes were from planktotrophic specimens (both species); dark circles mark haplotypes from lecithotrophic A. willowi, and green triangles mark haplotypes shared by specimens differing in development mode.