Journal
INTEGRATIVE AND COMPARATIVE BIOLOGY
Volume 52, Issue 1, Pages 138-150Publisher
OXFORD UNIV PRESS INC
DOI: 10.1093/icb/ics077
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Funding
- US National Science Foundation [IOS-1157279, OCE 06-48606, OCE 11-30072, DEB-0817084]
- Company of Biologists, Ltd.
- American Microscopical Society
- Society for Integrative and Comparative Biology
- SICB division DEDB
- SICB division DEE
- SICB division DIZ
- NSF [OCE 0095724, PEHS 0550468]
- Encyclopedia of Life
- Direct For Biological Sciences
- Division Of Environmental Biology [0817084] Funding Source: National Science Foundation
- Direct For Biological Sciences
- Division Of Integrative Organismal Systems [1157279] Funding Source: National Science Foundation
- Directorate For Geosciences
- Division Of Ocean Sciences [1130072] Funding Source: National Science Foundation
- Office Of The Director
- Office Of Internatl Science &Engineering [1130978] Funding Source: National Science Foundation
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Credible cases of poecilogony, the production of two distinct larval morphs within a species, are extremely rare in marine invertebrates, yet peculiarly common in a clade of herbivorous sea slugs, the Sacoglossa. Only five animal species have been reported to express dimorphic egg sizes that result in planktotrophic and lecithotrophic larvae: the spionid polychaete Streblospio benedicti and four sacoglossans distributed in temperate estuaries or the Caribbean. Here, we present developmental and genetic evidence for a fifth case of poecilogony via egg-size dimorphism in the Sacoglossa and the first example from the tropical Indo-Pacific. The sea slug Elysia pusilla produced both planktotrophic and lecithotrophic larvae in Guam and Japan. Levels of genetic divergence within populations were markedly low and rule out cryptic species. However, divergence among populations was exceptionally high (10-12% at the mitochondrial cytochrome c oxidase I locus), illustrating that extensive phylogeographic structure can persist in spite of the dispersal potential of planktotrophic larvae. We review reproductive, developmental, and ecological data for the five known cases of poecilogony in the Sacoglossa, including new data for Costasiella ocellifera from the Caribbean. We hypothesize that sacoglossans achieve lecithotrophy at smaller egg sizes than do related clades of marine heterobranchs, which may facilitate developmental plasticity that is otherwise vanishingly rare among animals. Insight into the environmental drivers and evolutionary results of shifts in larval type will continue to be gleaned from population-level studies of poecilogonous taxa like E. pusilla, and should inform life-history theory about the causes and consequences of alternative development modes in marine animals.
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