Abstract
BackgroundPoecilogony, the presence of two developmental modes in the same animal species, is a rare phenomenon. Few cases of poecilogony have been suggested for marine invertebrates including molluscs and even less stood extensive testing, mostly revealing a species pair with differing developmental modes. We studied a textbook example of poecilogony in the viviparous snail Planaxis sulcatus (Gastropoda: Planaxidae), for the first time throughout its entire distribution range.ResultsIn the Western Indian Ocean and Red Sea this intertidal species is observed to have large, shelled juveniles, whereas in the Indo-West Pacific planktotrophic veliger larvae are released from a subhaemocoelic brood pouch. We uncovered a shift in developmental modes across its range: from west to east successively earlier developmental stages are released. Furthermore, genetic data based on mitochondrial DNA suggests to recognize P. sulcatus as a single species rather than a group of cryptic species. A reconstruction of the ancestral area of P. sulcatus based on molecular data outlines the Western Indian Ocean and the Indo-West Pacific as area of origin.ConclusionThe findings supporting Planaxis sulcatus as a single widespread species and the geographical shift from one reproductive mode to another suggest for this species to truly represent a case of geographic poecilogony, i.e. differing developmental modes between populations of the same species. Furthermore, the results of our ancestral range estimation imply the release of planktotrophic larvae as the ancestral developmental mode.
Highlights
Poecilogony, the presence of two developmental modes in the same animal species, is a rare phenomenon
Based on records form the Iranian Gulf and Pakistan populations, it has been suggested that P. sulcatus releasing juveniles are adelphophagic, supplying nutrition eggs for juveniles within the brood pouch [15,16,17]
Brood pouches of IP populations contained large amounts of larvae
Summary
Poecilogony, the presence of two developmental modes in the same animal species, is a rare phenomenon. The marine snail Planaxis sulcatus (Born, 1780) is a textbook example for poecilogony [13] This viviparous intertidal species is widespread throughout the tropical Indo-West Pacific Region (IWP, geographic delimitations following [14]), with populations known from the Red Sea (RS), Western Indian Ocean (WIO) and IndoPolynesian (IP) provinces. Based on records form the Iranian Gulf and Pakistan populations, it has been suggested that P. sulcatus releasing juveniles are adelphophagic (offspring grow large by consuming other earlier developmental stages or other juveniles within the mother’s brood pouch), supplying nutrition eggs for juveniles within the brood pouch [15,16,17] In these populations it was found that broods do not represent single cohorts, but rather that eggs can be added sequentially [17]. No differences in larval morphology between veliger and juvenile releasing specimens has been recorded [15,16,17,18,19,20]
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