Sipunculan Larvae and "Cosmopolitan" Species

*Department of Marine Biology, Texas A&M University at Galveston, P.O. Box 1675, Galveston, TX 77553, USA
Integrative and Comparative Biology (Impact Factor: 2.93). 05/2012; 52(4):497-510. DOI: 10.1093/icb/ics082
Source: PubMed


Sipuncula is a relatively small taxon with roughly 150 recognized species. Many species are geographically widespread or "cosmopolitan." The pelagosphera larvae of some species are estimated to spend several months in the plankton. However, recent molecular evidence suggests that many of the "cosmopolitan" species actually represent species-complexes, some not even monophyletic. Herein, we present data on three sipunculan species with different developmental modes that occur both in the Sea of Japan and in the Northeast Pacific. The development of the three species-Phascolosoma agassizii, Thysanocardia nigra, and Themiste pyroides-is exceptionally well studied in both regions of the Pacific. Significant differences have been observed between the two regions with respect to egg size, developmental mode, and developmental timing. In general, eggs are larger and development slower in the Northeast Pacific when compared with the Sea of Japan. These differences have been explained as a result of phenotypic plasticity exhibited under different environmental conditions, in particular temperature, but we show that the populations of all three species are also remarkably distinct genetically and that gene flow between the two regions is extremely unlikely. In Thysanocardia nigra, we even found two very distinct genetic lineages within the same location in the Northeast Pacific. The amount of genetic divergence between populations from the Sea of Japan and those from the Northeast Pacific is not correlated with developmental mode. Themiste pyroides, the species with the most abbreviated development, actually has the least degree of genetic divergence between the regions. Analyses of molecular variance show that the majority of the observed variation in all three species is between the regions. We conclude that all three "cosmopolitan" species actually represent complexes of cryptic or pseudo-cryptic species. These examples demonstrate that a solid taxonomic framework based on molecular and morphological evidence is a prerequisite for evaluating relationships between dispersal capabilities, species' ranges, and the connectivity of populations.

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    • "They are also emerging as important non-model organisms for evolutionary and developmental biology, or evo-devo (Schulze and Rice, 2009b; Wanninger et al., 2005, 2009; Wanninger, 2008; Boyle and Seaver, 2010; Boyle and Rice, 2014). Fourth, due to an extended larval phase described for several species within multiple families (Scheltema and Hall, 1965, 1975; Rice, 1976, 1981; Scheltema and Rice, 1990), sipunculans constitute an interesting group for studying dispersal within and between widely separated oceanic regions, which is a topic addressed in several recent studies of cosmopolitanism in the marine realm (Staton and Rice, 1999; Kawauchi and Giribet, 2010; Kawauchi and Giribet, 2013; Schulze et al., 2012; Young et al., 2012; Lemer and Planes, 2014). These and similar studies rely upon robust phylogenetic hypotheses, from species to family-level relationships, to provide a stable evolutionary framework for critical re-interpretation of previous research, and to guide future investigations. "
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