Ianthella basta (Pallas, 1766)

Andreakis, Nikos, Luter, Heidi M. & Webster, Nicole S., 2012, Cryptic speciation and phylogeographic relationships in the elephant ear sponge Ianthella basta (Porifera, Ianthellidae) from northern Australia, Zoological Journal of the Linnean Society 166 (2), pp. 225-235 : 232-233

IN IANTHELLA BASTA

The marine regions of northern Australia and the GBR are characterized by biogeographic barriers that can act as promoters of population fragmentation and speciation, even on small geographic scales ( Wörheide et al., 2008). A well-known example is the Torres Strait, responsible for the formation of biodiversity hotspots, including elevated sponge species richness and refugia zones ( Wörheide et al., 2005). Ianthella basta is a widely distributed taxon with published records from the GBR, Torres Strait, the Dampier Archipelago, and the Indian Ocean ( Hooper & Wiedenmayer, 1994; Bergquist & Kelly-Borges, 1995; Hooper & Van Soest, 2002; Fromont, 2004). The combination of morphological plasticity and broad distribution range make I. basta an ideal model system for exploring ecological, biogeographic, and evolutionary processes in sponges.

Median-joining network reconstructions revealed three distinct haplotypes in northern Australia that were exclusively associated with geography. A fourth most-frequent haplotype was recovered from four out of the five sampling sites. DNA ITS2 sequences from the most frequent haplotype I were identical to the ITS2 sequences from specimens genetically and morphologically identified as Ianthella basta from Guam ( Erwin & Thacker, 2007), thus confirming Guam and northern Australia as the geographic distribution range of Ianthella basta s.s. Furthermore, the identical ITS2 sequences indicate a human-mediated introduction of ESU I into Guam, and is consistent with the previous recognition of this geographic region as a receiving site of non-indigenous marine species ( Paulay et al., 2002).

Despite the low sample number from Bonaparte Gulf, our results indicate considerable population structure across Torres Strait, characterized by variable levels of genetic differentiation ranging from monomorphic to highly variable geographic populations ( Tables 3 and 5). These phylogeographic patterns in I. basta across Torres Strait are consistent with historical population fragmentation through geographical isolation, and the secondary dispersal of a once widely distributed sponge population ( Mirams, Treml & Shields, 2011). On the other hand, the sympatric distribution of ESUs I/III and I/II in Kimberly coast and Orpheus Island, respectively, together with the congruence between nuclear and mitochondrial topologies, suggest a limited capability for interbreeding among the ESUs (sensu Mayr, 1942).

Torres Strait represents a geographic barrier between the Indian and Pacific oceans ( Voris, 2000), and is well known for acting as a land bridge during the periodic glacial cycles and sea level fluctuations over the past 250 000 years ( Voris, 2000). The phylogeography of I. basta is consistent with historic geographic isolation events and genetic differentiation observed in other marine species ( Reid et al., 2006; Bentlage & Wörheide, 2007; van Herwerden et al., 2009; Mirams et al., 2011). Given the absence of suitable fossil records or mutation rates useful for molecular clock calibrations for the taxa analysed in this study, we are unable to approximate the timing of diversification in I. basta . However, the high level of population differentiation together with the geographic structure for the COI haplotypes suggests a dynamic population fragmentation scenario. Following geographic isolation, selected, locally adapted haplotypes must have persisted in refugia zones such as the Kimberly coast and Orpheus Island, both regions known for their high sponge species richness ( Wörheide et al., 2005; Mirams et al., 2011). The lack of fossil records combined with seasonal changes in current direction and complex vortices that occur within Torres Strait ( Wolanski, Ridd & Inoue, 1988; Margvelashvili, Saint-Cast & Condie, 2008) make it difficult to establish the timing and direction of dispersal for ESU I. However, the limited but highly frequent nuclear types, the single mitochondrial genetic variant recovered from ESU I, and the geographic distance among sampling sites (i.e. from Davies Reef, where the ESU is abundant, to the Kimberly coast, where only one individual was recovered) is suggestive of a recent population expansion scenario.