Lepas testudinata, Aurivillius, 1894

Schiffer, Philipp H. & Herbig, Hans-Georg, 2016, Endorsing Darwin: global biogeography of the epipelagic goose barnacles Lepas spp. (Cirripedia, Lepadomorpha) proves cryptic speciation, Zoological Journal of the Linnean Society 177 (3), pp. 507-525 : 520

LEPAS TESTUDINATA

In general, L. testudinata seems to be a rare species. From our nuclear data (18S gene) we retrieved it as an in – group to L. anatifera . By contrast, Rees et al. (2014) found the species as a sister group to L. australis in their phylogram of barnacle phylogeny, based on 28S sequences. According to the morphological similarity of both species, we assume that singlegene genealogies are not suitable to resolve their relationships, and propose a future phylogenomic study. Mitochondrial data suggest the existence of two subgroups, in South African and western Australian waters. This is in contrast to the cool-water species L. australis , and to the warmer water species L. anatifera also. The different population structure remains unexplained.

LEPAS ANSERIFERA AND LEPAS PECTINATA

Our 18S data place L. anserifera as an out-group to L. anatifera and L. australis , and L. pectinata as an out-group to all three ( Fig. 3 ). Our data show that northern and southern hemisphere populations of L. pectinata are not genetically distinct. Previous observations from the Atlantic report finding the species in temperate and tropical waters from the north of Ireland to Cape Horn ( Gruvel, 1905; Young, 1990; see also data from the GBIF database). This means that a split into distinct antitropical populations ( Hinojosa et al., 2006: fig. 6; see our Fig. 1 ) is not realized, and that panmixis between oceans is likely. Tolerance of cooler waters is known for L. pectinata ( Zevina & Memmi, 1981; see also Hinojosa et al., 2006; our Fig. 1 ). This means that transit from the Indopacific into the Atlantic around South Africa should be possible, in spite of the Benguela upwelling system, whereas the plate tectonic-induced barrier at the Isthmus of Panama and the climateinduced barrier at Cape Horn will persist.

In L. anserifera a panmictic population in both hemispheres is documented by our samples from the north-equatorial Atlantic ( Cabo Verde, Senegal), from South Africa, and from Japan. It appears that similar ecological constrains explain the undifferentiated biogeographic distribution of L. pectinata and L. anserifera .

Concerning the Indopacific realm, the difference between L. anserifera and L. anatifera is striking. The latter, like other pelagic invertebrates, is strongly related to the hemispherical gyre systems, which are effectively separated by the eastwarddirected equatorial countercurrent; however, a study using tracer buoys and subsequently modelling the development of the huge garbage patches that accumulate in the gyre systems of the oceans on centennial timescales ( Van Sebille et al., 2012) demonstrated the leakiness of these systems and interchange between all of the oceans. Most important in this respect appear to be non-linear mesoscale eddies ( Chelton, Schlax & Samelson, 2011).

Trans-equatorial relationships are also well known for other Pacific marine organisms. In a study on the calcareous sponge Leucetta ‘ chagosensis ’ Dendy, 1913, Worheide, Hooper & Degnan (2002) proved a trans-equatorial clade, including individuals from the northern and central Great Barrier Reef as well as from Guam and Taiwan. Benzie & Williams (1997) also found strongly related haplotypes of the giant clam Tridacna maxima Roding, 1798 from the Philippines and the Great Barrier Reef. Finally, Williams & Benzie (1998) noted remarkable analogies in the starfish Linckia laevigata ( Linnaeus, 1758) between several south-western Pacific, Philippine, and Japanese occurrences. Trans-equatorial dispersal was also mentioned by Schwaninger (2008) concerning the Neogene dispersal of Membranipora , rafting on kelp southwards along the western coasts of the Americas, and later northwards in the Atlantic – a hypothesis in contrast to the current system stressed by Van Sebille et al. (2012). Thus, additional pathways seem to have existed in the Pacific seasonally, episodically, or during times of changed climate or sea level in the geological past. It might be speculated that during Pleistocene glacial periods and strongly restricted Indonesian throughflow towards the Indian Ocean, a western trans-hemispherical current was developed, comparable with the present-day Guiana Current along the western margin of the Atlantic. In fact, Benzie & Williams (1997) and Williams & Benzie (1998) showed that the routes of gene flow in the western Pacific are not consistent with present-day ocean currents. They inferred dispersal during lowered Pleistocene sea levels and colder climate, which affected wind and current systems in multiple ways, and the general extension of species originating in the western Pacific in the north-western direction, towards Southeast Asia.

Figures

Figure 1. Major oceanic current systems and the distribution of Lepas anatifera (blue), Lepas australis (green), and Lepas pectinata (orange, vertically ruled), after Hinojosa et al. (2006). Lepas anatifera is restricted to waters warmer than 15 °C, whereas L. australis is found in cooler water masses.

Figure 3. Phylogenetic (maximum-likelihood and Bayesian) tree based on the analysis of a fragment of the 18S ribosomal gene, including two out-group species. Posterior probabilities and bootstrap values are indicated; where support is maximal, only posterior probabilities are given. A neighbour net constructed in SplitsTrees indicates a clear tree-like signal in the sequence data, in conflict with the data for Lepas anserifera, which led us to analyse mitochondrial loci independently.