Abstract
Compared to the striking diversification and levels of endemism observed in many terrestrial groups within the Hawaiian Archipelago, marine invertebrates exhibit remarkably lower rates of endemism and diversification. Supralittoral invertebrates restricted to specific coastal patchy habitats, however, have the potential for high levels of allopatric diversification. This is the case of Ligia isopods endemic to the Hawaiian Archipelago, which most likely arose from a rocky supralittoral ancestor that colonized the archipelago via rafting, and diversified into rocky supralittoral and inland lineages. A previous study on populations of this isopod from Oʻahu and Kauaʻi revealed high levels of allopatric differentiation, and suggested inter-island historical dispersal events have been rare. To gain a better understanding on the diversity and evolution of this group, we expanded prior phylogeographic work by incorporating populations from unsampled main Hawaiian Islands (Maui, Molokaʻi, Lanaʻi, and Hawaiʻi), increasing the number of gene markers (four mitochondrial and two nuclear genes), and conducting Maximum likelihood and Bayesian phylogenetic analyses. Our study revealed new lineages and expanded the distribution range of several lineages. The phylogeographic patterns of Ligia in the study area are complex, with Hawaiʻi, Oʻahu, and the Maui-Nui islands sharing major lineages, implying multiple inter-island historical dispersal events. In contrast, the oldest and most geographically distant of the major islands (Kauaʻi) shares no lineages with the other islands. Our results did not support the monophyly of all the supralittoral lineages (currently grouped into L. hawaiensis), or the monophyly of the terrestrial lineages (currently grouped into L. perkinsi), implying more than one evolutionary transition between coastal and inland forms. Geometric-morphometric analyses of three supralittoral clades revealed significant body shape differences among them. A taxonomic revision of Hawaiian Ligia is warranted. Our results are relevant for the protection of biodiversity found in an environment subject to high pressure from disturbances.
Highlights
The Hawaiian Islands are well known for their rich biodiversity and high rate of endemism [1]
An additional interesting case of diversification in a coastal patchy habitat of the archipelago is that of oniscidean isopods in the genus Ligia, which most likely arose from a rocky supralittoral ancestor that arrived to the Hawaiian Archipelago via rafting, and diversified into rocky supralittoral and inland lineages
Given the low weights observed for these models under the Akaike Information Criterion (AIC) and AICc (Table S2), and that the 95% confidence interval included the Bayesian Information Criterion (BIC) selected model, we applied the latter in GARLI analyses
Summary
The Hawaiian Islands are well known for their rich biodiversity and high rate of endemism [1]. Endemism in Hawaiian marine invertebrates is strikingly lower than that in Hawaiian terrestrial organisms [14], and with the exception of intertidal Cellana limpets [15], there are no documented marine radiations within the Hawaiian Archipelago. Such disparity suggests that opportunities for allopatric differentiation within the archipelago have been very limited in the marine realm, while abundant in the terrestrial realm. Further phylogeographic analyses of Hawaiian Ligia including populations from previously unsampled islands are needed to better understand the biodiversity and evolution of this group
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