Sunday, March 18, 2018

[Arachnida • 2018] Repeated Diversification of Ecomorphs in Hawaiian Stick Spiders, Ariamnes spp.

Gillespie, Benjamin, Brewer, et al., 2018.

• Hawaiian stick spiders show adaptive radiation with repeated evolution of ecomorphs
• This phenomenon is found in only a few adaptive radiations of island insectivores
• Camouflage against a finite set of predators and wandering habit play key roles
• Limited pathways for the development of color contribute to deterministic evolution

Insular adaptive radiations in which repeated bouts of diversification lead to phenotypically similar sets of taxa serve to highlight predictability in the evolutionary process. However, examples of such replicated events are rare. Cross-clade comparisons of adaptive radiations are much needed to determine whether similar ecological opportunities can lead to the same outcomes. Here, we report a heretofore uncovered adaptive radiation of Hawaiian stick spiders (Theridiidae, Ariamnes) in which different species exhibit a set of discrete ecomorphs associated with different microhabitats. The three primary ecomorphs (gold, dark, and matte white) generally co-occur in native forest habitats. Phylogenetic reconstruction mapped onto the well-known chronosequence of the Hawaiian Islands shows both that this lineage colonized the islands only once and relatively recently (2–3 mya, when Kauai and Oahu were the only high islands in the archipelago) and that the distinct ecomorphs evolved independently multiple times following colonization of new islands. This parallel evolution of ecomorphs matches that of “spiny-leg” long-jawed spiders (Tetragnathidae, Tetragnatha), also in Hawaii. Both lineages are free living, and both have related lineages in the Hawaiian Islands that show quite different patterns of diversification with no evidence of deterministic evolution. We argue that repeated evolution of ecomorphs results from a rugged adaptive landscape, with the few peaks associated with camouflage for these free-living taxa against the markedly low diversity of predators on isolated islands. These features, coupled with a limited genetic toolbox and reduced dispersal between islands, appear to be common to situations of repeated evolution of ecomorphs.

Figure 2. Ecological Forms of the Hawaiian Ariamnes Colored boxes around images show the different ecomorphs: matte white, dark, and gold.
 (A) Ariamnes huinakolu; Kauai, Makalehas; July 2008. (B) A. sp.; Kauai, Pihea; November 2016. (C) A. kahili; Kauai, Wailua River; November 2016. (D) A. sp.; Oahu, Pahole; August 2008. (E) A. makue; Oahu, Kaala; November 2016. (F) A. uwepa; Oahu, Poamoho; November 2016. (G) A. corniger; East Maui; November 2016. (H) A. laau; East Maui; July 2013. (I) A. sp.; Molokai; November 2016. (J) A. waikula on web of Orsonwelles; Hawaii; July 2013. (K) A. hiwa; Hawaii; July 2014. (L) A. waikula; Hawaii, Saddle Road; July 2013.

Note that all of the gold forms—(C), (F), (I), and (L)—can exhibit color polymorphism, with red superimposed on the gold, as shown in (I). Photo credits: G. Roderick, (A–J), A. Rominger, (K), D. Cotoras, (L). Insets (B1, F1, and G1) show details of the guanine structure of the respective forms.

Ariamnes corniger, a stick spider from East Maui, Hawaiian Archipelago. This white matte ecomorph is cryptic against lichen.
 photo: George Roderick

Gold Molokai spider.
photo: George Roderick

An undescribed species of Ariamnes from Kauai, Hawaiian Archipelago. It is an example of the dark ecomorph.
photo: George Roderick 

Rosemary G. Gillespie, Suresh P. Benjamin, Michael S. Brewer, Malia Ana J. Rivera and George K. Roderick. 2018.  Repeated Diversification of Ecomorphs in Hawaiian Stick Spiders. Current Biology. DOI: 10.1016/j.cub.2018.01.083

How brightly colored spiders evolved on Hawaii again and again... and again   @physorg_com

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