First endemic arachnid from Isla Sala y Gómez (Motu Motiro Hiva), Chile: a new species of tube-dwelling spider (Araneae: Segestriidae)

The Isla Sala y Gómez or Motu Motiro Hiva is located 415 km northeast of Rapa Nui (Easter Island) and 3420 km from the coast of northern Chile. It is a small oceanic island (2.5 km2) dominated by volcanic rock with very little vegetal cover. Here, we describe the fi rst endemic arachnid for the island, Ariadna motumotirohiva sp. nov. Females are similar to those of Ariadna perkinsi Simon, 1900 from Hawaiʻi and Ariadna lebronneci Berland, 1933 from the Marquesas in the dorsal dark abdominal pattern, but they differentiate from the latter in the anterior receptaculum, promarginal cheliceral teeth and leg IV macrosetae. A recent survey of the arachnid fauna of Rapa Nui, which included Motu Nui and the rocky shores, did not record the presence of the family Segestriidae, neither has it been found during previous surveys. However, it is not possible to discard the possibility of a local extinction on Rapa Nui and survival on Sala y Gómez. This study suggests other endemic terrestrial arthropods could be present on this very small and remote island.


Introduction
Ariadna Audouin, 1826 is a genus in the tube-web family Segestriidae, currently comprising 107 species (World Spider Catalog 2019). Recently, Giroti & Brescovit (2018) presented a new taxonomic revision of the genus for the American continent, in which six species were recorded from Chile: Ariadna maxima (Nicolet, 1849), A. abrilae, A. araucana, A. changellkuk, A. levii, all described by Grismado (2008), and A. lalen Giroti & Brescovit, 2018. Currently, only A. maxima can be found on Chilean islands, most precisely on the Juan Fernández islands, a small archipelago near the Chilean coast. Similarly to other members of the Segestriidae, this genus inhabits silk tube-webs, with irradiate threads, often inside tree logs and bark, rock crevices, and human constructions (Giroti & Brescovit 2018). Some species are even found in desert areas of Namibia, where they build silk tubes burrowed in the sand (Costa et al. 1995;Costa & Conti 2013).
The Isla Sala y Gómez or Motu Motiro Hiva (26°28′ S, 105°21′ W) is located 415 km northeast of Rapa Nui (Easter Island) and 3420 km from the coast of northern Chile. It is 2.5 km 2 in area, has a horseshoe shape and measures 700 m east-west and 400 m north-south at its widest points (Fig. 1). The highest point reaches only 30 m above sea level (Castilla & Oliva 1988). This volcanic island is part of the same underwater mountain chain as Rapa Nui. The age of its lava fl ows ranges between 1.31 and 1.94 million years (Clark & Dymond 1977). The island's surface is dominated by volcanic rock with very little vegetal cover, composed of two vascular plants (Portulaca oleracea L. and Sesuvium portulacastrum (L.) L. ;Rehder 1980) and one fern (Asplenium obliquum G.Forst.; Skottsberg 1956). Most of the faunistic studies from the island refer to marine invertebrates (Rehder 1980;Castilla & Rozbaczylo 1988;Burukovsky 1990;Tabachnick 1990;Coloma et al. 2004) and fi shes (Parin 1991;Pequeño 2004). There are also records for 14 species of sea birds, with 10 of those nesting on site (Vilina & Gazitua 1999). The fi rst record for a terrestrial arthropod on the island corresponded to Cryptamorpha desjardinsi (Guérin-Méneville, 1844) (Silvanidae, Coleoptera; see Elgueta & Lazo 2013). This species is considered cosmopolitan and it is present on Rapa Nui (Campos & Peña 1973). Recently, Hershauer et al. (2020) published an arthropod inventory carried out in 2016. In this species list, they report the presence of 15 terrestrial morphospecies representing 10 different orders, which include three arachnids including a Segentriidae, but without defi nition of species. The remoteness of Sala y Gómez, combined with the diffi culties in accessing it are likely responsible for the limited number of terrestrial invertebrate records. In the present study, we describe the fi rst endemic arachnid for the island, a new segestriid species, based on female specimens collected in 2012.

Material and methods
Examinations and descriptions of the specimens were made under a Leica MZ6 stereo microscope. Descriptions, macrosetae pattern and female internal genitalia preparation follow Giroti & Brescovit (2018). Measurements are in millimeters (mm), except those of the internal female genitalia. External morphology photographs were taken with a Leica DFC 500 digital camera attached to a Leica MZ16A stereo microscope; extended focal range photos were composed with Leica Application Suite ver. 3.8. Internal female genitalia photographs were taken with a Leica DFC 425 digital camera attached to a Leica DM 2500 microscope and stacked using Combine ZP 1.0 software. Illustrations were made using a Leica DM 4000 B microscope with camera lucida attached. Geographical coordinates are given in degrees (DMS). The map on Fig. 1 was treated with Esri (www.esri.com) and QGIS (ver. 2.18.28).

Etymology
The name refers to the name given by the Rapa Nui people to the type locality, 'Motu Motiro Hiva'; noun in apposition.

Note
For comparison we used the female specimen of A. perkinsi described by Giroti & Brescovit (2018) (IBSP 209056) and the original description and illustrations provided by Berland (1933: 44) of A. lebronneci.

Distribution
Only known from the type locality (Fig. 1).

Discussion
A new species from the family Segestriidae is described based on female specimens collected on the remote Isla Sala y Gómez. The species is most similar to Ariadna perkinsi Simon, 1900 andAriadna lebronneci Berland, 1933 due to the dorsal dark abdominal pattern; however, it differentiates from them in the anterior receptaculum and the promarginal cheliceral teeth, and leg IV macrosetae, respectively. These morphological affi nities suggest an evolutionary proximity; however, explicit phylogenetic reconstructions including this species are still needed.
The colonization by the genus Ariadna of several remote archipelagos in the eastern Pacifi c (A. perkinsi in Hawaiʻi, A. lebronneci in the Marquesas, A. maxima in the Juan Fernández, A. tarsalis in the Galápagos, and A. weaveri in the Revillagigedo Islands; Grismado 2008;Giroti & Brescovit 2018;World Spider Catalog 2019), may have been mediated by rafting. The large size of these spiders when adult probably prevents mature specimens from performing ballooning (see Sheldon et al. 2017), although there is an exception to this claim found in Eresidae (Schneider et al. 2001). Currently, a single species is described for each group of islands, implying that the genus is not prone to diversifying within an island, or that there are cryptic species yet to be described.
Ariadna motumotirohiva sp. nov. is present on a single island, the closest land mass is Rapa Nui at 415 km. This distance is about 50 km greater than the separation between Kauaʻi and Maui in Hawaiʻi, or Isla Darwin and Isla San Cristóbal in the Galápagos; each island pair having populations of the respective species from the archipelago (Giroti & Brescovit 2018). However, there is the important difference of only having water between Sala y Gómez and Rapa Nui; while for the other examples, there are other islands in between, facilitating a potential stepping stone dispersal. Because of the high number of terrestrial extinctions on Rapa Nui (Wynne et al. 2014), the lack of knowledge and the depopulate condition of Sala y Gómez, it is diffi cult to argue that both islands correspond to an archipelago system, although for marine organisms this idea has been suggested (Rehder 1980). For this reason, the marine environment of Sala y Gómez might resemble the one on Rapa Nui before humans. Does the same situation apply to the terrestrial environment? It is important to consider the intrinsic particularities of Sala y Gómez, which create strong constraints for the species that inhabit it; particularly, the lack of permanent sources of fresh water, the reduced vegetation and the complete exposure to the marine environment. Therefore, the fairest comparison with Rapa Nui will correspond to the rocky shores or, even more similarly, to the two islets on the south of the island (Motu Nui and Motu Iti).
A recent survey of the arachnid fauna of Rapa Nui, which included Motu Nui and the rocky shores, did not record the presence of Segestriidae, neither was it described from previous collections (Cotoras et al. 2017a). However, it is not possible to discard the possibility of local extinction on Rapa Nui and survival on Sala y Gómez. The only endemic spider described for Rapa Nui, Tetragnatha paschae (Tetragnathidae), potentially went extinct during the fi rst half of the 20 th century (Cotoras et al. 2017b), implying that other unrecorded extinctions may also have occurred.
Neighboring small islets could function as relictual sites for the species originally present on the main island. This has been shown for plants in Hawaiʻi (Wood 2012) and for the Lord Howe Island stick insect (Priddel et al. 2003). The situation of the latter bears similarities to that of A. motumotirohiva sp. nov., due to the almost complete lack of vegetation on Ball's pyramid, the islet where the stick insect was re-discovered.
This study provides the fi rst record of an endemic arachnid on Isla Sala y Gómez, suggesting that other endemic terrestrial arthropods could be present on this very small and remote island. Only paleontological studies of sediment cores from the Rapa Nui lakes could reveal whether A. motumotirohiva sp. nov. is a relictual species previously also present on Rapa Nui or an extreme case of local insular endemism.