Ingi Agnarsson

Undersampling bias: the null hypothesis for singleton species in tropical arthropod surveys

1. Frequency of singletons - species represented by single individuals - is anomalously high in most large tropical arthropod surveys (average, 32%). 2. We sampled 5965 adult spiders of 352 species (29% singletons) from 1 ha of lowland tropical moist forest in Guyana. 3. Four common hypotheses (small body size, male-biased sex ratio, cryptic habits, clumped distributions) failed to explain singleton frequency. Singletons are larger than other species, not gender-biased, share no particular lifestyle, and are not clumped at 0.25-1 ha scales. 4. Monte Carlo simulation of the best-fit lognormal community shows that the observed data fit a random sample from a community of approximately 700 species and 1-2 million individuals, implying approximately 4% true singleton frequency. 5. Undersampling causes systematic negative bias of species richness, and should be the default null hypothesis for singleton frequencies. 6. Drastically greater sampling intensity in tropical arthropod inventory studies is required to yield realistic species richness estimates. 7. The lognormal distribution deserves greater consideration as a richness estimator when undersampling bias is severe.

From a comb to a tree: phylogenetic relationships of the comb-footed spiders (Araneae, Theridiidae) inferred from nuclear and mitochondrial genes.

The family Theridiidae is one of the most diverse assemblages of spiders, from both a morphological and ecological point of view. The family includes some of the very few cases of sociality reported in spiders, in addition to bizarre foraging behaviors such as kleptoparasitism and araneophagy, and highly diverse web architecture. Theridiids are one of the seven largest families in the Araneae, with about 2200 species described. However, this species diversity is currently grouped in half the number of genera described for other spider families of similar species richness. Recent cladistic analyses of morphological data have provided an undeniable advance in identifying the closest relatives of the theridiids as well as establishing the familys monophyly. Nevertheless, the comb-footed spiders remain an assemblage of poorly defined genera, among which hypothesized relationships have yet to be examined thoroughly. Providing a robust cladistic structure for the Theridiidae is an essential step towards the clarification of the taxonomy of the group and the interpretation of the evolution of the diverse traits found in the family. Here we present results of a molecular phylogenetic analysis of a broad taxonomic sample of the family (40 taxa in 33 of the 79 currently recognized genera) and representatives of nine additional araneoid families, using approximately 2.5kb corresponding to fragments of three nuclear genes (Histone 3, 18SrDNA, and 28SrDNA) and two mitochondrial genes (16SrDNA and CoI). Several methods for incorporating indel information into the phylogenetic analysis are explored, and partition support for the different clades and sensitivity of the results to different assumptions of the analysis are examined as well. Our results marginally support theridiid monophyly, although the phylogenetic structure of the outgroup is unstable and largely contradicts current phylogenetic hypotheses based on morphological data. Several groups of theridiids receive strong support in most of the analyses: latrodectines, argyrodines, hadrotarsines, a revised version of spintharines and two clades including all theridiids without trace of a colulus and those without colular setae. However, the interrelationships of these clades are sensitive to data perturbations and changes in the analysis assumptions.

Phylogeography of a successful aerial disperser: the golden orb spider Nephila on Indian Ocean islands

BackgroundThe origin and diversification patterns of lineages across the Indian Ocean islands are varied due to the interplay of the complex geographic and geologic island histories, the varying dispersal abilities of biotas, and the proximity to major continental landmasses. Our aim was to reconstruct phylogeographic history of the giant orbweaving spider (Nephila) on western Indian Ocean islands (Madagascar, Mayotte, Réunion, Mauritius, Rodrigues), to test its origin and route of dispersal, and to examine the consequences of good dispersal abilities for colonization and diversification, in comparison with related spiders (Nephilengys) inhabiting the same islands, and with other organisms known for over water dispersal. We used mitochondrial (COI) and nuclear (ITS2) markers to examine phylogenetic and population genetic patterns in Nephila populations and species. We employed Bayesian and parsimony methods to reconstruct phylogenies and haplotype networks, respectively, and calculated genetic distances, fixation indices, and estimated clade ages under a relaxed clock model.ResultsOur results suggest an African origin of Madagascar Nephila inaurata populations via Cenozoic dispersal, and the colonization of the Mascarene islands from Madagascar. We find evidence of gene flow across Madagascar and Comoros. The Mascarene islands share a common ancestral COI haplotype closely related to those found on Madagascar, but itself absent, or as yet unsampled, from Madagascar. Each island has one or more unique haplotypes related to the ancestral Mascarene haplotype. The Indian Ocean N. inaurata are genetically distinct from the African populations.ConclusionsNephila spiders colonized Madagascar from Africa about 2.5 (0.6-5.3) Ma. Our results are consistent with subsequent, recent and rapid, colonization of all three Mascarene islands. On each island, however, we detected unique haplotypes, consistent with a limited gene flow among the islands subsequent to colonization, a scenario that might be referred to as speciation in progress. However, due to relatively small sample sizes, we cannot rule out that we simply failed to collect Mascarene haplotypes on Madagascar, a scenario that might imply human mediated dispersal. Nonetheless, the former interpretation better fits the available data and results in a pattern similar to the related Nephilengys. Nephilengys, however, shows higher genetic divergences with diversification on more remote islands. That the better disperser of the two lineages, Nephila, has colonized more islands but failed to diversify, demonstrates how dispersal ability can shape both the patterns of colonization and formation of species across archipelagos.

Biogeography and diversification of hermit spiders on Indian Ocean islands (Nephilidae: Nephilengys)

The origin of the terrestrial biota of Madagascar and, especially, the smaller island chains of the western Indian Ocean is relatively poorly understood. Madagascar represents a mixture of Gondwanan vicariant lineages and more recent colonizers arriving via Cenozoic dispersal, mostly from Africa. Dispersal must explain the biota of the smaller islands such as the Comoros and the chain of Mascarene islands, but relatively few studies have pinpointed the source of colonizers, which may include mainland Africa, Asia, Australasia, and Madagascar. The pantropical hermit spiders (genus Nephilengys) seem to have colonized the Indian Ocean island arc stretching from Comoros through Madagascar and onto Mascarenes, and thus offer one opportunity to reveal biogeographical patterns in the Indian Ocean. We test alternative hypotheses on the colonization route of Nephilengys spiders in the Indian Ocean and simultaneously test the current taxonomical hypothesis using genetic and morphological data. We used mitochondrial (COI) and nuclear (ITS2) markers to examine Nephilengys phylogenetic structure with samples from Africa, southeast Asia, and the Indian Ocean islands of Madagascar, Mayotte, Réunion and Mauritius. We used Bayesian and parsimony methods to reconstruct phylogenies and haplotype networks, and calculated genetic distances and fixation indices. Our results suggest an African origin of Madagascar Nephilengys via Cenozoic dispersal, and subsequent colonization of the Mascarene islands from Madagascar. We find strong evidence of gene flow across Madagascar and through the neighboring islands north of it, while phylogenetic trees, haplotype networks, and fixation indices all reveal genetically isolated and divergent lineages on Mauritius and Réunion, consistent with female color morphs. These results, and the discovery of the first males from Réunion and Mauritius, in turn falsify the existing taxonomic hypothesis of a single widespread species, Nephilengys borbonica, throughout the archipelago. Instead, we diagnose three Nephilengys species: Nephilengys livida (Vinson, 1863) from Madagascar and Comoros, N. borbonica (Vinson, 1863) from Réunion, and Nephilengys dodo new species from Mauritius. Nephilengys followed a colonization route to Madagascar from Africa, and on through to the Mascarenes, where it speciated on isolated islands. The related golden orb-weaving spiders, genus Nephila, have followed the same colonization route, but Nephila shows shallower divergencies, implying recent colonization, or a moderate level of gene flow across the archipelago preventing speciation. Unlike their synanthropic congeners, N. borbonica and N. dodo are confined to pristine island forests and their discovery calls for evaluation of their conservation status.

The phylogenetic placement and circumscription of the genus Synotaxus (Araneae:Synotaxidae), a new species from Guyana, and notes on theridioid phylogeny

The genus Synotaxus Simon, 1895 is reviewed. Modern systematic work has challenged the classical placement of Synotaxus as an argyrodine theridiid. Its placement is evaluated in a phylogenetic analysis containing a wide selection of theridiids, including all major argyrodine genera. For the analysis, two published matrices are fused to produce a dataset containing 83 orbicularian taxa and 302 morphological characters. Although the two matrices share only 10 taxa and 33 characters, a single most parsimonious tree is obtained. The results are congruent with results from each independent matrix, and unambiguously corroborate the placement of Synotaxus outside Theridiidae. Some superficial similarities, such as the elongate abdomen extending far beyond the spinnerets, are clearly convergent in the two taxa. Synotaxus, furthermore, lacks the suite of synapomorphies defining Theridiidae. Thus, its transfer out of Theridiidae is corroborated, and a sister relationship with Chileotaxus Platnick, 1990 is proposed, based on similarities in web and somatic morphology. A synapomorphy-based circumscription of the genus is given. Synotaxus waiwai, sp. nov. is described, Synotaxus monoceros (Caporiacco, 1947), previously only known from males, is redescribed and synonymised with S. pupularum, Exline & Levi, 1965, syn. nov., previously only known from females. A description of the webs of both species is given, as well as that of Chileotaxus sans Platnick, 1990. Many further Synotaxus species remain to be described.

Madagascar: an unexpected hotspot of social Anelosimus spider diversity (Araneae: Theridiidae)

Abstract.u2002 The spider genus Anelosimus Simon, 1891 (Theridiidae) currently contains over forty described species, found worldwide in tropical to warm temperate areas. American Anelosimus are all social, a rare trait among spiders, but social behaviour has not been reported for Anelosimus species elsewhere. Old World Anelosimus are poorly known, both behaviourally and taxonomically, and no Anelosimus species have yet been described from sub‐Saharan Africa or Madagascar. Based on a preliminary phylogenetic analysis we predicted sociality in an undescribed Madagascar species because it grouped among social New World species. An expedition to Madagascar then found no less than five undescribed periodic‐social (subsocial) Anelosimus species in Périnet reserve. A sixth species from the same locality is known from museum specimens and the Anelosimus diversity of Périnet is comparable with the most diverse single locality in the Americas. Subsocial species play a key role in understanding the evolution of permanent sociality (quasisociality). This increased pool of available subsocial study species demonstrates the utility of phylogenies as predictors of traits in species thus far unstudied. Here, A. andasibesp.n., A.u2003may Agnarsson sp.n., A. nazarianisp.n., A. salleesp.n., A. salutsp.n. and A. vondronasp.n. are described. Anelosimus locketi Roberts, 1977 from Aldabra Atoll is a junior synonym of A. decaryi ( Fage, 1930 ) comb.n. from Madagascar. Preliminary data on the behaviour of the new species are given, indicating a level of sociality similar to the American A.‘arizona’1. The phylogenetic analysis supports the monophyly of the Madagascar group and places it as sister to a clade containing the eximius lineage from the Americas, and a pair of undescribed Tanzanian species.

Molecular insights into the phylogenetic structure of the spider genus Theridion (Araneae, Theridiidae) and the origin of the Hawaiian Theridion‐like fauna

The Hawaiian happy face spider (Theridion grallator Simon, 1900), named for a remarkable abdominal colour pattern resembling a smiling face, has served as a model organism for understanding the generation of genetic diversity. Theridion grallator is one of 11 endemic Hawaiian species of the genus reported to date. Asserting the origin of island endemics informs on the evolutionary context of diversification, and how diversity has arisen on the islands. Studies on the genus Theridion in Hawaii, as elsewhere, have long been hampered by its large size (> 600 species) and poor definition. Here we report results of phylogenetic analyses based on DNA sequences of five genes conducted on five diverse species of Hawaiian Theridion, along with the most intensive sampling of Theridiinae analysed to date. Results indicate that the Hawaiian Islands were colonised by two independent Theridiinae lineages, one of which originated in the Americas. Both lineages have undergone local diversification in the archipelago and have convergently evolved similar bizarre morphs. Our findings confirm para‐ or polyphyletic status of the largest Theridiinae genera: Theridion, Achaearanea and Chrysso. Convergent simplification of the palpal organ has occurred in the Hawaiian Islands and in two continental lineages. The results confirm the convergent evolution of social behaviour and web structure, both already documented within the Theridiidae. Greater understanding of phylogenetic relationships within the Theridiinae is key to understanding of behavioural and morphological evolution in this highly diverse group.

The Generation of a Biodiversity Hotspot: Biogeography and Phylogeography of the Western Indian Ocean Islands

The importance of islands in revealing evolutionary processes was highlighted already at the birth of evolutionary biology as a science (Darwin 1859; Darwin and Wallace 1858). Since the thrilling discoveries revealed by Darwin’s work on the Galapagos (Darwin 1909) and Wallace’s work in the Malay (Indonesian) archipelago (Wallace 1876), island biogeography has experienced an explosive growth. The discipline has provided many elegant examples of the evolutionary mechanisms involved in generating biodiversity, especially the interplay of geological processes and colonization and isolation (Emerson 2008; Gillespie, Claridge, and Goodacre 2008; Parent, Caccone, and Petren 2008; Ricklefs and Bermingham 2008). Islands have provided particularly strong insights into adaptive radiations (Camacho-Garcia and Gosliner 2008; Blackledge and Gillespie 2004; Cowie and Holland 2008; Gillespie and Roderick 2002; Losos and DeQueiroz 1997; Schluter 2000), the processes of colonization and extinction (Ricklefs and Bermingham 2008; Goldberg, Lancaster, and Ree 2011), the formation of species (Emerson 2008; Pickford et al. 2008; Gillespie and Roderick 2002; Schluter and Nagel 1995; Vences et al. 2009), and convergent evolution and formation of ecomorphs (Bossuyt and Milinkovitch 2000; Gillespie 2004, 2005; Losos 1988; Wildman et al. 2007; Burridge 2000; Rothe et al. 2011). Naturally, islands have also played a key role in revealing the causes and consequences of long distance dispersal, in particular, the ecological and evolutionary consequences of varying dispersal propensities of different lineages, and the evolutionary changes in dispersal propensity, such as the loss of dispersal ability following island colonization (Cowie and Holland 2006, 2008; Hedges and Heinicke 2007; Holland and Cowie 2006; Byrne et al. 2011; Bell et al. 2005; Darwin 1909; Clark 1994; Gillespie et al. 2012).

Systematics of new subsocial and solitary Australasian Anelosimus species (Araneae : Theridiidae)

Abstract. Species of the cobweb spider genus Anelosimus range from solitary to subsocial to social, and sociality has evolved repeatedly within the genus. Thus, this genus allows studies of the traits that play a role in social evolution. However, taxonomic knowledge of Anelosimus is geographically narrow and nearly all sociobiological studies have been done in the Americas. Only one behaviourally unknown species has been described from all of Australasia. Here, I describe seven new Anelosimus from Papua New Guinea (Anelosimus potmosbi, sp. nov., Anelosimus pomio, sp. nov., Anelosimus eidur, sp. nov. and Anelosimus luckyi, sp. nov.), Bali (Anelosimus bali, sp. nov.), Australia (Anelosimus pratchetti, sp. nov.) and an unknown locality (Anelosimus terraincognita, sp. nov.), ranging from solitary to subsocial. A phylogenetic analysis supports the inclusion of these species in Anelosimus, and suggests that solitary Papuan species represent a second reversal from subsocial behaviour. Both solitary species inhabit the beachfront, a habitat that appears not to be conducive to social behaviour in spiders. Subsocial species, as in other parts of the world, are found in montane tropical forests of Papua New Guinea, and at relatively high latitudes in Australia. Thus, a global ecological pattern of sociality in Anelosimus is emerging as taxonomic, phylogenetic and ethological knowledge extends beyond the Americas.

How Did the Spider Cross the River? Behavioral Adaptations for River-Bridging Webs in Caerostris darwini (Araneae: Araneidae)

Background Interspecific coevolution is well described, but we know significantly less about how multiple traits coevolve within a species, particularly between behavioral traits and biomechanical properties of animals “extended phenotypes”. In orb weaving spiders, coevolution of spider behavior with ecological and physical traits of their webs is expected. Darwins bark spider (Caerostris darwini) bridges large water bodies, building the largest known orb webs utilizing the toughest known silk. Here, we examine C. darwini web building behaviors to establish how bridge lines are formed over water. We also test the prediction that this spiders unique web ecology and architecture coevolved with new web building behaviors. Methodology We observed C. darwini in its natural habitat and filmed web building. We observed 90 web building events, and compared web building behaviors to other species of orb web spiders. Conclusions Caerostris darwini uses a unique set of behaviors, some unknown in other spiders, to construct its enormous webs. First, the spiders release unusually large amounts of bridging silk into the air, which is then carried downwind, across the water body, establishing bridge lines. Second, the spiders perform almost no web site exploration. Third, they construct the orb capture area below the initial bridge line. In contrast to all known orb-weavers, the web hub is therefore not part of the initial bridge line but is instead built de novo. Fourth, the orb contains two types of radial threads, with those in the upper half of the web doubled. These unique behaviors result in a giant, yet rather simplified web. Our results continue to build evidence for the coevolution of behavioral (web building), ecological (web microhabitat) and biomaterial (silk biomechanics) traits that combined allow C. darwini to occupy a unique niche among spiders.